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CLIMACLIMACLIMACLIMACLIMATTTTTOLOLOLOLOLOGYOGYOGYOGYOGY

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Atmosphere is a thick gaseous envelope thatsurrounds the earth and extends thousands ofkilometers above the earth's surface. Much of thelife on the earth exists because of the atmosphereotherwise the earth would have been barren. Infact, atmosphere directly or indirectly influencesthe vegetation pattern, soil type and topographyof the earth.

Composition of the Atmosphere

The atmosphere is a mixture of several gases.It contains huge amount of solid and liquidparticles collectively known as aerosols. Pure dryair consists mainly of Nitrogen, Oxygen, Argon,Carbon Dioxide, Hydrogen, Helium and Ozone.Besides, water vapour, dust particles, smoke, salts,etc. are also present in the air in varying quantities.

The chemical composition of atmosphere upto an altitude of about 90 km is uniform in termsof major gases-Nitrogen and Oxygen. This layeris called Homosphere. Above 90 km, theproportion of the gases changes with progressiveincrease in the proportion of lighter gases. Thislayer is known as Heterosphere.

Nitrogen and Oxygen comprise 99% of thetotal volume of the atmosphere. But Nitrogen doesnot easily enter into chemical union with othersubstances. It serves mainly as an agent of dilutionand remains chemically inactive. Oxygencombines with all the elements easily and is mostcombustible. Carbon dioxide constitutes a smallpercentage of the atmosphere. It can cause thelower atmosphere to be warmed up by absorbingheat from the incoming short wave solar radiationfrom the sun and reflecting the long waveterrestrial radiation back to the earth's surface.Carbon dioxide is utilized by the green plants inthe process of photosynthesis.

Water vapour and dust particles are theimportant variables of weather and climate. Theyare the source of all forms of condensation andprincipal absorbers of heat received from the sunand radiated from the earth. Water vapourcomprises 3-4% of the total volume of air.

However, the amount of water vapour presentin the atmosphere decreases from the equatortowards the poles. Nearly 90% of the total watervapour lies below 6 km of the atmosphere.

Structure of the Atmosphere

The atmosphere consists of almost concentriclayers of air with varying density and temperature.

a) Troposphere:

• Lowest layer of the atmosphere.

• The height of troposphere is 16 km thickover the equator and 10 km thick at thepoles.

• All weather phenomena are confined totroposphere (e.g. fog, cloud, frost, rainfall,storms, etc.)

• Temperature decreases with height in thislayer roughly at the rate of 6.5° per 1000metres, which is called normal lapse rate.

• Upper limit of the troposphere is calledtropopause which is about 1.5 km.

b) Stratosphere:

• The stratosphere is more or less devoid ofmajor weather phenomenon but there iscirculation of feeble winds and cirruscloud in the lower stratosphere.

CLIMATOLOGY

Component Per Cent by Volume

Nitrogen 78.08 %

Oxygen 20.94 %

Argon 0.93%

Carbon dioxide 0.03%

Neon 0.0018%

Helium 0.0005%

Ozone 0.00006%

Hydrogen 0.00005%

EARTH'S ATMOSPHERE

CHRONICLEIAS ACADEMYA CIVIL SERVICES CHRONICLE INITIATIVE

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• Jet aircrafts fly through the lowerstratosphere because it providesconducive flying conditions.

• Ozone layer lies within the stratospheremostly at the altitude of 15 to 35 km aboveearth's surface.

• Ozone layer acts as a protective cover asit absorbs ultra-voilet rays of solarradiation.

• Depletion of ozone may result in rise oftemperature of ground surface and loweratmosphere.

• Temperature rises from -60°C at the baseof the stratosphere to its upper boundaryas it absorbs ultra-voilet rays.

• Upper limit of the Stratosphere is calledstratopause.

c) Mesosphere

• Mesosphere extends to the height of 50-90 km.

• Temperature decreases with height. Itreaches a minimum of -80°C at an altitudeof 80-90 km

• The upper limit is called mesopause.

d) Thermosphere

• It lies at 80 km to 640 km above the earth'ssurface.

• It is also known as ionosphere.

• Temperature increases rapidly withincreasing height.

• It is an electrically charged layer. This layeris produced due to interaction of solarradiation and the chemicals present, thusdisappears with the sunset.

• There are a number of layers inthermosphere e.g. D-layer, E-layer, F-layer and G-layer.

• Radio waves transmitted from earth arereflected back to the earth by these layers.

e) Exosphere

• This is the uppermost layer of theatmosphere extending beyond theionosphere.

• The density is very low and temperaturebecomes 5568°C.

• This layer merges with the outer space.

About Ionosphere

At heights of 80 km (50 miles), the gas is sothin that free electrons can exist for short periodsof time before they are captured by a nearbypositive ion. The number of these free electrons issufficient to affect radio propagation. This portionof the atmosphere is ionized and contains plasmawhich is referred to as the ionosphere. In plasma,the negative free electrons and the positive ionsare attracted to each other by the electromagneticforce, but they are too energetic to stay fixedtogether in an electrically neutral molecule.

The Ultraviolet (UV), X-Ray and shorterwavelengths of solar radiation ionizes theatmosphere. In this process the light electronobtains a high velocity so that the temperature ofthe created electronic gas is much higher (of theorder of thousand K) than the one of ions andneutrals. The reverse process to Ionization isrecombination, in which a free electron is"captured" by a positive ion, occursspontaneously. The balance between these twoprocesses determines the quantity of ionizationpresent.

Ionization depends primarily on the Sun andits activity. The amount of ionization in theionosphere varies greatly with the amount ofradiation received from the Sun. Thus there is adiurnal (time of day) effect and a seasonal effect.

The ionosphere is broken down into the D, Eand F regions.

The D region is the lowest in altitude, itdoesn't have a definite starting and stoppingpoint, but includes the ionization that occursbelow about 90km. Ionization here is due toLyman series-alpha hydrogen radiation at awavelength of 121.5 nanometres (nm) ionizingnitric oxide (NO). In addition, with high Solaractivity hard X-rays (wavelength < 1 nm) mayionize (N2, O2). Recombination is high in the Dlayer, the net ionization effect is low, but loss ofwave energy is great due to frequent collisions ofthe electrons. As a result high-frequency (HF)radio waves are not reflected by the D layer butsuffer loss of energy therein.

The E region peaks at about 105 km. It absorbssoft x-rays. Normally, this layer can only reflectradio waves having frequencies lower than about10 MHz. At night the E layer rapidly disappearsbecause the primary source of ionization is nolonger present.


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The F layer or region, also known as theAppleton layer extends from about 200 km tomore than 500 km above the surface of Earth. Itis the densest point of the ionosphere, whichimplies signals penetrating this layer will escapeinto space. At higher altitudes the amount ofoxygen ions decreases and lighter ions such asHydrogen and Helium become dominant, thislayer is the topside ionosphere. Here extremeultraviolet (UV, 10-100 nm) solar radiation ionizesatomic oxygen. The F layer consists of one layerat night, but during the day, a deformation oftenforms in the profile that is labeled F1. The F2 layerremains by day and night responsible for mostskywave propagation of radio waves, facilitatinghigh frequency (HF, or shortwave) radiocommunications over long distances.

Actually when a radio wave reaches theionosphere the electric field in the wave forces

the electrons in the ionosphere into oscillation atthe same frequency as the radio wave. Some ofthe radio-frequency energy is given up to thisresonant oscillation. The oscillating electrons willthen either be lost to recombination or will re-radiate the original wave energy. Total refractioncan occur when the collision frequency of theionosphere is less than the radio frequency, andif the electron density in the ionosphere is greatenough.

The critical frequency is the limiting frequencyat or below which a radio wave is reflected by anionospheric layer at vertical incidence. If thetransmitted frequency is higher than the plasmafrequency of the ionosphere, then the electronscannot respond fast enough, and they are notable to re-radiate the signal. On a more practicalnote, the D and E regions reflect AM radio wavesback to Earth.

CONCEPT OF WEATHER AND CLIMATE

Earth's climate has been changing for billions ofyears

Climatologists have used various techniquesand evidence to reconstruct a history of theEarth's past climate. They have found that duringmost of the Earth's history global temperatureswere probably 8 to 15 degrees Celsius warmerthan today. In the last billion years of climatichistory, warmer conditions were broken by glacialperiods starting at 925, 800, 680, 450, 330, and 2million years before present.

The period from 2,000,000 - 14,000 B.P.(before present) is known as the Pleistocene orIce Age. During this period, large glacial ice sheetscovered much of North America, Europe, andAsia for extended periods of time. The extent ofthe glacier ice during the Pleistocene was notstatic. The Pleistocene had periods when theglacier retreated (interglacial) because of warmertemperatures and advanced because of coldertemperatures (glacial). During the coldest periodsof the Ice Age, average global temperatures wereprobably 4 - 5 degrees Celsius colder than theyare today.

By 5000 to 3000 BC average globaltemperatures reached their maximum levelduring the Holocene and were 1 to 2 degreesCelsius warmer than they are today.Climatologists call this period the ClimaticOptimum. During the Climatic Optimum, manyof the Earth's great ancient civilizations began

Weather refers to the sum total of theatmospheric conditions in terms of temperature,pressure, wind, moisture etc of a given place andtime. It is very dynamic as it may change severaltimes even in a day. Weather changes each daybecause the air in our atmosphere is alwaysmoving, distributing energy from the Sun. In mostplaces in the world, the types of weather eventsalso vary throughout the year as season's change.

Climate on the other hand, is the averageweather condition or atmospheric condition of aregion over a considerable period of time. Theregional climate is the average weather in a placeover more than thirty years. The climate of a regiondepends on many factors including the amountof sunlight it receives, its height above sea level,the shape of the land, and how close it is to oceans.Since the equator receives more sunlight than thepoles, climate varies depending on distance fromthe equator.

However, Global climate is a description ofthe climate of a planet as a whole, with all theregional differences averaged. Overall, globalclimate depends on the amount of energy receivedby the Sun and the amount of energy that istrapped in the system. These amounts aredifferent for different planets. Scientists who studyEarth's climate and climate change study thefactors that affect the climate of the whole planet.While the weather can change in just a few hours,climate changes over longer time-frames.


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and flourished. In Africa, the Nile River had threetimes its present volume, indicating a much largertropical region.

From 3000 to 2000 BC a cooling trendoccurred. This cooling caused large drops in sealevel and the emergence of many islands(Bahamas) and coastal areas that are still abovesea level today. A short warming trend took placefrom 2000 to 1500 BC, followed once again bycolder conditions. Colder temperatures from 1500- 750 BC caused renewed ice growth in continentalglaciers and alpine glaciers, and a sea level dropof between 2 to 3 meters below present day levels.

During the period from 750 BC - 800 ADwarming up of atmosphere took place. Duringthe time of Roman Empire (150 BC - 300 AD) acooling began that lasted until about 900 AD. Atits height, the cooling caused the Nile River (829

AD) and the Black Sea (800-801 AD) to freeze.

From 1550 to 1850 AD global temperatureswere at their coldest since the beginning of theHolocene. Scientists call this period the Little IceAge. During the Little Ice Age, the averagesannual temperature of the Northern Hemispherewas about 1.0 degree Celsius lower than today.During the period 1580 to 1600, the westernUnited States experienced one of its longest andmost severe droughts in the last 500 years. Coldweather in Iceland from 1753 and 1759 caused25% of the population to die from crop failureand famine.

The period 1850 to present is one of generalwarming. Many scientists believe the warmertemperatures of the 20th and 21st centuries arebeing caused by the human enhancement of theEarth's greenhouse effect.

The source of all energy on the earth is theSun. The Sun acts as a great engine that driveswinds, ocean currents, exo-genetic or denudationprocesses and sustains life on the earth. Theenergy radiated from the Sun comes from thenuclear fusion process taking place in its core,where the temperature is about 15,000,000ºC.The solar energy is transmitted in the form ofshort wave electromagnetic radiations. Theytravel at the speed of light (about 2, 98,000 kmper second).

Insolation

Insolation is a measure of solar radiationenergy received on a given surface area in a giventime. Solar radiation is radiant energy emitted bythe sun from a nuclear fusion reaction that createselectromagnetic energy. The spectrum of solarradiation is close to that of a black body with atemperature of about 5800 K. About half of theradiation is in the visible short-wave part of theelectromagnetic spectrum. The other half is mostlyin the near-infrared part, with some in theultraviolet part of the spectrum.

On an average the earth receives 1.94 caloriesper sq. cm per minute at the top of its atmosphere.

Factors affecting the distribution of insolation:

• Distance between the Earth and the Sun-The solar output received at the top of theatmosphere varies slightly in a year due tothe variations in the distance between the

earth and the Sun. During its revolutionaround the Sun, the earth is farthest fromthe sun (152 million km) on 4th July. Thisposition of the earth is called aphelion. On3rd January, the earth is nearest to the sun(147 million km). This position is calledperihelion. Therefore, the energy receivedby the earth on 3rd January is slightly morethan the amount received on 4th July.

• Angle of inclination of the sun's rays- Itdepends on the latitude of a place. The higherthe latitude the less is the angle it makes withthe surface of the Earth, resulting in slantingSun rays. Hence, the area covered by slantingrays is always higher than that of the verticalrays. Thus, the energy gets distributed in alarger area and the net energy received perunit area decreases. Thus the amount ofinsolation received at the Earth's surfacedecreases from equator towards the poles.The total amount of insolation received atthe equator is about four times higher thanthat of the poles.

• Length of the day- During the summerseason days are longer than the night. Thesituation is reversed in the winter season.The longer the day, the more is the insolationreceived by earth.

• Altitude of land- Insolation heats up theland which in turn warms the air above it byconduction and convection. As higher landis further away from the main heat source,

INSOLATION AND TEMPERATURE


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it is relatively cooler. Also the densityof air decreases with height.

• Sunspots- Sunspots are created in thesolar outer surface due to periodicdisturbances and explosions. Thesunspots are cyclic in nature. Theincrease and decrease of number ofsunspots is completed in cycle of 11years. The amount of insolationreceived at earth's surface increasesand decreases with number ofsunspots.

Heat Budget

The Earth's climate is a solar poweredsystem. The earth intercepts only ½ of thebillionth fraction of the energy radiatedfrom the Sun. The Earth in turn radiates back theenergy received from the sun in the form of longwave terrestrial radiations. As a result, the earthneither warms up nor does it get cooled over aperiod of time. It maintains its temperature.

This can happen only because the amountof heat received in the form of insolationequals the amount lost by the earth throughterrestrial radiation. This is known as heat budgetof the earth.

Let the solar energy radiated be taken as 100units. Out of total incoming radiation enteringthe earth's atmosphere 35 per cent is sent back tospace through scattering by dust particles (6%),reflection from clouds (27%) and from the groundsurface (2%), 51 per cent is received by the earth'ssurface and 14 per cent is absorbed by theatmospheric gases and water vapour.

Incoming shortwave solar radiation: equals to100 units

a) Amount lost to space through scattering andreflection equals to 35% comprises of

• Clouds = 27%

• Reflected by ground = 2%

• Scattered by dust particles = 6%

b) Heat received by earth equals to 51%comprises of

• Through direct radiation = 34%

• Received as diffuse day light = 17%

c) Absorption by the atmospheric gases andwater vapour equals to 14%

After receiving energy from the Sun the Earthalso radiates energy out of its surface into theatmosphere through long-wave radiation. As wehave seen above 51 per cent of heat is absorbedby the earth. Thus at the time of outgoingradiations 23 per cent of the energy is lost throughdirect long-wave terrestrial radiation (in which6% absorbed by atmosphere and 17% goes directlyto space). About 9 per cent out of 51 per cent isspent in convection and 19 per cent is spentthrough evaporation. Thus the total energyreceived by atmosphere from sun and the earthbecomes 48 per cent.

The atmosphere receives a total of 14 + 34 =48 units and this amount is radiated back to spaceby the atmosphere. The total loss of energy tospace thus amounts to 100 units: 35 units reflectedby the atmosphere, 17 units lost as terrestrialradiation and 48 units from the atmosphere. Inthis manner, no net gain or loss of energy occursin the earth's surface.

Outgoing long-wave terrestrial radiation

a) Reflected by Earth which was equal to 51per cent as shown above

• 23% from radiation

• 9% through convection

• 19% through evaporation

b) 48% absorbed in atmosphere moved throughradiation back into space.

Although the Earth and its atmosphere as awhole have a radiation balance, there arelatitudinal variations. The heat energy istransferred from the lower latitudes to the higher


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latitudes through winds and ocean currents. Inthe low latitudes (between 400°N and 400°S) heatgained by short wave radiation is far more thanthe heat loss by long waves through the earth'sradiation. In contrast in the higher latitudes moreheat is lost by outgoing long wave than it isreceived in short waves. In view of the imbalancesat high and low latitudes, there is large-scaletransfer of heat from tropics to high latitudes byatmospheric and oceanic circulation.

Nowadays Green House Gases are disturbingthe Earth's heat budget. Carbon dioxide forcesthe Earth's energy budget out of balance byabsorbing thermal infrared energy (heat) radiatedby the surface. It absorbs thermal infrared energywith wavelengths in a part of the energy spectrumthat other gases, such as, water vapor, do not.Carbon dioxide is a very strong absorber ofthermal infrared energy with wavelengths longerthan 12-13 micrometers, which means thatincreasing concentrations of carbon dioxidepartially "close" the atmospheric window. Inother words, wavelengths of outgoing thermalinfrared energy that our atmosphere's mostabundant greenhouse gas-water vapor-wouldhave let escape to space are instead absorbed bycarbon dioxide.

The absorption of outgoing thermal infraredby carbon dioxide means that Earth still absorbsabout 70 percent of the incoming solar energy,but an equivalent amount of heat is no longerleaving. The exact amount of the energyimbalance is very hard to measure, but it appearsto be a little over 0.8 watts per square meter.

When increasing greenhouse gasconcentrations bumps the energy budget out ofbalance, it doesn't change the global averagesurface temperature instantaneously. It may takeyears or even decades for the full impact of aforcing to be felt. This lag between when animbalance occurs and when the impact on surfacetemperature becomes fully apparent is mostlybecause of the immense heat capacity of the globalocean. The heat capacity of the oceans gives theclimate a thermal inertia that can make surfacewarming or cooling more gradual, but it can'tstop a change from occurring.

However, as long as greenhouse gasconcentrations continue to rise, the amount ofabsorbed solar energy will continue to exceed theamount of thermal infrared energy that canescape to space. The energy imbalance willcontinue to grow, and surface temperatures willcontinue to rise. This leads to Global Warming.

Temperature

Temperature is the measurement of availableheat energy in a system. It is a measure of hotnessand coldness of the body. The atmosphere is notheated directly by insolation or the Sun's rays;rather it is heated from below by the warmedsurface of the earth, i.e. from terrestrial radiation.

The lower levels of the atmosphere are heatedby conduction. The earth's surface is heatedduring day time after receiving solar radiation.The air coming in contact with the warmerground surface is also heated because of transferof heat from the ground surface through themolecules to the air.

The upper levels of the earth get heated byconvection. The air coming in contact with thewarmer surface of the earth gets heated andexpands in volume. The warmer air rose up andforms vertical circulation of air. This mechanismtransports heat from the ground surface to theatmosphere, thus helps in heating up of theatmosphere.

Factors controlling the distribution oftemperature:

• Latitude: In general, average temperaturedecreases form the equator towards the polesbecause the sun rays become more and moreoblique poleward.

• Altitude: the temperature decreases withincreasing height from the earth's surface atan average rate of 6.5 °C per 1000 m. Thelower layer of air contains more vapourhence it absorbs more heat radiated from theearth's surface than the upper air layers.

• Mountain Ranges: In certain areas of theworld the existence of high ranges ofmountains acts as formidable barrier to thefree circulation of air in the lower reaches ofthe atmosphere. The Himalayas, for example,prevents the monsoon conditions extendingfurther north into the interior of Asia andprevents the extremely cold anticyclonicwinter conditions in Central Asia frompenetrating Indian subcontinent. Similarlythe Western Cordillera (including the RockyMountains) of North America prevents theintrusion of the westerly maritime influenceof the Pacific into the heartland of the Prairiesand Great Plains.

• Distance from the Sea: The Sea heats upand cools down much more slowly than the


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land. The effects of this phenomenon aremore noticeable in the temperate latitudeswhere the warming effect of the seaparticularly affects coastal regions in winter.In general, the sea has a moderating effecton the temperatures of the coastal areasthroughout the year. On the other hand,regions deep within the interior of landmasses experience extreme temperatures.This phenomenon is known as continentality.

• Ocean Currents: Ocean currents influencethe temperature of the coastal regionsparticularly where onshore winds carrythe influence of warm currents towardscoastal regions in winter. Cold currentshave a cooling effect on the nearby coastsbut have a lesser effect than warmcurrents due to the fact that they often flowbelow offshore winds.

• Clouds: The presence or absence of cloudsin the atmosphere over different regions ofthe earth's surface has a significant bearingon the temperature of the regions. Cloudshave the effect of reducing the amount bothof insolation, which reaches the surface ofthe earth and of outgoing radiation from theearth's surface. As a result, tropical rainforests with dense cloud cover have very littlerange of temperature, where as, the hotdeserts, which have comparatively less cloudcover, have both high diurnal and annualtemperature ranges.

• Prevailing Winds: Prevailing winds alsoaffect the temperature conditions of theareas. The moderating effects of oceansare brought to the adjacent lands throughwinds. On the contrary, off shore windstake the effects of warm or cold currentsaway from land.

• Local Weather: Local weather comprisesdifferent types of storms, cloudiness,precipitation and other weather conditions.In the equatorial regions, despite the verticalrays of the Sun, large amount of cloudinessobstructs the solar radiation from reachingthe earth surface. It is due to the clear skythat near the Tropic of Cancer and the Tropicof Capricorn the amount of solar radiationincident on the earth exceeds that reachingthe equatorial regions. Thus, in thesubtropical high pressure belt the surfacewater temperature in the oceans is a littlehigher. Besides, the incidence of daily

afternoon rains in the equatorial regionsdoes not allow the temperatures to risefurther, whereas the extremely dryweather and cloudless skies prove helpfulin raising the temperatures in thesubtropical regions. In the same way inregions of stormy weather the ocean watertemperatures are relatively lower.

Concept of Inversion of Temperature

The temperature decreases with thealtitudes in the troposphere at an averagerate of 6.5 °C per 1000m, this is known asnormal lapse rate. But sometimes thetemperature increases upward upto a fewkilometers from the earth's surface. This isknown as inversion of temperature i.e. presenceof warm layer of air above the cold layer of air.

Types of inversion of temperature:

1. Ground surface inversion: The mostcommon condition for inversion oftemperature is through the cooling of the airnear the ground at night. Once the sun goesdown, the ground loses heat very quickly,and this cools the air that is in contact withthe ground. However, since air is a very poorconductor of heat, the air just above thesurface remains warm. Conditions thatfavour the development of a strong surfaceinversion are calm winds, clear skies, andlong nights. Calm winds prevent warmerair above the surface from mixing down tothe ground, and clear skies increase the rateof cooling at the Earth's surface. Long nightsallow for the cooling of the ground to continueover a longer period of time, resulting in agreater temperature decrease at the surface.Since the nights in the winter time are muchlonger than nights during the summer time,surface inversions are stronger and morecommon during the winter months. Duringthe daylight hours, surface inversionsnormally weaken and disappear as the sunwarms the Earth's surface.

2. Upper air inversion: The thermal upperair inversion is caused by the presence ofozone layer in the stratosphere. Theozone layer absorbs most of theultraviolet rays radiated from the sunthus the temperature of this layer becomeshigher than the other layers.


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Consequences of Temperature Inversions

a) Fog is formed due to the presence of warm airabove the cold air. The cold air cools the warmair from below thus forms the tiny dropletsaround dust particles and smokes duringwinter season that result in formation of fog.

b) The urban smog is formed by theintensification of fog by pollution. Whensmog gets mixed with sulphur dioxide it

becomes poisonous and deadly for humanbeings.

c) Inversion of temperature leads to formationof frost. Frost is economically unfavourableweather phenomenon as it damages fruitorchids and crops.

d) The inversion of temperatures createsanticyclonic conditions thus inhibits rainfalland encourages dry conditions.

small changes in outside air pressure cause thecapsule to expand or contract. The size of theaneroid cell is then calibrated and any change inits volume is transmitted by springs and levers toan indicating arm that point to the correspondingatmospheric pressure.Standard Sea-level Pressure

For climato-logical and meteoro-logicalpurposes, standard sea-level pressure is said tobe 76.0 cm or 29.92 inches or 1013.2 millibars.Scientists often use the kilopascal (kPa) as their

preferred unit for measuring pressure. 1 kilopascalis equal to 10 millibars. Another unit of forcesome-times used by scien-tists to measureatmo-spheric pressure is the Newton. Onemillibar equals 100 newtons per square meter(N/m2).

Pressure Belts of the World

a) Equatorial Low Pressure Belt:

At the Equator heated air rises leaving alow-pressure area at the surface. This lowpressure area is known as equatorial low

Air Pressure

Air is a mixture of several gases. Gas moleculesare in constant state of collision and move freely.Pressure of air at a given place is defined as aforce exerted against surface by continuouscollision of gas molecules. Air pressure is thusdefined as total weight of a mass of column of airabove per unit area at sea level. The amount ofpressure exerted by air at a particular point isdetermined by temperature and density which ismeasured as a force per unit area.

Measuring Atmospheric Pressure

• Torricelli Barometer

The instrument that measures air pressure iscalled a barometer. The first measurement ofatmospheric pressure began with a simpleexperiment performed by Evangelista Torricelliin 1643. In his experiment, Torricelli immersed atube, sealed at one end, into a container ofmercury. Atmospheric pressure then forced themercury up into the tube to a level that wasconsiderably higher than the mercury in thecontainer. Torricelli determined from thisexperiment that the pressure of the atmosphereis approximately 30 inches or 76 centimeters (onecentimeter of mercury is equal to 13.3 millibars).He also noticed that height of the mercury variedwith changes in outsideweather conditions.

• Aneroid Barometer

It is the most commontype barometer used inhomes. Inside this instrumentis a small, flexible metalcapsule called an aneroidcell. In the construction of thedevice, a vacuum is createdinside the capsule so that

PRESSURE AND WIND BELTS


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pressure. This area extends between 50°N and50°S latitudes. The zone shifts along with thenorthward or southward movement of sunduring summer solstice and winter solsticerespectively. The pressure belt is thermallyinduced because the ground surface gets heatedduring the day. Thus warm air expands, rises upand creates low pressure.

b) Sub-tropical High Pressure Belt:

The warm air risen up at the equator due toheating reaches the troposphere and bendtowards the pole. Due to coriolis force the airdescends at 30-35º latitude thus creates the beltof sub-tropical high pressure. The pressure belt isdynamically induced as it owes its origin to therotation of the earth and sinking and settling ofwinds. This zone is characterized by anticyclonicconditions which cause atmospheric stability andaridity. Thus the hot deserts of the world arepresent in this region extending between 25-35degrees in both the hemisphere.

c) Sub-Polar Low Pressure Belt:

This belt is located between 60-65 degreeslatitudes in both the hemisphere. This pressurebelt is also dynamically induced. As shown in thefigure the surface air spreads outward from thiszone due to rotation of the earth thus produceslow pressure. The belt is more developed andregular in the southern hemisphere than thenorthern due to over dominance of water in theformer.

d) Polar High Pressure Belt:

High pressure persists at the pole due to lowtemperature. Thus the Polar High Pressure Belt isthermally induced as well as dynamically inducedas the rotation of earth also plays a minor role.

Coriolis Force

The rotation of the Earth creates force, termedCoriolis force, which acts upon wind. Insteadof wind blowing directly from high to lowpressure, the rotation of the Earth causes windto be deflected off course. In the NorthernHemisphere, wind is deflected to the right ofits path, while in the Southern Hemisphere itis deflected to the left. Coriolis force is absentat the equator, and its strength increases asone approaches either pole. Furthermore, anincrease in wind speed also results in a strongerCoriolis force, and thus in greater deflectionof the wind.

Shifting of Pressure Belts

By late June, when the sun is overhead at theTropic of Cancer, the doldrums low pressure beltsmove significantly northwards from the equatorwith a resultant shift of other belts in the NorthernHemisphere. Similarly, in late December, whenthe sun is overhead at the Tropic of Capricorn,the belts move southwards.

Winds

When the movement of the air in theatmosphere is in a horizontal direction over thesurface of the earth, it is known as the wind.Movement of the wind is directly controlled bypressure.

Horizontally, at the Earth's surface windalways blows from areas of high pressure toareas of low pressure usually at speedsdetermined by the rate of air pressure changebetween pressure centres. Wind speed is afunction of the steepness or gradient ofatmospheric air pressure found between high andlow pressure systems. When expressedscientifically, pressure change over a unit distanceis called pressure gradient force and the greaterthis force the faster the winds will blow.

I. Planetary winds:

Planetary winds are major component of thegeneral global circulation of air. These are knownas planetary winds because of their prevalencein the global scale throughout the year. Planetarywinds occur due to temperature and pressurevariance throughout the world.

The planetary winds are discussed below:

(a) Trade wind

Winds blowing from the Subtropical HighPressure Belt or horse latitudes towards theEquatorial Low Pressure Belt or the ITCZ are thetrade winds. In the Northern Hemisphere, thetrade winds blow from the northeast and areknown as the Northeast Trade Winds; in the


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Southern Hemisphere, the winds blow from thesoutheast and are called the Southeast TradeWinds.

The weather conditions throughout thetropical zone remain more or less uniform. Thisbelt is subjected to seasonal variation due tonorthward and southward movement of sun.

The equatorward part of the trade wind arehumid because they are characterized byatmospheric instability thus causes heavyprecipitation.

(b) Westerly Wind

The Westerlies are the prevailing winds inthe middle latitudes between 35º and 65º latitude,blowing from the high pressure area in the SubTropical High Pressure Belt i.e. horse latitudestowards the sub polar low pressure belt. Thewinds are predominantly from the south-west tonorth-east in the Northern Hemisphere and fromthe north-west to south-east in the SouthernHemisphere.

The Westerlies are strongest in the winterseason and times when the pressure is lower overthe poles, while they are weakest in the summerseason and when pressures are higher over thepoles. The Westerlies are particularly strong,especially in the Southern Hemisphere, as thereis less land in the middle latitudes to obstruct theflow. The Westerlies play an important role incarrying the warm, equatorial waters and windsto the western coasts of continents, especially inthe Southern Hemisphere because of its vastoceanic expanse.

(c) Polar Wind

The winds blowing in the Arctic and theAntarctic latitudes are known as the Polar Winds.They have been termed the 'Polar Easterlies', asthey blow from the Polar High Pressure belttowards the Sub-Polar Low-Pressure Belts. In theNorthern Hemisphere, they blow in general fromthe north-east, and are called the North-East PolarWinds; and in the Southern Hemisphere, theyblow from the south-east and are called the South-East Polar Winds. As these winds blow from theice-capped landmass, they are extremely cold.They are more regular in the Southern Hemispherethan in the Northern Hemisphere.

II. Periodic Winds:

Land and sea breezes and monsoon windsare winds of a periodic type. Land and sea breezes

occur daily, whereas the occurrence of monsoonwinds is seasonal. Following are periodic winds:

(a) Monsoon winds

(b) Land and Sea Breeze

(c) Mountain and Valley Breeze

(a) Monsoon Winds

Monsoons are regional scale wind systemsthat predictably change direction with thepassing of the seasons. Like land and seabreezes, these wind systems are created bythe temperature contrasts that exist between thesurfaces of land and ocean. However, monsoonsare different from land and sea breezes bothspatially and temporally. Monsoons occur overdistances of thousands of kilometers, and theirtwo dominant patterns of wind flow act over anannual time scale.

Summer Monsoon

During the summer, monsoon winds blowfrom the cooler ocean surfaces onto the warmercontinents. In the summer, the continents becomemuch warmer than the oceans because of anumber of factors. These factors include:

(i) Specific heat differences between land andwater.

(ii) Greater evaporation over water surfaces.

(iii) Subsurface mixing in ocean basins, whichredistributes heat energy through a deeperlayer.

Precipitation is normally associated with thesummer monsoons. Onshore winds blowinginland from the warm ocean are very high inhumidity, and slight cooling of these air massescauses condensation and rain. In some cases, thisprecipitation can be greatly intensified byorographic uplift. Some highland areas in Asiareceive more than 10 meters of rain during thesummer months.

Winter Monsoon

In the winter, the wind patterns reverses, asthe ocean surfaces are now warmer. With littlesolar energy available, the continents begincooling rapidly as longwave radiation is emittedto space. The ocean surface retains its heat energylonger because of water's high specific heat andsubsurface mixing. The winter monsoons bringclear dry weather and winds that flow from landto sea.


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World Monsoonal Climate

The Asiatic monsoon is the result of acomplex climatic interaction between thedistribution of land and water, topography,and tropical and mid-latitudinal circulation.In the summer, a low-pressure centre formsover Northern India and Northern SoutheastAsia because of higher levels of received solarinsolation. Warm moist air is drawn into thethermal lows from air masses over the IndianOcean. Summer heating also causes thedevelopment of a strong latitudinal pressuregradient and the development of an easterlyjet stream at an altitude of about 15 kilometersat the latitude of 25° north. The jet streamenhances rainfall in Southeast Asia, in theArabian Sea, and in South Africa. Whenautumn returns to Asia the thermal extremesbetween land and ocean decrease and thewesterlies of the mid-latitudes move in. Theeasterly jet stream is replaced with strongwesterly winds in the upper atmosphere.Subsidence from an upper atmosphere cold lowabove the Himalayas produces outflow that creates

a surface high-pressure system that dominates theweather in India and Southeast Asia.

Besides, Asian continent, monsoon windsystems also exist in Australia, Africa, SouthAmerica, and North America.

(b) Land and Sea Breezes:

A land breeze is created when the land iscooler than the water such as at night and thesurface winds have to be very light. When thishappens the air over the water slowly begins torise, as the air begins to rise, the air over thesurface of the ocean has to be replaced, this isdone by drawing the air from the land over thewater, thus creating a sea breeze.

A sea breeze is created when the surface ofthe land is heated sufficiently to start rising of the

air. As air rises, it is replaced by air from the sea;you have now created a sea breeze. Sea breezestend to be much stronger and can produce gustywinds as the sun can heat the land to very warmtemperatures, thereby creating a significanttemperature contrast to the water.

(c) Mountain and Valley winds:

Mountain-valley breezes are formed by thedaily difference of the thermo effects betweenpeaks and valleys. In daytime, the mountainsideis directly heated by the sun, the temperature ishigher, air expands, air pressure reduces, andtherefore air will rise up the mountainside fromthe valley and generate a valley breeze. The valleybreeze reaches its maximum force at around 2p.m. After this time, the breeze decreases in


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power and come to a complete stop by sunset. Bynightfall, the mountainside region is able todissipate heat more quickly, due to its higheraltitude and therefore temperature drops rapidly.Cold air will then travel down the mountainsidefrom the top and flow into the valley, forming amountain breeze.

In daytime, valley breezes carry water vapourto the peak, which will often condense into clouds;these are the commonly seen peak and flag clouds.When the mountain breeze travels down andgathers in the valleys, water vapour willcondense. Therefore in valley or basin regions,there are usually clouds and fog before sunrise.During late spring or early autumn, the cold airtrapped in the valley and basin will often generate

frost. That's why farmers usually plant cold-sensitive plants such as tangerines and coffee onmountainsides.

III. Local Winds

These local winds blow in the various regionof the world.

Hot Winds

Sirocco - Sahara Desert

Leveche - Spain

Khamsin - Egypt

Harmattan - Sahara Desert

Santa Ana - USA

Zonda - Argentina

Brick fielder - Australia

Cold Winds

Mistral - Spain and France

Bora - Adriatic coast

Pampero - Argentina

Buran - Siberia

Descending Winds

Chinook - USA

Fohn - Switzerland

Berg - Germany

Norwester - New Zealand

Samun - Persia (Iran)

Nevados - Ecuador

JET-STEAMS

The JET STREAMS located in the uppertroposphere (9 - 14 km) are bands of high speedwinds (95-190 km/hr). The term was introducedin 1947 by Carl Gustaf Rossby. Average speed isvery high with a lower limit of about 120 Kms inwinter and 50 km per hours in summer. The twomost important types of jet streams are the PolarJet Streams and the Subtropical Jet Streams. Theyare found in both the Northern and SouthernHemispheres.

Polar Jet Stream: These jet streams are createdwhen cold air from the Polar Regions meets

warmer air from the equator. This temperaturegradient as a result forms a pressure gradientthat increases wind speed. During the winter,these jet streams bring winter storms andblizzards to the United States and in summer,they become weaker and move towards highlatitudes. They are found in both the Northernand Southern Hemispheres.

Sub-Tropical Jet Stream: These jet streams areformed as warm air from the equator movestowards the poles that form a steep temperatureincline along a subtropical front that like the polar


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jet streams produce strong winds. In SoutheastAsia, India, and Africa, these jet streams helpbring about the region's monsoon or rainy climate.As these jet streams warm the air above theTibetan highlands, a temperature and pressuregradient is formed as the air from the ocean iscooler than that above the continental high lands.This as a result forms on-shore winds thatproduces the monsoon.

Formation of Jet Stream

Warm air masses in the south meet cool airmasses from the north and create temperatureand air pressure gradients. In wind speed, thepressure difference between a high and lowpressure zone can be very large, thereby creatinghigh winds. Pressure and temperature differencesin the jet stream can be large as a global warmfront from the south and a cold front from thenorth meet.

EL NIÑO AND LA NINA SITUATION

than in the east. Thus forms warmer, deeperwaters in the western Pacific and cooler, shallowerwaters in the east near the coast of South America.The different water temperature of these areasaffects the types of weather these two regionsexperience.

In the east the water cools the air above it,and the air becomes too dense to rise to produceclouds and rain. However; in the western Pacificthe air is heated by the water below it, increasingthe buoyancy of the lower atmosphere thusincreasing the likelihood of rain. This is why heavyrain storms are typical near Indonesia while Peruis relatively dry.

El Niño condition

El Nino happens when weakening tradewinds (which sometimes even reverse direction)allow the warmer water from the western Pacificto flow toward the east. This flattens out the sealevel, builds up warm surface water off the coastof South America, and increases the temperatureof the water in the eastern Pacific.

Normal condition

In general, the water on the surface of theocean is warmer than at the bottom because itis heated by the sun. In the tropical Pacific, windsgenerally blow in easterly direction. These winds

tend to push the surface water toward the westalso. As the water moves west it heats up evenmore because it's exposed longer to the sun.

Meanwhile in the eastern Pacific along thecoast of South America an upwelling occurs.Upwelling is the term used to describe whendeeper colder water from the bottom of the oceanmoves up toward the surface away from theshore. This nutrient-rich water is responsible forsupporting the large fish population commonlyfound in this area. Indeed, the Peruvian fishinggrounds are one of the five richest in the world.

Because the trade winds push surface waterwestward toward Indonesia, the sea level isroughly half a metre higher in the western Pacific


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An El Nino condition results from weakenedtrade winds in the western Pacific Ocean nearIndonesia, allowing piled-up warm water to flowtoward South America.

Since fish can no longer access this rich foodsource, many of them die off. That is why, theseconditions are called "El Nino", or "the ChristChild", which is what Peruvian fisherman callthe particularly bad fishing period aroundDecember. More importantly, the different watertemperatures tend to change the weather of theregion.

What happens to the ocean also affects theatmosphere. Tropical thunderstorms are fueledby hot, humid air over the oceans. Hotter the air,the stronger and bigger the thunderstorms. As thePacific's warmest water spreads eastward, thebiggest thunderstorms move with it.

The clouds and rainstorms associated withwarm ocean waters also shift toward the east.Thus, rains which normally would fall over the

tropical rain forests of Indonesia start falling overthe deserts of Peru, causing forest fires anddrought in the western Pacific and flooding inSouth America. Moreover the Earth's atmosphere

reponds to the heating of El-Nino byproducing patterns of high and lowpressure which can have a profoundimpact on weather far away from theequatorial Pacific. For instance, highertemperatures in western Canada andthe upper plains of the United Stateswhereas colder temperatures in the

southern United States. The east coast of southernAfrica often experiences drought during El Nino.

La Nina

If, on the other hand, the surface trade windsstrengthen and with it the east-west slopes andalong with it the east-west oceanic temperaturegradients the resulting weather pattern leads toan anti-El Niño, that is often referred to as LaNiña. Such events are characterized by a highpositive Southern Oscillation Index (i.e. anincreased westward pressure gradient over theequatorial Pacific), stronger surface trade windsover the central Pacific, and cooler SSTs in theeastern equatorial Pacific. Such a weatherpattern, on the other hand, is associated withincreased cyclone activity in the western Pacific,off shore of eastern Australia, the Phillipines, andthe western Atlantic region.

CLOUDS AND PRECIPITATION

Moisture, or water vapour, is an extremelyimportant constituent of the atmosphere. Watervapour present in the air is known as Humidity.The nature and amount of precipitation, theamount of loss of heat through radiation fromthe earth's surface, latent heat of atmosphere etcdepend on the amount of water vapour presentin the atmosphere.

Water vapour is derived through the processof evaporation from oceans, seas, rivers, lakesetc. Humidity capacity that is content of watervapour in the air is directly positively related withtemperature i.e. higher the temperature higherthe humidity capacity. Oceanic and coastal areasrecord higher humidity capacity. It decreasesfrom equator to pole.

Humidity of a place can be expressedin three ways

Absolute Humidity: The measure of watervapour content of the atmosphere which may be

expressed as the actual quantity of water vapourpresent in a given volume of air is called AbsoluteHumidity. The absolute humidity is measured interms of grain per cubic metre air. If the absolutehumidity of air at a given place is 10 gm/cu m. itmeans that 10 grams of water vapour are presentin a cubic metre of air. Absolute humidity of theair changes from place to place and from time totime. The ability of air to hold water vapourdepends entirely on its temperature. Warm aircan hold more moisture than the cold air.

Specific Humidity: Another way to expresshumidity as the weight of water vapour per unitweight of air or the proportion of the mass ofwater vapour to the total mass of air is called thespecific humidity. Specific humidity is not affectedby changes in pressure of temperature.

Relative Humidity: A more useful measure ofhumidity of the atmosphere is called the relativehumidity. This is a ratio expressed as a percentagebetween the actual quantity of water vapour


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present in the air at a given temperature and themaximum quantity of water vapour that theatmosphere can hold at that temperature.Relative humidity determines the amount andrate of evaporation and hence it is an importantclimatic factor. With the same quantity of watervapour, relative humidity will decrease withincrease of temperature and vice versa.

Condensation: Concept Relatedto Climatology

Condensation is the transformation ofgaseous form of water into solid form i.e. ice andliquid form i.e. water.

Mechanism of Condensation

The mechanism of condensation dependsupon the amount of relative humidity present inthe air. When the air achieves 100 per cent relativehumidity, it is called as saturated air (no morewater vapour can added to it).

The temperature at which air becomessaturated is known as dew point andcondensation starts only at this point. If thetemperature at dew point is above freezing pointcondensation occurs in the form of fog, rainfall,etc. On the other hand if the dew point is belowfreezing point condensation occurs in the formof snow, frost, etc.

How saturation of air is achieved

Method 1: When the absolute humidity at a giventemperature is raised equal to the humidityretaining capacity of the air.

Method 2: When the temperature of the air isreduced to such an extent that the humiditycapacity becomes equal to its absolute humidity.

Concept of Adiabatic change of temperature

If the change in temperature of air takes placeby the ascent and descent of air and no additionor subtraction of heat occurs then this process isknown as adiabatic change of temperature.

When the air is warmer than the surroundingair-mass, it ascends. Due to upward movementof air, volume increases and the temperaturedecreases. As the dew point is achieved,condensation process starts and leads toformation of clouds, fog and ultimately leads toprecipitation. Thus instability of air causesdifferent weather phenomenon. In the processexplained above there is no addition or

subtraction of heat and the process is solely dueadiabatic change of temperature.

On the other hand if the dew point is notachieved, the air becomes colder thansurrounding air. Thus the air descends andbecomes cooler. This leads to stability of air andweather phenomenon get hampered.

Clouds and its types

A cloud is a visible mass of liquid droplets orfrozen crystals made of water and/or variouschemicals suspended in the atmosphere abovethe surface of a planetary body.

In general, clouds form when rising air iscooled to its dew point (the temperature at whichthe air becomes saturated). Water vapournormally begins to condense on condensationnuclei such as dust, ice, and salt in order to formclouds. If sufficient condensation particles arenot present, the air will become supersaturatedand the formation of cloud or fog will be inhibited.

The cloud formation is generally due to adiabaticcooling.

Sources of water vapour

The main sources from which water vapouris added to the air are: wind convergence overwater or moist ground into areas of upwardmotion, precipitation, daytime heating leadingto evaporation of water from the surface ofoceans, water bodies or wet land andtranspiration from plants.


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Significance of clouds

Clouds are very significant because:

• They cause all forms of precipitation.

• They play a major role in the heat budgetof the earth.

• They reflect, absorb some part of incomingsolar radiation as well as some part oflong-wave terrestrial radiation re-radiated by the earth.

Types of clouds

1. Cirrus clouds: These clouds form above20,000 feet (6,000 meters) and since thetemperatures are extremely low at suchhigh elevations, these clouds are primarilycomposed of ice crystals. High-level cloudsare typically thin and white in appearance,but can appear in a magnificent array ofcolours when the sun is low on the horizon.

2. Cirro-cumulus: These clouds appear as small,rounded white puffs. The small ripples inthe cirrocumulus sometimes resemble thescales of a fish. A sky with cirrocumulusclouds is sometimes referred to as a "mackerelsky."

3. Cirro-stratus: These clouds are thin, sheet-like high clouds that often cover the entiresky. They are so thin that the sun and themoon can be seen through them.

Cirrus clouds Cirro-cumulus

Cirro-stratus

4. Altocumulus clouds: These are middle levelclouds that are made up of water dropletsand appear as grey, puffy masses, sometimesrolled out in parallel waves or bands.

5. Altostratus clouds: These are grey or blue-grey middle level clouds composed of icecrystals and water droplets. These cloudsusually cover the entire sky. In the thinnerareas of the cloud, the sun may be dimlyvisible as a round disk. Altostratus cloudsoften forms ahead of storms that producescontinuous precipitation.

Altocumulus clouds

Altostratus clouds

6. Stratus clouds: These are uniform greyishclouds that often cover the entire sky. Theyresemble fog that does not reach the ground.Usually no precipitation falls from stratusclouds, but sometimes they may drizzle.When a thick fog "lifts," the resulting cloudsare low stratus.

7. Nimbostratus clouds: These form a darkgrey, "wet" looking cloudy layer associatedwith continuously falling rain or snow. Theyoften produce precipitation that is usuallylight to moderate.

Stratus clouds


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Nimbo-Stratus clouds

8. Cumulonimbus clouds: These arethunderstorm clouds that form if cumuluscongestus clouds continue to grow vertically.Their dark bases may be no more than 300 m(1000 ft) above the Earth's surface. Their topsmay extend upward to over 12,000 m (39,000ft). Tremendous amount of energy is releasedby the condensation of water vapour withina cumulonimbus. Lightning, thunder, andeven violent tornadoes are associated withthe cumulonimbus.

9. Cumulus clouds: These are puffy clouds thatsometimes look like pieces of floating cotton.The base of each cloud is often flat and maybe only 1000 m (330 ft) above the ground.The top of the cloud has rounded towers.When the top of the cumulus resembles thehead of a cauliflower, it is called cumuluscongestus or towering cumulus. These cloudsgrow upward, and they can develop into agiant cumulonimbus, which is athunderstorm cloud.

Cumulonimbus clouds

Cumulus clouds

Fog

Fog is a cloud in contact with the groundwhich reduces visibility to less than 1 km. Fog isformed when water vapour condenses oncondensation nuclei (or particles) in the air nearthe ground. When conditions are right these waterparticles continue to attract more water vapourand grow until the particles become visible.

There are however, several conditions thatinitiate the process of fog formation. Fog usuallydevelops when relative humidity is near 100%and when the air temperature and dew pointtemperature are close to one another or less than4°F (2.5 °C). As a result, the water vapourcondenses to form water droplets and fog.

Types of Fog:

a) Radiation Fog

Radiation fog forms when moist air is cooledbelow its dew-point by contact with a cold landsurface that is losing heat by radiation. The idealconditions are:

• High relative humidity at low levels sothat overnight cooling will be sufficientfor the air temperature to fall to below itsdew point temperature resulting incondensation occurring.

• Cloudless, or near cloudless skies, to allowa large heat loss at the ground, andsubsequent cooling of the air in contactwith the ground.

• Light winds to promote mixing of thiscooled air through a few hundred feet ofthe surface (a calm wind tends to restrictthe fog to low-lying pockets).

b) Advection Fog

Advection fog develops when warm moistair moves over a cooler surface resulting in thecooling of the air to below its dew pointtemperature, and subsequent saturation andcondensation. Radiation processes frequentlyassist in the formation and maintenance of thistype of fog, but it is still usually called anadvection fog. A certain amount of turbulenceis needed for proper development of advectionfog, thus wind between 6-15 knots arecommonly associated with advection fog. Notonly does the turbulence facilitate coolingthrough a thicker layer, but it also carries thefog to greater heights. Unlike radiation fogs,advection fogs are often thick and persistent.


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c) Sea Fog

Sea fogs are usually advection fogs. Theydevelop when moist air that has been lying overa warm water surface moves over a colder watersurface, resulting in the cooling of this air to belowits dew point temperature.

d) Steam Fog

Steam fog is caused by evaporation fromwater into overlying colder air, causes the air tobecome saturated and condensation to occur. Theconvection currents above the water give rise tothe steaming appearance. The fog may remain insitu but any light wind may advect it manykilometres. This is a common occurrence in areaswhere large, shallow and warm waterways anddams exist. In coastal areas where cool landbreezes have opportunity to flow across warmseas, steam fog can be extensive and lift into lowstratus.

e) Frontal Fog

Frontal fog occurs at the boundary of two airmasses rather than within a single air mass. Itmainly develops due to precipitation falling fromrelatively warm air above a frontal surface,evaporating into drier and cooler air below,causing this air to saturate and condense. Suchfogs usually form rapidly and are very extensive.

Impacts of fogs

Fog occurrence impacts a wide variety ofhuman activities worldwide. The fog dropletscollect the pollutants present near groundsurface. These pollutants contain toxic gases likesulphur dioxide, ammonia, hydrochloric acid,nitrogen oxides and radioactive isotopes (naturaland anthropogenic). These pollutants frequentlystick to the fog droplets, posing threats to healthof the people breathing in such environment. Thereduced visibility caused by fog, results intoeconomic losses due to its impact on aviation,land and marine transportation.

Precipitation

Precipitation is water released from clouds inthe form of rain, freezing rain, sleet, snow, or hailon the Earth's surface.

When air is lifted in the atmosphere, it expandsand cools and leads to the formation of clouds.The clouds floating overhead contain watervapour and cloud droplets, which are small drops

of condensed water. For precipitation to happen,first tiny water droplets must condense on eventinier dust, salt, or smoke particles, which act asa nucleus. Some vapour freezes into tiny icecrystals which attract cooled water drops. Thedrops freeze to the ice crystals, forming largercrystals known as snowflakes. When thesnowflakes become heavy, they fall as it exceedsthe cloud updraft speed. When the snowflakesmeet warmer air on the way down, they meltinto raindrops.

Rainfall and Its Types

Rain is the most common form ofprecipitation. When minute droplets of water arecondensed from water vapour in the atmosphereon to nuclei, they may float in the atmosphere asclouds. If the droplets coalesce, they will formlarger drops which will be enough to overcomeby gravity and will fall as RAIN to the surface ofthe earth.

There are three main types of rainfall

a) CONVENTIONAL RAINFALL: It occurswhen moist air, having been warmed byConduction from a heated surface, expands,rises and is adiabatically cooled to the DewPoint. Cumulus clouds develop and may fallaccompanied by Thunder.


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Convectional rainfall occurs commonlyduring the afternoon near the equator, dueto high temperature and high humidity.

b) OROGRAPHIC RAINFALL: This type ofrainfall occurs when air is forced to ascend

the side of a mountain range. When landbarriers such as mountain ranges, hillyregions or even escarpments of plateaus liein the path of prevailing winds, large portionof the atmospheric air is forced to rise abovethese barriers. This resultant precipitation istermed as orographic. Because the air hasdeposited on the windward side of themountain, there will normally less rainfallon the leeward side which is known as RainShadow Area.

c) FRONTAL RAINFALL: Cyclonic or Frontalprecipitation results when the warm, moistair mass (warm front) meets a cool and dryair mass (cold front). The molecules in the

cold air are more tightly packed together (i.e.,more dense), and thus, the cold air is heavierthan the warm air. The warmer air mass isforced up over the cool air. As it rises, thewarm air cools, the water vapour in the aircondenses, and forms clouds and results inprecipitation.

This type of system is called FrontalPrecipitation because the moisture tends tooccur along the front of the air mass.

Global Distribution of Rainfall

The global distribution of precipitation isinfluenced by the general circulation of theatmosphere, proximity to large bodies of water,and topography.

Factors controlling the distribution of rainfallover the earth's surface are the belts of converging-ascending air flow, air temperature, moisture-bearing winds, ocean currents, distance inlandfrom the coast, and mountain ranges.

The Earth's atmosphere is known to haveregions characterized by large scale rising air,and other regions with descending air; these varyby latitude and by season. Rising air is foundprimarily near the equator and in the mid-latitudes (40° to 60° North and South latitude),so these tend to be wet areas. Descending airdominates in the subtropics (20° to 30° Northand South latitude) and the poles. The globaldistribution of precipitation shows that thewettest areas on Earth are in the "rising air" zones,while the driest areas (subtropical deserts andthe even drier polar areas) are in the "descendingair" belts.

The Earth revolves around the Sun thus theorientation of its axis relative to the Sun changes.This causes the apparent position of the Sunrelative to the Earth to change, and creates distinctseasons. Between March and September, the axisof Earth is tilted toward the Sun, and hence theSun shines more directly over the NorthernHemisphere, resulting in more sunlight, moreheat, and the warmer temperatures of Northernsummer. In the other 6 months, the Earth's axisis tilted away from the Sun, and the Sun shinesmore directly over the Southern Hemisphere,bringing summer to countries south of the Equator(and winter to the north).

The "rising" and "sinking" zones movenorthward and southward with the Sun's path.Thus, the wet area near the Equator movesnorthward into the Northern Hemisphere in itssummer and southward into the Southern Hemi-sphere during its summer. Similarly, the dry zonesand wet zone at higher latitudes shift northwardand southward throughout the year.

The result of these shifting zones is latitudebands with distinctive precipitationcharacteristics:

0-5° latitude: wet throughout the year (risingzone)


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5-20° latitude: wet summer (rising zone), drywinter (sinking zone)

20-30° latitude: dry all year (sinking zone)

30-50° latitude: wet winter (rising zone), drysummer (sinking zone)

50-60° latitude: wet all year (rising zone)

60-70° latitude: wet summer (rising zone), drywinter (sinking zone)

70-90° latitude: dry all year (sinking zone)

If the Earth had no mountains, and oceanswere homogeneous with respect to their heatcontent, the climate would occur in latitude bandslike those listed above. However, due to presence ofmountains, strong influence on precipitation exerts.

When moving air encounters a hill ormountain, it is forced to rise. As the rising aircools and condenses, precipitation is heaviest onthe upwind side of a mountain, where the air isrising. This process is known as orographic lifting.On the downwind side, air descends, warms,and becomes drier thus forms rain shadowarea. One example of a mountainous area

receiving frequent rain is Mt. Waialeale onKauai, Hawaii. It is a sharp peak directlyin the path of steady trade winds whichblow from the northeast most of the time. Onthe upwind (northeast) side of Waialeale, the airrises and condenses; resulting in almost constantclouds-nearly every day thus experiences rain.Waialeale is among the wettest places on Earth.In contrast, on the downwind (southwest) sideof Mt. Waialeale, the air descends, warms, anddries, in an area known as a "rain shadow." Theresult is a semiarid area with less than 51centimeters (20 inches) of rain a year.

The other factor that affects the rainfalldistribution is presence of water bodies i.e. oceans,lakes, rivers, etc.

Zonal distribution of rainfall

• Equatorial zone of maximum rainfall: thiszone extends upto 10° latitudes on either sideof the equator characterized by warm andmoist air masses. Rainfall occurs mainly dueto convectional rainfall and mean annualranges from 1750 mm to 2000 mm.

• Trade wind rainfall zone: This zone extendsbetween 10-20 degree latitudes in both thehemispheres and is characterized by north-east and south-east trade winds. These windsyield rainfall in the eastern part of thecontinent as they flow over oceans andbrings moisture. Thus, the western part ofthe continents becomes extremely dry anddeserts.

• Subtropical zone of minimum rainfall: Thiszone extends between 20-30 degree latitudesin both the hemisphere where descendingair from above induces high pressure and

winds thus results in anticyclonic conditions.

• Medi ter raneanrainfall zone: Thisrainfall zone extendsbetween 30-40 degreelatitudes in both thehemispheres whererainfall occurs throughWesterlies and cyclonesduring winter seasonwhile summer seasonsare dry.

• Mid-latitudinalzone: This zoneextends between 40-50

degrees latitudes in both the hemisphereswhere rainfall occurs due to Westerlies andTemperate Cyclones.

• Polar zone: It extends from 60 degrees topole in both the hemispheres. Precipitationmainly occurs in the form of snow.

Terminologies

Hydrologic cycle: The exchange of waterbetween the oceans, the atmosphere andcontinents through evaporation, transpiration,


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condensation and precipitation is calledhydrologic cycle.

Humidity: It refers to the invisible amount ofwater vapour present in the air.

Dew point: The temperature at which saturationoccurs in a given sample of air is known is dewpoint.

Evaporation: It is the process by which water istransformed from liquid to gaseous form.

Condensation: It is the process of change of statefrom gaseous to liquid or solid state.

Latent heat: The amount of energy emitted orabsorbed when a body changes its state or phase,without any change of temperature within thatbody.

Adiabatic change: It refers to the changes oftemperature which occurs in a mass of gas (air)when it is compressed (heated) or expanded(cooled) without the aid of any external sourcesof heating and cooling.

Dew: The moisture deposited in the form of waterdroplets on the surface of vegetation and otherobjects located near to ground level.

White Frost: When the dew point is below thefreezing point, minute crystals of ice are depositedinstead of droplets of water. These crystals of iceare called white frost.

Fog: It is a cloud with its base at or very near theground. The visibility is less than 1km.

Mist: It is a kind of fog in which the visibility ismore than 1km, but less than 2 kms.

Cloud: It is a mass of minute droplets of water ortiny crystals of ice formed by condensation of thewater vapour in free air at considerable elevation.

Precipitation: Condensation of water vapour inthe air in the form of water droplets and ice andtheir falling on the ground is called precipitation.

Sleet: Sleet is frozen raindrops and re-frozenmelted snow water.

Rain Shadow area: The leeward slope, where norain occurs and remains dry is known as rainshadow area.

Absolute Humidity: The weight of actual amountof water-vapour present in unit volume of air.

Specific Humidity: The weight of water-vapourper unit weight of air or the proportion of themass of water-vapour to the total mass of air inunits of weight.

Relative Humidity: The ratio of the air's actualwater-vapour content to its water-vapourcapacity at a given temperature. The relativehumidity of the saturated air is 100 per cent.

Haze: Haze is formally formed by water particlesthat have condensed around nuclei in theatmosphere, but may also be a result of particlesof smoke dust or salt in the air visibility to fewerthan 2 km but over 1 km.

Frost: Frost forms when air temperatures havefallen below the freezing point. Conditionsfavourable for frost are a mass of dry cool polarair followed by clear, calm nights during whichthe surface air may be reduced to below freezing.Frost is common in temperate lands. It is veryharmful to plants. Most crops are sensitive to frost.

Hail: When there are strong ascending currents,condensation takes place at high levels attemperatures below freezing point. The iceparticles grow in size as they do not fall downdue to the force of ascending currents. When theice particles get larger in size, they fall down owingto their weight. This form of precipitation isknown as hail and the large ice particles areknown as hailstones.

AIR MASS

An air mass is a large body of air of relativelysimilar temperature and humidity characteristics,covering thousands of square kilometers. Thevertical distribution of temperature and moisturecontent of the air mass are two basic propertieswhich controls the weather conditions of the areaaffected by that air mass.

Generally, air masses are classified accordingto the characteristics of their source region orarea of formation. An air mass originates when

atmospheric conditions remain stable anduniform for over a long period so that the airlying above the area attains the temperature andmoisture characteristics of the ground surface.

The primary classification of air masses isbased on the characteristics of the source region,giving Arctic (A), Polar (P) or Tropical air (T),and on the nature of the surface in the sourceregion: continental (c) or maritime (m).


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• Warm Air Mass is that whose temperatureis greater than the surface temperature ofthe areas over which it moves.

Thus this indicates that the surfaceunderlying the air mass is cold. Due to thepresence of cool surface the warm air massgets cooled from below. The lower layerbecomes stable and stops the verticalmovement of air. Due to this

No adiabatic cooling of air formation ofcloud halts precipitation stops createsanti-cyclonic stable conditions.

Warm air mass can further be divided ascontinental warm air mass and maritime warmair mass (based on the source of origin whethercontinent or ocean respectively).

• Cold Air Mass originates in the polar andArctic regions. Temperature and specifichumidity is very low. Its temperature islower than the surface temperature of theareas over which it moves.

Thus the air mass is warmed frombelow and becomes unstable. Due toheating up of air from below, the airrises vertically and because of adiabaticcooling condensation process starts.This will lead to formation of cloudsand finally will result in precipitation.

But the precipitation will occur only whenthe air mass is above the warm ocean asit will get the unobstructed supply ofmoisture whereas if lies above warmcontinent then leads to clear weather.

Adiabatic cooling of air formation ofcloud precipitation occurs createsunstable conditions.

Cold air mass can further be divided ascontinental cold air mass and maritime cold airmass (based on the source of origin whethercontinent or ocean respectively).

FRONTS AND ITS TYPES

Air Mass Symbol Characteristics/Comments

Continental Arctic cA Form exclusively in the Arctic and Antarctic regions anddescent toward the equator. Bitterly cold and extremely dryin the winter, cool and dry during the summer.

Continental Polar cP Form over dry lands. Cold and dry during the winter, mildand dry during the winter.

Continental Tropical cT Typically hot and dry during the summer and mild and dryduring the winter.

Maritime Polar mP Marine type humidities with cool or cold weather. Typicallyprovide for miserable, damp, grey days. Mild to cold andhumid with low stratus clouds and precipitation is often therule with Maritime Polar air masses.

Maritime Tropical mT Hot, humid, sticky weather. A good example of when mT airmasses affect the United States is during the summer with theBermuda High phenomena. A southerly flow of hot, humid,sticky weather is circulated northward into the US.

It is a transition zone between two air massesof different densities. It is formed when two airmasses with contrasting physical characteristicslike temperature, humidity, pressure, etc.converge. The type of front depends on both thedirection in which the air mass is moving and thecharacteristics of the air mass. There are four typesof fronts as described below: cold front, warmfront, occluded front and stationary front.

• Cold fronts

Cold fronts occur when a colder air massbecomes more active and forcibly uplifts thewarmer air.

As the warm air rises, it cools, the watervapour in the air condenses out and clouds startto form. Rain associated with cold fronts is usuallyheavy but short lived and only affects a smalldistance (about 50 to 70 km). A cold front is


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associated with bad weather characterized bythick clouds, heavy downpour with hunderstormsand lightning.

• Warm Fronts

A warm front is the transition zone in theatmosphere where an advancing warmsubtropical, moist air mass replaces a retreatingcold, dry polar air mass. High altitude cirrus,cirrostratus and middle altitude altostratusclouds are formed. These clouds produceprecipitation in the form of snow or rain.Between the nimbostratus clouds and thesurface location of the warm front, low altitudestratus clouds are found.

Weather Prior to the Passing Contact with After the Passing of thePhenomenon of the Front the Front Front

Temperature Warm Cooling suddenly Cold and getting colder

Atmospheric Decreasing steadily Levelling off then Increasing steadilyPressure increasing

Winds South to southeast Variable and gusty West to northwest

Precipitation Showers Heavy rain or snow, Showers then clearinghail sometimes

Clouds Cirrus and cirrostratus Cumulus and Cumuluschanging later to cumulonimbuscumulus andcumulonimbus

Weather conditions associated with a cold front.

Weather Prior to the Passing Contact with After the Passing of thePhenomenon of the Front the Front Front

Temperature Cool Warming suddenly Warm then levelling off

Atmospheric Decreasing steadily Levelling off Slight rise followed by aPressure decrease

Winds South to southeast Variable South to southwest

Precipitation Showers, snow, steel Light drizzle Noneor drizzle

Clouds Cirrus and cirrostratus Status sometimes Clearing with scatteredaltostratus, cumulonimbus stratus, sometimes scatterednimbostratus and cumulonimbusthen stratus

Weather conditions associated with a warm front.


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• Occluded Fronts

Occluded front is formed when cold frontovertakes warm front and warm air is completelydisplaced from the ground surface. The temperaturedrops as the warm air mass is occluded, or "cut off,"from the ground and pushed upwards.

• Stationary Fronts

A stationary front if formed when a cold frontor warm front stops moving. This happens whentwo masses of air are pushing against each otherbut neither is powerful enough to move the other.Winds blowing parallel to the front instead ofperpendicular can help it stay in place.

CYCLONES

Cyclones are well developed low-pressuresystems surrounded by closed isobars havingincreasing pressure outside and closed aircirculation towards the centre such that the airblows inward in anticlockwise direction in thenorthern hemisphere and clockwise in thesouthern hemisphere.

A. Tropical cyclones

Tropical cyclones are intense cyclonic stormsthat develop over the warm oceans of the tropics.Surface atmospheric pressure in the centre oftropical cyclones tends to be extremely low. Moststorms have an average pressure of 950 millibars.

The main characteristics of tropical cyclonesare:-

• Have winds that exceed 34 knots (39 mi/hr)

• Blow clockwise in the SouthernHemisphere and

• Counter-clockwise about their centres inthe Northern Hemisphere

This is one of the most devastating naturalcalamities. They are known as Cyclones in theIndian Ocean, Hurricanes in the Atlantic,Typhoons in the Western Pacific and South ChinaSea, and Willy-Willies in the Western Australia.

Tropical cyclones originate and intensify overwarm tropical oceans. The conditions favourablefor the formation and intensification of tropicalstorms are:

(i) Large sea surface with temperaturehigher than 27° C;

(ii) Presence of the Coriolis force;

(iii) Small variations in the vertical windspeed;

(iv) A pre-existing weak low - pressure areaor low - level - cyclonic circulation;

(v) Upper divergence above the sea levelsystem.

The energy that intensifies the storm comesfrom the condensation process in the toweringcumulonimbus clouds, surrounding the centreof the storm. With continuous supply of mois-ture from the sea, the storm is further streng-thened. On rea-ching the land the mois-turesupply is cut off and the storm dissipates. Theplace where a tropical cyclone crosses the coastis called the landfall of the cyclone. The cyclones,which cross 200N latitude generally, recurve andthey are more destructive.

The eye is aregion of calmwith subsidingair. Around theeye is the eyewall, wherethere is a strongs p i r a l l i n gascent of air togreater heightreaching thetropopause. The wind reaches maximum velocityin this region, reaching as high as 250 km per hour.

Torrential rain occurs here. From the eye wallrain bands may radiate and trains of cumulus


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and cumulonimbus clouds may drift into the outerregion. The diameter of the storm over the Bay ofBengal, Arabian Sea and Indian Ocean is 600 -1200 km. The cyclone creates storm surges andthey inundate the coastal low lands.

Tropical cyclones Damage and Destruction

Tropical cyclones are the most deadly anddestructive type of severe weather event on ourplanet. The damage that tropical cyclones inflictis caused by high speed winds, heavy rainfall,storm surge, and tornadoes. Wind speed intropical cyclones is usually directly related toatmospheric pressure. The lower the pressure thefaster the winds blow. Wind speed also varieswithin the storm. High winds inflict damage byblowing down objects, creating choppy wavesand high seas, and by inundating coastal areaswith seawater. Rainfall within tropical cyclonescan often exceed 60 centimeters (24 inches) in a24-hour period. If this rainfall occurs on land,flooding often occurs.

B. Temperate cyclones

The systems developing in the mid and highlatitude, beyond the tropics are called the middlelatitude or temperate cyclones.

Extra tropical cyclones form along the polarfront. Two air masses of contrasting physicalproperties: one air mass is polar in character andis cold, denser and north-easterly in directionwhile the other air mass is tropical in origin and iswarm, moist, lighter and south westerly in direction.

When south westerly warm air mass entersthe territory of cold polar air mass along the polarfront, the warm air glides over the cold air and asequence of clouds appear over the sky ahead of

the warm front and cause precipitation. The coldfront approaches the warm air from behind andpushes the warm air up. As a result, cumulusclouds develop along the cold front. The cold frontmoves faster than the warm front ultimatelyovertaking the warm front. The warm air iscompletely lifted up and the front is occludedand the cyclone dissipates.

They cover a larger area and can originateover the land and sea. Whereas the tropicalcyclones originate only over the seas and onreaching the land they dissipate. The extratropical cyclone affects a much larger area ascompared to the tropical cyclone. The windvelocity in a tropical cyclone is much higher andit is more destructive. The extra tropical cyclonesmove from west to east but tropical cyclones,move from east to west.

The life-cycle of a mid-latitude cyclone can bedivided into 6 stages:

• stationary front, with opposing shearacross the front;

• cyclone formation (cyclogenesis) beginsas a cyclonic wave develops and amplifies;

• distinct poleward moving warm andequatorward moving cold fronts developforming low pressure at apex;

• cold front begins to overtake the warmfront (occlusion begins);

• Occluded front develops and cyclonereaches maximum intensity;

• Eventually all warm sector is forced aloftand cold air surrounds bottom of cyclone,and pressure gradients weakens andcyclone dissipates.


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An anticyclone is a region of high atmosphericpressure related to the surrounding air, generallythousands of kilometres in diameter and alsoknown as a high or high-pressure system. Windsin an anticyclone form a clockwise out-spiral inthe Northern Hemisphere; whereas they form ananti-clockwise out-spiral in the Southern Hemi-sphere. The subsiding air compresses as it

descends, thus causes adiabatic warming. Theeventually warmer and drier air suppresses cloudformation and thus anticyclones are usuallyassociated with fine weather in the summer anddry, cold, and sometimes foggy weather in the winter.Calm settled weather is usually synonymous withanticyclones in temperate latitudes. Anticyclonesare typically relatively slow moving features.

ANTICYCLONES

Idealized illustration showing expected airflow aloft around low-and high-pressure centers.

THUNDERSTORMS

Thunderstorms are formed when moist,unstable air is lifted vertically into the atmosphere.The lifting of the air results in condensation andthus releases latent heat. The process to initiatevertical lifting can be caused by:

(1) Unequal warming of the surface of the Earth.

(2) Orographic lifting due to topographicobstruction of air flow.

(3) Dynamic lifting because of the presence of afrontal zone.

Immediately as lifting begins, the rising parcelof warm moist air begins to cool because ofadiabatic expansion. At a certain elevation thedew point is reached resulting in condensationand the formation of a cumulus cloud. For thecumulus cloud to form into thunderstorm,continued uplift must occur in an unstableatmosphere. With the vertical extension of theair parcel, the cumulus cloud grows into a

cumulonimbus cloud. Cumulonimbus clouds canreach heights of 20 kilometers above the Earth'ssurface. Severe weather associated with somethese clouds includes hail, strong winds, thunder,lightning, intense rain, and tornadoes.

Air Mass Storm

The most common type of thunderstorm isthe air mass storm. Air mass thunderstormsnormally develop in late afternoon hours whensurface heating produces the maximum numberof convection currents in the atmosphere. Thelife cycle of these weather events has three distinctstages.

1st Stage- Cumulus Stage: The first stage of airmass thunderstorm development is called thecumulus stage. In this stage, parcels of warmhumid air rise and cool to form clusters of puffywhite cumulus clouds. The clouds are the resultof condensation and deposition which, releaseslarge quantities of latent heat. The added heat


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energy keeps the air inside the cloud warmer thanthe air around it. The cloud continues to developas long as more humid air is added to it frombelow. Updrafts dominate the circulation patternswithin the cloud.

2nd Stage- Mature Stage: When the updraftsreach their maximum altitude in the developingcloud, usually 12 to 14 kilometers, they changetheir direction 180° and become downdrafts. Thismarks the mature stage. With the downdrafts,precipitation begins to form through collision andcoalescence. The storm is also at its most intensestage of development and is now a cumulonimbuscloud. The top of the cloud takes on the familiaranvil shape, as strong stratospheric upper-levelwinds spread ice crystals at the top of the cloudhorizontally. At its base, the thunderstorm isseveral kilometers in diameter. The mature airmass thunderstorm contains heavy rain, thunder,lightning, and produces wind gusts at the surface.

3rd Stage- Dissipating Stage: The maturethunderstorm begins to decrease in intensity andenters the dissipating stage after about half anhour. Air currents within the convective stormare now mainly downdrafts as the supply ofwarm moist air from the lower atmosphere isdepleted. Within about 1 hour, the storm isfinished and precipitation is stopped.

Thunderstorms form from the equator to asfar north as Alaska. They occur most commonlyin the tropics where convectional heating ofmoist surface air occurs year round. Manytropical land based locations experience over100 thunderstorm days per year.Thunderstorm formation over tropical oceans isless frequent because these surfaces do not warmrapidly. Outside the tropics, thunderstormformation is more seasonal occurring in thosemonths where heating is most intense.

CLIMATIC ZONES OF THE WORLD

The world has several climatic zones. Theseare summarized on the map below.

1. Tropical Moist Climate (Af)

This climate is located upto 50 to 100latitudes on both the hemispheres. The zone issubjected to seasonal shifting due to northwardand southward movement of Sun. The tropicalclimate is characterized by two major properties- uniformly high temperature throughout theyear and uniformly adequate rainfall throughout

the year by convectional rainfall. The total annualrainfall is often more than 250 cm. Humidity isbetween 77% and 88%.

The equatorial climate is found in - TheAmazon Basin in South America, Congo Basinin Africa, Guinea coast in Africa, Java, Sumatra,Malaysia, etc.

The climates on eastern sides of continentsare influenced by maritime tropical air masses.These air masses flow out from the moist western


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sides of oceanic high-pressure cells, and bringlots of summer rainfall. The summers are warmand very humid. It also rains a lot in the winter.

A tropical rain forest has more kinds of treesthan any other area in the world. Scientists havecounted about 100 to 300 species in one 2 1/2-acre (1-hectare) area in South America. Seventypercent of the plants in the rainforest are trees.

About 1/4 of all the medicines we use comefrom rainforest plants. Curare comes from atropical vine, and is used as an anesthetic and torelax muscles during surgery. Quinine, from thecinchona tree, is used to treat malaria. A personwith lymphocytic leukemia has a 99% chancethat the disease will go into remission because ofthe rosy periwinkle. More than 1,400 varieties oftropical plants are thought to be potential curesfor cancer.

All tropical rain forests resemble one anotherin some ways. Many of the trees have straighttrunks that don't branch out for 100 feet or more.There is no sense in growing branches below thecanopy where there is little light. The majority ofthe trees have smooth, thin bark because there isno need to protect the them from water loss andfreezing temperatures. It also makes it difficultfor epiphytes and plant parasites to get a hold onthe trunks. The bark of different species is so similarthat it is difficult to identify a tree by its bark.Many trees can only be identified by their flowers.

This climatic region is characterized by broadleaf evergreen dense forests comprising ofmahogany, rosewood, bamboos, sandal etc.

• Average temperature: 18° C (°F)

• Annual Precipitation: 262 cm. (103 in.)

• Latitude Range: 10° S to 25° N

• Global Position: Amazon Basin; CongoBasin of equatorial Africa; East Indies,from Sumatra to New Guinea.

2. Wet-Dry Tropical Climate (Aw) (savanna)

This type of climate is located between 50 -200 latitudes on either side of the equator. Thisclimatic type is bounded by tropical rainforestclimate towards the equator and by dry climatetowards the poles.

The Savanna type is found in the southerncontinents and all the regions are to the south ofthe Tropic of Cancer.

South America: Cuba, Jamaica and the islandsin the Pacific.

Africa: The Sudan, large parts of the newlyformed Republics - Senegal, Guinea, Mali,Niger, Chad and also in Ghana, Togo, Kenya,Zimbabwe, Tanzania, Angola and Uganda.

Australia: The northern region and Queensland.

The Savanna climate is characterized bydistinct wet and dry seasons, mean hightemperature throughout the year and highinsolation. There is sunshine for 13 to 14 hoursand humidity is low, the air is hot, dry and dusty.The average monthly temperature during the dryseason ranges between 22°C and 37°C. Thehighest temperatures are recorded just before therainy season, for example, in the Sudan the hottestmonths are April and May. There is a generallowering of temperature during the rainy seasonwhen the temperature ranges between 21° and26°C. Rainfall varies from 25 cm. to 150 cm andis usually unreliable. Coastal regions on thewindward side of the mountains get heavier rain.Rainfall decreases as one goes either towardsnorth (in the Northern Hemisphere) or towardssouth (in the Southern Hemisphere).

Plants of the savannas are highly specializedto grow in this environment of long periods ofdrought. They have long tap roots that can reachthe deep water table, thick bark to resist annualfires, trunks that can store water, and leaves thatdrop off during the winter to conserve water.The grasses have adaptations that discourageanimals from grazing on them; some grasses aretoo sharp or bitter tasting for some animals, butnot others, to eat. The side benefit of this is thatevery species of animal has something to eat.Different species will also eat different parts ofthe grass. Many grasses grow from the bottomup, so that the growth tissue doesn't get damagedby grazers. Many plants of the savanna also havestorage organs like bulbs and corms for makingit though the dry season.

The species found in savannas vary by thegeographic location of the biome. Animals nativeto African savannas include African elephants,zebras, horses, and giraffes. Many animals in thesavanna are herbivores, which means they eatplants, and there is plenty of grass in the savanna.During the rainy months animals thrive in thesavanna, but the rainy season is only for half theyear. During the dry season, surface water fromthe rain is quickly absorbed into the ground bythirsty soils. The competition for water duringthe dry season is so intense that most birds andmany of the large mammals migrate elsewhere


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in search of water. Depending on the severity ofthe drought, the migration may be to a placenearby, or far away. The dry season is oftenassociated with fires. Many insects with short lifespans die in these fires, but the birds and largeranimals are usually able to fly or run to safety.Although small burrowing animals probablycan't outrun the flames, they often survive thefire by digging deep into the ground andremaining there until the flames are gone.

• Temperature Range: 16° C

• Annual Precipitation: 0.25 cm. (0.1 in.).All months less than 0.25 cm. (0.1 in.)

• Latitude Range: 15° to 25° N and S

• Global Range: West Africa, southernAfrica, South America and the north coastof Australia

3. Hot Desert Climate

This type of climate is located between thelatitudinal belts of 150 - 300 in both thehemispheres. The arid deserts lie close to the tropicof cancer and Tropic of Capricorn in the westernmargins of continents.

The climatic zone lies in - The Sahara, theArabia, the Thar, Mohave and Sonoran (SouthWestern U.S.A.), Kalahari and Namib (SouthWestern Africa), Simpson, Gibson, Great Sandy(Australia)

The climate is dominated by the subsidence ofair masses and marked stability of the sub-tropicalanticyclones and hence nearly rainless. The highesttemperatures in the world are recorded here(Aziziya 58.7° C). The greatest daily ranges oftemperature of (15° C) are seen here. These areasreceive the lowest annual rainfall (12 to 15 cm). Coldcurrents also influence the climate on the westernmargins of continents. The aridity is intensifiedbecause of these currents which chill the air andfurther stabilize it.

The vegetation found here is cactus, thornyplants, shrubs, herbs.

Deserts plants have many adaptations tosurvive in such a dry environment. They are goodat storing and finding water. Some plants haveseeds that can stay dormant in the sand for a longtime, until there is enough rain for them to grow.Cacti have waxy coating, water can't escape andtheir spines protect them from being desert dinner.Their roots are shallow, and widely spread sothat any rain can be absorbed immediately. Some

other plants are creosote bush, sagebrush, andocotillo. Coastal deserts house a variety of plants.These plants must adapt to minimal rainfall byhaving extensive root systems that come up tothe surface to absorb any possible rainfall, and gofar down to absorb any water saturated in theground. These plants also have very thick leavesthat can absorb and store water whenever it isavailable. The plants that live in coastal desertsinclude salt bush, rice grass, black sage andchrysothamnus. Plants can even live in colddeserts. Plants in cold deserts include algae,grasses, and plants with spiny thin leaves. Usuallythese plants grow only in the summer.

Some animals that live in the hot desert arecold-blooded, like snakes, insects, and lizards.Mammals that live in the desert are usually small,such as the kangaroo rat and kit fox.

• Temperature Range: 16° C

• Annual Precipitation: 0.25 cm (0.1 in). Allmonths less than 0.25 cm (0.1 in).

• Latitude Range: 15° - 25° N and S.

• Global Range: southwestern United Statesand northern Mexico; Argentina; NorthAfrica; South Africa; central part ofAustralia.

4. Steppe Climate

This type of climatic zone is found between40° and 55° North and South. They lie far awayfrom the influence of the sea, in the heart ofcontinents.

The areas are - Prairies (North America),Pampas (South America), Velds (South Africa),Downs (Australia) and Steppes (Russia)

The climate is not controlled by the subsidingair masses or the sub-Tropical Air masses, likedeserts. Instead these are dry lands principallybecause of their position in the deep interiors oflarge land masses away from the oceans. Thetemperature in summer varies from 18° C to 24°C and in winter from - 4° C to 2°C. The range oftemperature is large. Rainfalls in spring and earlysummer and vary between 23 cm. and 65 cm. Itis of convectional type but very light.

Grasses dominate temperate grasslands. Treesand large shrubs are rarely found in grasslandareas. There are many species of grasses that livein this biome, including, purple needlegrass, wildoats, foxtail, ryegrass, and buffalo grass. Treesappear only on the slopes of mountains. With


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land surface. Rainfall is both orographic andcyclonic in nature. In winter season the wind isoff shore and hence is cool and dry. But someparts like Madras coast get rain during this seasonbecause winds are on shore there.

Seasonality of its precipitation is the hallmarkand most well-known characteristic of themonsoon climate. Many think that the term"monsoon" means wet weather, when in fact itdescribes an atmospheric circulation pattern.

Though the annual amount of precipitationis quite similar to that of the rain forest, monsoonprecipitation is concentrated into the high-sun season.

Maritime equatorial and maritime tropical airmasses travel from the ocean on to land duringthe summer, where they are uplifted by eitherconvection or convergence of air to inducecondensation.

Locally, Orographic (Relief) uplift is animportant mechanism for promotingprecipitation. As air travels into the Indiansubcontinent, it is uplifted by the Himalayas,causing cloud development and precipitation.

The low-sun season is characterized by a shortdrought season when high pressure inhibitsprecipitation formation.

In the case of the Asian monsoon, thereplacement of the thermal low with thesubsidence of the Siberian High suppresses uplift.Air masses that dominate this period are dry giventheir continental origin or stability.

A distinct dry season from October to May,when the temperature is lower, the interior ofAsia is a region of high pressure. Wind blowsover the land in a north east direction , carryinglittle or no moisture. These cool , dry North EastMonsoon winds blow towards areas of lowpressure and do not bring rain.

A wet season from June to September, whenthe wind changes in direction, the wind blows inthe region of low pressure. Winds blow acrossthe equator and blow over the oceans, they arewarmer and carry a lot of moisture. They bringalot of rain. Total rainfall can reach upto 600 mm

6. Mediterranean Climate:

This type of climate has developed between300 - 400 latitudes in both the hemispheres. Thisis a wet-winter, dry-summer climate. Extremelydry summers are caused by the sinking air of the sub-tropical highs and may last for up to five months.

underground stems and buds, grasses are noteasily destroyed by fire. Shrubs and trees that livein temperate grasslands are not as good as grassesat coping with the flames, and often are destroyedby fire. Wildflowers also grow well in temperategrasslands. Popular flowers that you might findgrowing on grasslands are asters, blazing stars,goldenrods, sunflowers, clovers, and wild indigos.

The dominant vertebrates in grasslands areherbivorous or plant-eating grazers calledungulates. Ungulates are mammals with hoofs,like horses and deer. Their long legs help themrun fast to escape grassland predators. Thetemperate grassland does not have much animaldiversity, especially compared to the Savannah.Some animals that inhabit temperate grasslandsin North America are bison, antelope, birds,gophers, prairie dogs, coyotes, and insects.

• Temperature Range: 24° C (43° F).

• Annual Precipitation: less than 10 cm (4in) in the driest regions to 50 cm (20 in) inthe moister steppes.

• Latitude Range: 35° - 55° N.

• Global Range: Western North America(Great Basin, Columbia Plateau, GreatPlains); Eurasian interior, from steppesof eastern Europe to the Gobi Desert andNorth China.

5. Monsoon Climate

Monsoon climate is generally related to thoseareas which register complete seasonal reversalof wind direction and are associated with tropicaldeciduous forests. The region lies between 10°Nto 30°N and 10°S to 30°S latitude.

Climatic zone areas are - Eastern Brazil (S.America), Central American countries, Natalcoast (South Africa), India, Pakistan andBangladesh, South East Asia, including Burma,Thailand, Vietnam and the Philippines, etc., Partsof East Africa including Malagasy, North Australia.

The annual average temperature is about 26°Cand the annual range is about 3°C. The maximumtemperatures occur in May before the summerrainfall maximum in June and July. The annualrainfall amounts to about 300 cm. Thecharacteristic feature of this type of climate is areversal in the wind direction with the change ofseason. During the summer season, the wind ison shore, bring large amount of moisture to the


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This climatic region includes European,Asiatic and African lands bordering the Mediter-ranean Sea. This climate owes its origin to theseasonal shifting of wind and pressure belts dueto northward and southward migration of the sun.

In winter they are under the influence ofwesterlies which are moisture laden thus bringsrainfall in winters whereas they come under theinfluence of subtropical high pressure belt in summersthus associated with anti cyclonic conditions.

Plants have adapted to the extreme differencein rainfall and temperature between winter andsummer seasons. Sclerophyll plants range informations from forests, to woodland, and scrub.Eucalyptus forests cover most of the chaparralbiome in Australia. Fires occur frequently inMediterranean climate zones.

• Temperature Range: 7° C (12° F)

• Annual Precipitation: 42 cm (17 in).

• Latitude Range: 30° - 50° N and S

• Global Position: central and southernCalifornia; coastal zones bordering theMediterranean Sea; coastal WesternAustralia and South Australia; Chileancoast; Cape Town region of South Africa.

7. Taiga Climate

This climate type has been named after theconiferous forest cover of the same name foundin the region. The region extends from 50-55degrees to 60-70 degrees latitudes in northernhemisphere.

It stretches as an almost continuous belt acrosssouthern Canada, northern Europe and Russia.The Tundra region lies on the north and theTemperate Grasslands on the south.

The areas are - Southern Alaska, SouthernCanada, parts of Norway, Sweden, Finland, NorthernRussia, Northern Siberia, Sakhalin Island.

Winters are very cold and severe for 6 to 7months with temperatures below freezing. In thisregion lies Verkhoyansk the "cold pole" becomescolder than the Arctic region. Oimyakon to thesouth-east has recorded even lower wintertemperatures. It has January temperatures of -

50°C and in July the temperature sometimesreaches 15°C, Summers are short lasting for 3 or4 months but the days are long; at 60°N the sunshines for over 18 hours.

Rainfall varies from 25 to 100cm. There is morerainfall near the coast. Most of the rain comes fromcyclonic weather. It falls throughout the year butmaximum in summer in frequent showers. In winterit takes the form of snow, which may remain, onthe ground from 5 to 7 months.

The vegetation associated with this climatetype is the soft-wood coniferous forests.

• Temperature Range: 41° C (74° F), lows; -25° C (-14° F), highs; 16° C (60° F).

• Average Annual Precipitation: 31 cm (12 in).

• Latitude Range: 50° - 70° N and S.

• Global Position: central and westernAlaska; Canada, from the Yukon Territoryto Labrador; Eurasia, from northern Europeacross all of Siberia to the Pacific Ocean.

8. Tundra Climate

The tundra climate is found along arcticcoastal areas. Polar and arctic air massesdominate the tundra climate. The winter seasonis long and severe. A short, mild season exists,but not a true summer season. Moderating oceanwinds keep the temperatures from being as severeas interior regions.

Precipitation totals 6-10 inches of rain a year,which includes melted snow. This is almost aslittle as the world's driest deserts. Coupled withstrong and drying winds, the tundra is an extremeweather biome. The tundra seems like a wet andsoggy place because the precipitation that fallsevaporates slowly, because of the poor drainagecaused by the permafrost.

• Temperature Range: -22° C to 6° C (10°F to 41° F).

• Average Annual Precipitation: 20 cm (8 in).

• Latitude Range: 60° - 75° N.

• Global Position: arctic zone of NorthAmerica; Hudson Bay region; Greenlandcoast; northern Siberia bordering the ArcticOcean.

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