ATMOSPHERIC ATMOSPHERIC TEMPERATURETEMPERATURE

WHAT IS TEMPERATURE?

Temperature is a measure of the average heat or thermal energy of the particles in a substance. Since it is an average measurement, it does not depend on the number of particles in an object. In that sense it does not depend on the size of it. For example, the temperature of a small cup of boiling water is the same as the temperature of a large pot of boiling water. Even if the large pot is much bigger than the cup and has millions and millions more water molecules.

It is expressed according to a comparative scale and shown by a thermometer or perceived by touch.

DIFFERENCE BETWEEN HEAT & TEMPERATURE!•Heat is the amount of thermal energy in an object because of its moving molecules. Thermal (heat) energy is the total energy of the particles that make up an object•Temperature is a measure of thermal energy or how fast molecules are moving in an object. The more you heat something, the faster the molecules move. This is what causes temperature to rise. Temperature is a measure of the average kinetic energy of each particle within an object.

The Tool used for measuring temperature is called a

Thermometer

What is a Thermometer?An instrument for measuring and indicating temperature, typically one consisting of a narrow, hermetically sealed glass tube marked with graduations and having at one end a bulb containing mercury or alcohol that expands and contracts in the tube with heating and cooling. An analog thermometer consists of a sealed tube with markings on it. These markings are increasing temperatures in Celsius or Fahrenheit or Kelvin. The markings on a clinical and weather thermometer are different, but they work in the same way. The name is made up of two smaller words: "Thermo" means heat and "meter" means to measure.

HOW TO MEASURE TEMPERATURE?Three temperature scales (units): Kelvin (K), Celsius (C), Fahrenheit (F)

All scales are relativedegrees F = 9⁄5 degrees C + 32degrees K = degrees C + 273.15

How does a Thermometer work?The glass tube of a thermometer usually contains mercury. Mercury is perfect to test temperature because it changes from a solid to liquid very easily.When the metal tip of the thermometer comes into contact with the material it is testing, it conducts heat energy to the mercury. The mercury turns into liquid and so it expands. It begins to rise up the tube. Where it stops, is where you can take the temperature reading on the scale.

How does a Digital Thermometer work?Digital thermometers are different because they use a computer chip to tell the temperature instead of mercury. You would normally have to wait three whole minutes for the mercury to heat up and give you a reading.It takes only 30 seconds for a digital thermometer to give you a reading because the heat-sensitive tip is able to accurately tell the computer chip inside what the temperature is.Your normal body temperature would be 98.6 °F on the Fahrenheit scale. On the Celsius scale, it would be 37 C. The Celsius scale is used in most countries, aside from the USA, because it is a part of a metric system.

FAHRENHEIT SCALEThis scale is mostly understood and used in the US. Weather reports and the news media usually state temperatures in degrees Fahrenheit. It is named after the German physicist Gabriel Fahrenheit. Reference points on this scale include sea level freezing and boiling points of pure water – which are 32 F & 212 F.For converting Celsius to Fahrenheit :F = (C X 1.8) + 32 OR (C X 9/5) +32

CELSIUS SCALEIn most countries the Celsius scale is used – which is named after the Swedish astronomer Andres Celsius.It is a component of the International system of measurement (S.I) because it is a decimal scale with 100 units (degrees) between the freezing and boiling points of water at sea level.For converting Fahrenheit to Celsius :C = (F-32) X 5/9 OR (F-32) / 1.8

KELVIN SCALEFor scientific purposes – the Kelvin scale named after the British physicist Lord Kelvin – has long been used because it measures what are called absolute temperatures – which means that the scale begins at absolute zero O K – the lowest possible temperature. The scale maintains a 100 unit range between the freezing and boiling points of water. There are no negative values. Because the scale is usually used by the scientific community – we only use it for comparison to the Fahrenheit and Celsius scale.C = K - 273.15

THERMOMETERS USED FOR WEATHER OBSERVATIONS

Energy Transfer as HeatThree mechanisms of energy transfer as heat are conduction, convection, and radiation.Conduction is the transfer of heat through matter by molecular activity.Convection is the transfer of heat by mass movement or circulation within a substance.Radiation is the transfer of energy (heat) through space by electromagnetic waves that travel out in all directions.

Radiation

All objects, at any temperature, emit radiantenergy.Hotter objects radiate more total energy per unit area than colder objects do.The hottest radiating bodies produce the shortest wavelengths of maximum radiation.Objects that are good absorbers of radiation are good emitters as well.

AdvectionWhen the dominant direction of energy transfer in a moving fluid is horizontal (sideways) – the term advection is used. In the atmosphere the wind may transfer warm or cool air horizontally from one place to another through the process of advection. Some wind systems develop as part of a large atmospheric convection cells – the horizontal component of air movement within such a convection cell is properly called advection.

THE VERTICAL DISTRIBUTION OF TEMPERATURE

PRINCIPAL TEMPERATURE CONTROLS1. Latitude – Insolation is the single most important influence on temperature variations. The intensity of insolation decreases as one moves away from the equator. In addition day length and sun angle change through out the year – increasing seasonal effect with increasing latitude. From equator to poles - earth ranges from continually warm to seasonally variable to continually cold

2. Altitude/Elevation – In the troposphere temperature decreases with increase in altitude. At high elevations, air temperatures are generally cooler and show a greater day-to- night range. The sun would be cold in space, because there is no air for it to heat. As the air is thinner the higher up a mountain you go, the temperature drops. The higher the air pressure, the higher the temperature and vice versa. As the air pressure is so much thinner, the temperature is therefore lower.

Adiabatic Temperature ChangesAs air is heated it expands becoming less dense, and as a result, lighter. Because it is lighter, it rises upwards above the cooler air. As it does so, this air continues to expand. This is because there is less pressure higher in the atmosphere, allowing the air molecules to spread out more.In order to spread out, these molecules require energy. As they do so, they become less agitatedand vibrate slower. As a result, the temperature of these air molecules drops, despite the fact that no heat has been removed from them.

This process is referred to as adiabatic cooling.As the air cools down, it again begins to fall towards the surface of the Earth. As it sinks deeper into the atmosphere, the pressure from the weight of the air above it pushes air molecules closer together, causing them to become more agitated and heating them up again. As a result, their temperature rises, even though no heat has been added. This process is referred to as adiabatic warming.

Dry air is air that contains no water.

Moist air is air that contains water in vapor form.

The moisture in the air is usually referred to as humidity.

The stability of dry air - dry convection.The stability of air under vertical displacement is determined by the outcome of a small change in an air parcels elevation:If the environment (the surrounding atmosphere) is such that vertically displaced parcels continue to rise on their own, even when the lifting exerted on them stops, the environment is referred to as unstable.If vertically displaced parcels sink back to their initial elevation after the lifting ceases, the environment is stable.If vertically displaced parcels remain where they are after being lifted, the environment is neutral.

Three Categories of Stability: Stable: Returns to its original position

after displacement

Neutral: Remains in new position after being displaced

Unstable: Moves further away from its original position after being displaced

What are Lapse Rates Parcel lapse rate – the rate at which Parcel lapse rate – the rate at which

temperature changes as the parcel temperature changes as the parcel is lifted to a higher altitudeis lifted to a higher altitude

Environmental lapse rate – the rate Environmental lapse rate – the rate at which the air surrounding the at which the air surrounding the parcel changes as altitude increasesparcel changes as altitude increases

The three fundamental stability conditions of the atmosphere are:

(1) Absolute stability - when the environmental lapse rate is less than the

wet adiabatic rate (2) Absolute instability - when the

environmental lapse rate is greater than the dry adiabatic rate

(3) Conditional instability - when moist air has an environmental lapse rate between the dry and wet adiabatic rates.

Stability Conditions

What is the difference between stable and unstable airIf a rising parcel of air is cooler than the surrounding atmosphere it will tend to sink back to its original position. This is because cool air is more dense or heavier than warmer air. This is referred to as stable air. If a rising parcel of air is warmer than the surrounding atmosphere it will continue to rise. This is because warm air is less dense or lighter than cool air. This is referred to as unstable air.

In general, when stable air is forced aloft, the associated clouds have little vertical thickness, and precipitation, if any, is light. In contrast, clouds associated with unstable air are towering and frequently accompanied by heavy rain. Any factor that causes air near the surface to become warmed in relation to the air aloft increases the air's instability. The opposite is also true; any factor that causes the surface air to be chilled results in the air becoming more stable.

Most processes that alter stability result from temperature changes caused by horizontal or vertical air movement, although daily temperature changes are important too. Changes in stability occur as air moves horizontally over a surface having markedly different temperatures. On a smaller scale, the loss of heat from cloud tops during evening hours adds to their instability and growth. Furthermore, in general, subsidence ( a general downward airflow) generally stabilizes the air, while upward air movement enhances instability.

Two ways to saturate the air (or raise the relative humidity)1. Add more water vapor to it2. Decrease the temperature

This is because warm air is capable of “holding” more water vapor molecules than cold air.

(Remember the water vapor molecules are moving faster in warm air and less likely to stick and condense)

DALR

Air in parcel must be unsaturated (RH < 100%)

Rate of adiabatic heating or cooling = 9.8°C for every 1000 meter (1 kilometer) change in elevationParcel temperature decreases by about 10° if parcel is raised by 1km, and increases about 10° if it is lowered by 1km

SALR/MALRAs rising air cools, its RH increases because the temperature approaches the dew point temperature, TdIf T = Td at some elevation, the air in the parcel will be saturated (RH = 100%)If parcel is raised further, condensation will occur and the temperature of the parcel will cool at the rate of about 6°C per 1km in the mid-latitudes

DALR VS MALRThe MALR is less than the DALR because of latent heating

As water vapor condenses into liquid water for a saturated parcel, LH is released, lessening the adiabatic cooling

The higher the air temperature, the more water vapor will be present in the air at saturation

Saturation is when evaporation = condensation

Dew Point = the temperature to which air must be cooled in order to become saturated When air temperature

= Dew point temperature RH = 100%

Temperature InversionUnder normal circumstances, the air near the surface of the Earth is warmer than the air above it. Typically air is hottest at the ground. Air is fairly transparent to sunlight, most of which passes straight through it without heating it. When the sunlight reaches the ground it is almost entirely absorbed, heating the land. Some of this energy is then re-radiated as black body, and due to the average temperatures involved, much of this is in the infrared. Unlike the original sunlight, infrared light interacts more strongly with air, which is then heated from below.Hot air, however, rises. This leads to constant

convection which draws the warmer air up, to be replaced with cooler air which is then heated. It is this process that leads to cloud building, thermals, and other convection related atmospheric behavior.However, it is sometimes possible to find situations where the gradient is inverted, so that the air gets actually colder as you approach the surface of the Earth. This is called a temperature inversion. It is most commonly created by the movement of air masses of different temperature moving over each other. A warm air mass moving over a colder one can "shut off" the convection effects, keeping the cooler air mass trapped below. 

The weather plays an important role in the formation and disappearance of air pollution. During winters, air quality has been observed to decline very quickly after long clear nights with weak winds. Then pollutants from different sources are emitted into the air, but because of poor mixing circumstances near the ground, pollutants released into the atmosphere's lowest layer are trapped at breathing level and can reach unhealthy levels in a few hours. Winters are characterized by short days and low solar activity. The snow-covered ground is cold and its white colour reflects almost all heat coming in. When the sun goes down, the ground loses heat very quickly and this cools the air above the

ground. Nights in the summertime are much shorter than nights during the wintertime when cooling of the ground can continue over a longer period of time. Weak winds prevent air mixing near the surface and clear skies increase the rate of cooling at the Earth's surface. Stable conditions inhibit vertical and horizontal mixing near the ground and consequently, favour the development of a strong surface temperature inversion or radiation inversion (see picture above). The condition like this is called an inversion because it is the reverse of a normal air pattern (i.e., warmer air below and cooler air above). 

How do inversions impact air quality?Winter temperature inversions play a significant role in the winter pollution episodes in Nordic urban sites. An inversion can prevent the rise and dispersal of pollutants from the lower layers of the atmosphere, because warm air above cooler air acts like a lid, preventing vertical mixing and trapping the pollution material e.g. at the breathing level. Traffic emissions especially have a great impact on air quality at the breathing level, because they are released near the ground.The strength and duration of the inversion and

elevation of the release compared to the inversion elevation has a large influence on the air quality. Air pollution will continue to accumulate until inversion disappears. Traffic particularly and other sources add more pollutants to the air. A strong and low height inversion will lead to high pollutant levels, while a weak inversion will lead to lower levels. In other words, the smaller is the mixing volume; the higher is the pollution concentration. Inversions are also stronger and more common during the winter months. In summer, inversions are less frequent and weaker.

3. CLOUD COVERAt any one time – 50% of the earth is covered with clouds. This causes a moderation of temperature and this varies with cloud type – height and density. The moisture in the clouds reflects – absorbs and liberates large amounts of energy released upon condensation and cloud formation. At night clouds act as insulation and radiate LW energy – preventing rapid energy loss. During the day – clouds reflect insolation due to their high albedo value. In general they lower daily maximum temperature and raise nighttime minimum temperature. Clouds are the most variable factor influencing the earth’s radiation budget.

Land-Water Heating DifferenceThe continents heat up faster than the oceans, and they cool down faster too. There are a few reasons for the land-ocean cooling differences, and they all have to do with how heat is absorbed and transported.(1) Specific Heat Capacity. Water has a higher heat capacity than land. So it takes more heat to raise the temperature of one gram of water by one degree than it does to raise the temperature of land. 1 calorie of solar energy (any type of energy really) will warm one gram of water by 1 degree Celcius, while the same calorie would raise the temperature of a gram of granite by more than 5 degrees C.

(2) Transparency. The heat absorbed by the ocean is spread out over a greater volume because the oceans are transparent (to some degree). Since light can penetrate the surface of the water the heat from the sun is dispersed over a greater depth.(3) Evaporation. The oceans loose a lot of heat from evaporation. While there is some evaporation from wet soils and transpiration by plants, the land does not have anywhere near as much available moisture to cool it down.(4) Currents. Not only do the oceans absorb heat over a greater depth, but they can also move that energy around with their currents. The solar energy absorbed at the equator gets transported towards the poles, while

the colder polar water gets transported the other way. Currents help average out ocean temperatures. Gulf stream being an example.

(5) Movement – Land is rigid and solid – whereas water is fluid and capable of movement. Differing temperatures and currents result in mixing of cooler and warmer waters and that mixing spreads the available energy over an even greater volume than if the water were still. Surface and deeper waters mix and redistribute energy. Both land and oceans radiate LW radiation at night but land loses its energy more rapidly than does the moving oceanic energy reservoir.

World air temperature patterns for World air temperature patterns for January January

World air temperature patterns for July

Summary – Marine effects Vs Continental effectsHigh ocean temperatures produce higher evaporation rates and more energy is lost from the oceans as latent heat. As the water vapor content of the overlying air mass increases - the ability of the air to absorb LW radiation also increases. Therefore the air mass becomes warmer – the warmer the air and the ocean become – the more evaporation that occurs – increasing the amount of water vapor entering the air mass. More water vapor leads to cloud formation – which reflects insolation and produces lower temperatures.

The interior landmass, will cool and heat much more rapidly then the ocean. This effect leads to a wider annual range of temperature for the landmass then that of the ocean. The more moderate temperature range (due to the longer cooling and warming periods) found in the ocean does influence the mainland landmasses. While all land cools and warms quicker then water, the mainland temperature is moderated by the water air temperature that moves from the ocean the land. This means that to locations on land, with the same latitude, can have vastly different temperatures.

DAILY AND YEARLY CYCLES OF TEMPERATUREDaily Cycle of Air Temperature/Diurnal CycleAt the Earth's surface quantities of insolation and net radiation undergo daily cycles of change because the planet rotates on its polar axis once every 24 hours. Insolation is usually the main positive component making up net radiation. Variations in net radiation are primarily responsible for the particular patterns of rising and falling air temperature over a 24 hour period. The following three graphs show hypothetical average curves of insolation, net radiation, and air temperature for a typical land based location at 45°

of latitude on the equinoxes and solstices.INSOLATION

Hourly variations in insolation received for a location at 45° North latitude over a 24 hour period.In the diurnal cycle – there is one maximum and one minimum daily

NET RADIATION

Hourly variations in net radiation for a location at 45° North latitude over a 24 hour period.

TEMPERATURE

Hourly variations in surface temperature for a location at 45° North latitude over a 24 hour period.

The relative placement of the temperature profiles for the various dates correlates to the amount of net radiation available for daily surface absorption and heat generation. The more energy available, the higher up the Y-Axis the profile is on the graph. September Equinox is warmer than the March Equinox because of the heating that occurred in the previous summer months. For all dates, minimum temperature occurs at sunrise. Temperature drops throughout the night because of two processes. First, the Earth's radiation balance at the surface becomes negative after sunset. Thus, the surface of the Earth stops heating up as solar radiation is not being absorbed. Secondly, conduction and convection transport heat

energy up into the atmosphere and the warm air that was at the surface is replaced by cooler air from above because of atmospheric mixing. Temperature begins rising as soon as the net radiation budget of the surface becomes positive. Temperature continues to rise from sunrise until sometime after solar noon. After this time, mixing of the Earth's surface by convection causes the surface to cool despite the positive addition of radiation and heat energy.

Annual Cycle of Air TemperatureAs the Earth revolves around the Sun, locations on the surface may under go seasonal changes in air temperature because of annual variations in the intensity of net radiation. Variations in net radiation are primarily controlled by changes in the intensity and duration of received solar insolation which are driven by variations in day length and angle of incidence. Now we examine how changes in net radiation can effect mean monthly temperatures for the following five locations: Manaus – Brazil, Bulawayo – Zimbawe, Albuquerque – USA, London – England & Fairbanks - USA

Manaus, Brazil - 3°South, 60°West

Monthly variations in net radiation and average monthly temperature for Manaus, Brazil.

At Manaus, values of monthly net radiation average about 135 Watts per square meter. Monthly variation in net radiation is only about 35 Watts over the entire year. Two peaks in net radiation are visible on the graph. Both of these peaks occur during the equinoxes when the height of the Sun above the horizon is at its maximum (90° above the horizon). Minimum values of net radiation correspond to the time of the year when the Sun reaches its minimum height of only 66.5° above the horizon at solar noon. Because of the consistent nature of net radiation, mean monthly air temperature only varies by 2° Celsius over the entire year.

Bulawayo, Zimbabwe - 20° South, 29° East

Monthly variations in net radiation and average monthly temperature for Bulawayo, Zimbabwe.

Net radiation at Bulawayo has a single peak and trough over the one year period graphed. This pattern is primarily controlled by variations in the intensity and duration of incoming solar insolation. During the December solstice, the Sun reaches its highest altitude above the horizon and day length is at a maximum (13 hours and 12 minutes). The lowest values of net radiation occur around the June solstice when the Sun reaches its lowest altitude above the horizon and day length is at a minimum (10 hours and 48 minutes) in the Southern Hemisphere. Monthly temperature variations follow the monthly change in net radiation. When received at the Earth's surface much of this energy is used to create sensible heat.

Albuquerque, USA - 35° North, 107° West

Monthly variations in net radiation and average monthly temperature for Albuquerque, USA.

At Albuquerque, maximum net radiation occurs in May. The timing of this peak roughly coincides with the June solstice when day lengths are at their longest and solar heights are their greatest. However, monthly temperature variations do not mirror the changes in net radiation exactly. Peak monthly temperatures occur about two months after the net radiation maximum. This lag is probably caused by the delayed movement of stored heat energy in the ground into the atmosphere. Minimum monthly temperatures do coincide with the lowest values of net radiation which occur during the December solstice.

London, England - 52° North, 1° East

Monthly variations in net radiation and average monthly temperature for London, England.

The annual patterns of net radiation and mean monthly temperature for London are quite similar to those already described for Albuquerque. London does, however, experience a greater annual variation in net radiation. This greater variation can be explained by the effect increasing latitude has on annual variations of insolation. During the winter months outgoingLW radiation actually exceeds incoming insolation producing negative net radiation values. This was not seen in Albuquerque. The variation in monthly mean temperature is also less extreme in London when compared to Albuquerque. Intuitively, one would expect London to have a greater annual change in temperature because of the greater variation in net

radiation over the year. However, London's climate is moderated by the frequent addition of latent heat energy from seasonal precipitation.

Fairbanks, USA - 65° North, 148° West

Monthly variations in net radiation and average monthly temperature for Fairbanks, USA.

Of the five locations examined, Fairbanks has the greatest variations in mean monthly temperature. Fairbanks is also the coldest of the climates examined. This is primarily due to the fact that during six months of the year net radiation is negative because outgoing long wave exceeds incoming insolation. Fairbanks also receives the least cumulative amount of net radiation over the entire year. Mean month temperature is at its maximum in July which is one month ahead of the peak in net radiation.

WHAT ARE ISO-THERMS?Isotherms are lines on a weather map that connect points where the temperature is the same.

Isotherms generally trend east and west and show a decrease in temperatures from the tropics toward the poles.Isotherm: is a iso line that connects points of equal

temperature. Isotherm maps allows Geographers to study the spatial

analysis of temperatures.

Measurement of air Measurement of air temperaturetemperature

Air temperatures are now automatically recorded by thermometers at a uniform height above the ground.

The daily cycle of air The daily cycle of air temperaturetemperature

◆ Because the earth rotates on its axis, incoming solar energy at a location can vary widely throughout the 24-hour period.

◆ Insolation is greatest in the middle of the daylight period, when the sun is at its highest position in the sky, and falls to zero at night.

Daily insolation and net Daily insolation and net radiationradiation

◆ The daily cycle of temperature is controlled by the daily cycle of net radiation.

◆ At the equinox, insolation begins at about sunrise (6 a.m.),rises to a peak value at noon, and declines to zero at sunset (6 p.m.).

◆ At the June solstice, insolation begins about two hours earlier (4 a.m.) and ends about two hours later (8 p.m.).

◆ At the December solstice, insolation begins about two hours later than the equinox curve (8 a.m.) and ends about two hours earlier (4 p.m.).

◆ When net radiation is positive, the surface gains heat, and when negative, it loses heat.

◆ Net radiation begins the 24-hour day as a negative value-a deficit-at midnight. The deficit continues into the early morning hours. Net radiation shows a positive value- a surplus-shortly after sunrise and rises sharply to a peak at noon.

Daily temperatureDaily temperature

The minimum daily temperature usually occurs about a half hour after sunrise. Air temperature rises sharply in the morning hours and continues to rise long after the noon peak of net radiation. Air temperature rises as long as net radiation is positive. Temperatures are lowest just after sunrise and highest in midafternoon.

Urban and rural temperature Urban and rural temperature contrastscontrasts

◆ Urban surfaces lack moisture and so are warmer than rural surfaces during the day. At night, urban materials conduct stored heat to the surface, also keeping temperatures warmer.

The urban heat islandThe urban heat island

◆ As a result of the above effects, air temperatures in the central region of a city are typically several degrees warmer than those of the surrounding suburbs and countryside. This is called a heat island.

◆ The heat island persists through the night because of the availability of a heat stored in the ground during the daytime hours.

◆ Another important factor in warming the city is fuel consumption. In summer, city temperatures are raised through the use of air conditioning.

Temperature Inversion and Temperature Inversion and FrostFrost

◆ In a temperature inversion, air temperature increases with altitude.

◆ Low-level temperature inversions often occur over snow-covered surfaces in winter.

◆ Inversions can also result when a warm air layer overlies a colder one. This type of inversion is often found along the west coasts of major continents.

Frost◆ If the temperature of the lowermost air

falls below the freezing point, for sensitive plants during the growing season, this temperature condition is called a killing frost.

The annual cycle of air The annual cycle of air temperaturetemperature

The annual cycle of net radiation, which results from the variation of insolation with the seasons, drives the annual cycle of air temperatures.

Land and water contrastsLand and water contrasts

◆ Land-water contrasts keep air temperatures at coastal locations more constant than at interior continental locations.

◆ Oceans heat and cool more slowly than continents.

◆ The surface of any extensive, deep body of water heats more slowly and cools more slowly than the surface of a large body of land when both are subjected to the same intensity of insolation.

The average daily cycle of air temperature for four different months shows the effect of continental and maritime location. Daily and seasonal ranges are great at El Paso, a station in the continental interior, but only weakly developed at North Head, Washington, which is on the Pacific coast. The seasonal effect on overall temperatures is stronger at El Paso.

Annual temperature cycleAnnual temperature cycle

P63 figure 3.16 Annual cycles of insolation (a) and monthly mean air temperature (b) for two stations at lat. 50°N: Winnipeg, Canada, and Scilly Islands, England.

Insolation is identical for the two stations. Winnipeg temperatures clearly show the large annual range and earlier maximum and minimum that are characteristic of its continental location. Scilly Islands temperatures show its maritime location in the small annual range and delayed maximum and minimum.

World Patterns of air World Patterns of air temperaturetemperature

◆ Isotherms: Lines drawn to connect locations having the same temperature.

◆ Maps of isotherms show centers of high and low temperatures as well as temperature gradients.

Factors controlling air Factors controlling air temperature patternstemperature patterns

◆ Global air temperature patterns are controlled primarily by latitude, coastal-interior location, and elevation.

◆ Temperatures decrease from the equator to the poles.

◆ Large landmasses located in the subarctic and arctic zones develop centers of extremely low temperatures in winter.

◆ Temperatures in equatorial regions change little from January to July.

◆ Isotherms make a large north-south shift from January to July over continents in the midlatitude and subarctic zones.

◆ Highlands are always colder than surrounding lowlands.

◆ Areas of perpetual ice and snow are always intensely cold.

The annual range of air The annual range of air temperaturestemperatures

◆ The annual range increases with latitude, especially over northern hemisphere continents.

◆ The greatest ranges occur in the subarctic and arctic zones of Asia and North America.

◆ The annual range is moderately large on land areas in the tropical zone, near the tropics of cancer and Capricorn.

◆ The annual range over oceans is less than that over land at the same latitude.

◆ The annual range is very small over oceans in the tropical zone.