Weather and climate

What is precipitation?

• Precipitation is any form of moisture which falls to the earth. This includes rain, snow, hail and sleet. Precipitation occurs when water vapour cools. When the air reaches saturation point (also known as condensation point and dew point) the water vapour condenses and forms tiny droplets of water. These tiny droplets of water from clouds.

• All rain is the same. It happens as the result of warm, moist air being cooled, leading to condensation and in turn rain.

Relief Rainfall

• Stage 1.Warm wet air is forced to rise over high land.

• Stage 2.As the air rises it cools and condenses. Clouds form and precipitation occurs.

• Stage 3.The drier air descends and warms.

• Stage 4.Any moisture in the air (e.g. cloud) evaporates.

Convectional Rainfall

• Stage 1.The sun heats the ground and warm air rises.

• Stage 2As the air rises it cools and water vapour condenses to form clouds.

• Stage 3.When the condensation point is reached large cumulonimbus clouds are formed.

• Stage 4. Heavy rain storms occur. These usually include thunder and lightening due to the electrical charge created by unstable conditions.

Frontal Rainfall

• Stage 1.An area of warm air meets and area of cold air.

• Stage 2.The warm air is forced over the cold air

• Stage 3.Where the air meets the warm air is cooled and water vapour condenses.

• Stage 4.Clouds form and precipitation occurs

Climatic Zones• The world has several climatic zones. These are summarised on the map

below.

• The classification is based on maximum and minimum temperatures and the temperature range as well as the total and seasonal distribution of precipitation.

Summary of climatic zones:

• Polar - very cold and dry all year

• Temperate - cold winters and mild summers

• Arid - dry, hot all year

• Tropical - hot and wet all year

• Mediterranean - mild winters, dry hot summers

• Mountains (tundra) very cold all year

What is pressure?

• Pressure is the weight of air pressing down on the earth's surface. Pressure varies from place to place and and results in pressure systems.

What is low pressure?

• At times of low pressure the air is usually rising. As the air rises, it cools, condenses and forms clouds. Areas of low pressure are known as depressions.

• Depressions, or 'lows', bring rain, strong winds and changeable conditions. Changeable weather is a feature of British weather, depressions are responsible for much of this.

• Strong winds blow in an anticlockwise direction.

Front

• Depressions occur when warm air meets cold air. The point where warm air meets cold air is called a front. Along the front there is usually cloud and rainfall. This occurs because the warm air cools and condenses when it meets the cold air

What is high pressure?

• Areas with above average pressure are called anticyclones. Anticyclones occur when air is sinking. As a result there are usually few clouds in the air. In the UK anticyclones bring cold clear days in winter and hot and sunny days in summer.

• Light winds blow in a clock wise direction.

Weather experienced during a winter anticyclone

• In winter the skies are cloudless so heat is allowed to escape. Therefore temperatures are usually very cold. The ground cools rapidly at night so frost often forms. Fog can also form as the cold air makes water vapour condense into tiny droplets. Fog can last long into the day as there is insufficient heat from the sun to evaporate the water droplets away.

Weather experienced during a summer anticyclone

• Summer anticyclones bring very different weather. As the air descends it is heated causing water in the air to evaporate. Therefore there are few clouds in the air. The skies are clear allowing the suns rays to reach the surface of the earth. This causes temperatures to rise. Heat waves can occur if anticyclones remain over Britain for a number of weeks.

Air mass:

• A large body of air that has similar temperature, pressure and moisture properties.

Anticyclone:

• High pressure system in which air descends to give calm conditions and clear skies. Associated with summer heatwaves and winter frosts and fogs.

Atmosphere:

• The air surrounding the Earth and bound to it by gravity.

Atmospheric pressure:

• Pressure produced by the atmosphere on any surface by its weight.

Clear sky:

• Sky with a total cloud cover of less than one okta.

Climate:

• long-term (30 year) weather averages.

Cold front:

• The "leading edge" of a relatively cold air mass.

Continental climate:

• A climate with a high temperature range away from the influence of the sea. Winters will be colder and summers warmer compared to a coastal location for the same latitude.

Depression (cyclone, low-pressure):

• Area in the atmosphere in which the pressures are lower than those of the surrounding region at the same level. In its development a depression usually has the following phases. A wave (young) depression forms and moves along a front. Mature depressions have well-developed warm sectors and both cold and warm fronts. An occluded depression is that within which there has developed an occluded front.

Dew point (dew-point temperature):

• The temperature to which certain air must be cooled in order for saturation to occur. When this temperature is below 0 °C, frosts form.

Evaporation:

• The physical process by which a liquid or solid substance is transformed to a gas; the opposite of condensation.

Fog:

• Saturated air with visibility below one kilometre. Fog differs from cloud only in that the base of fog is at the Earth's surface while clouds are above the surface.

Front:

• The meeting point between two air masses of different density. Since the temperature is the most important regulator of the atmosphere density, a front almost invariably separates air masses of different temperature. When warmer air replaces the colder, it is a warm front, and a front is a cold one when the opposite occurs.

Humidity:

• Water vapor content of the air.

Isobar:

• A line of equal or constant pressure. Measured in millibars (mb).

Maritime climate:

• A climate with a low temperature range influenced by proximity to the sea. Winters will be warmer and summers cooler compared to a continental location for the same latitude.

Occluded front:

• A front that is formed as a cold front overtakes a warm front and lifts the warm air completely off the ground.

Okta:

• A measure of cloud cover (in fractions of eight) on a synoptic chart.

Precipitation:

• Any of all of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The forms of precipitation are: rain, drizzle, snow, hail, and ice pellets.

Prevailing wind

• Is the most common wind direction for a particular location.

Relief rainfall:

• Formed when air is forced to rise over relief features such as hills or mountains. Cooling and condensation occurs as the air rises.

Seasonality:

• Periodic fluctuations in the climate related to seasons of the year e.g. wet winters, drier summers.

Smog:

• A word currently used as a synonym for general air pollution. It was originally created by combining the words "smoke" and "fog."

Synoptic chart:

• A weather chart reflecting the state of the atmosphere over a large area at a given moment.

Temperature:

• A physical quantity characterizing the mean random motion of molecules in a physical body. In other words, it is a measure of the degree of hotness or coldness of a substance.

Temperature range:

• Maximum minus the minimum temperature for a particular location.

Warm front:

• The forward edge of an advancing warm air mass that is rising over cooler air in its path.

Warm sector:

• The zone of warm air within a depression.

Water vapour:

• Water substance in vapour (gaseous) form; one of the most important of all constituents of the atmosphere.

Weather:

• The state of the atmosphere, mainly with respect to its effects upon life and human activities. As distinguished from climate, weather consists of the short-term (minutes to about 15 days) variations of the atmosphere state.

Wind:

• Movement of air caused by changes in temperature and air pressure. Winds are always identified by the compass direction from which they blow.

Clouds

• A cloud is a visible aggregate of tiny water droplets and/or ice crystals in the atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied by precipitation; rain, snow, hail or sleet.

Introduction to Weather

• Meteorology is the study of weather. Weather is caused by the movement or transfer of energy. Energy is transferred wherever there is a temperature difference between two objects. Many weather phenomena result from a transfer of energy that occurs via the movement of air in the atmosphere. This is known as convection.

• Air contains water vapour from the evaporation of liquid water sources on the Earth's surface, including oceans, lakes and rivers, and from evapotranspiration by plants. When air is moved about the Earth, either vertically when uplifted or horizontally as part of air masses, it may cool and release water vapour as condensation in the form of clouds and eventually rain and other forms of precipitation, which is returned to Earth. This cycle of evaporation, condensation and precipitation between the Earth and the atmosphere is known as the water cycle.

Introduction to Weather• The physical transfer of heat and moisture by convective processes is the

basis for the formation of many meteorological patterns and features, including anticyclones, depressions, fronts, monsoons, thunderstorms, hurricanes and tornadoes. Heat however, may also radiate directly from a hot object to a colder one, without involving the movement of air. Many small-scale weather phenomena are the result of this form of heat transfer, including dew, frost and fog.

• Weather can be simply measured by observing and recording temperature, rainfall, pressure, humidity, sunshine, wind and cloudiness. It is also possible to identify and name different types of clouds, which are associated with different patterns of weather. Commonly observed cloud types include cirrus, cumulus, cumulonimbus and stratus. To make predictions and forecasts about what the weather will do in the future however, it helps to draw synoptic charts, composed of special weather symbols and isobars that reveal patterns of weather. The use of sophisticated technology such as weather radar and satellite imagery also assist with weather forecasting.

Anticyclones• The cloudy rainy weather of low-pressure depressions is due to rising air,

which is most pronounced near frontal regions. The anticyclone on the other hand is produced by a large mass of descending air. This takes place throughout a depth of the atmosphere up to 12km. This means that the air is very stable and atmospheric pressure is high. In addition, winds associated with an anticyclone are usually very light if present at all, especially close to the centre of the high-pressure system.

• Subsidence warms the air by compression. Any clouds present quickly evaporate as the temperature of the air rises above its dew point. For this reason, anticyclones usually bring fine, dry and settled weather, particularly in the summer.

• Sometimes, subsidence and compression of the air can produce a temperature inversion at one or two thousand metres above the ground. Such phenomena act as caps to rising air heated by the ground under the influence of the Sun, preventing extensive air cooling and cloud formation. Unfortunately, if the air is moist below the temperature inversion, a dreary formless layer of cloud can form which becomes difficult to disperse owing to the light winds.

Anticyclones• Winter anticyclones, if clear of cloud, bring with them further problems. A short

cloudless day is the forerunner of a long night with more radiation cooling than a low-angle Sun can counteract the next day. The second night of cooling therefore starts with a lower air temperature than the first. Such conditions, if persistent, can lead to successive nights of frost, which become progressively harder. When the air is particularly moist, cooling at night soon results in fog. Britain in particular can experience episodes of anticyclonic fog from late September through to May.

• Anticyclones move, but not quite in the same purposeful way as travelling depressions. They nudge their way into position and can be incredibly stubborn about leaving, perhaps persisting for weeks, diverting depressions to different routes. Such persistent anticyclones are known as "blocking highs". In winter they can lead to long spells of very cold weather, especially if their airflow comes from Russia and Siberia. In summer they can lead to long hot spells and sometimes drought.

• A ridge of high pressure is a wedge-shaped extension of an anticyclone or belt of high pressure. The weather associated with ridges is similar to that in an anticyclone. In temperate latitudes as in the British Isles, ridges of high pressure often occur between two depressions and move with them. They give rise to intervals of fair weather between the cloud and rain of the low-pressure systems.

Clouds

• A cloud is a visible aggregate of tiny water droplets and/or ice crystals suspended in the atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied by precipitation; rain, snow, hail or sleet.

• All clouds form as a consequence of rising air. Sometimes air is forced to rise over mountains. More usually, warm air, being less dense, will rise above cold air. At fronts for example, warm air masses rise over cold air masses when they converge. At much smaller scales, columns of rising warm air may be generated by daytime heat from the Sun.

• When air rises, it expands, causing cooling and a drop in temperature. As the temperature falls, the humidity (or water vapour content) of air increases towards 100%. Finally, after sufficient cooling, the air becomes saturated, and water vapour begins to condense out as tiny water droplets, forming cloud.

Cold Fronts

• Cold fronts are usually associated with depressions. A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. At a cold front cold air following warm air undercuts the warm air, heaving it upwards. The air associated with a cold front is usually unstable and conducive to cumulonimbus cloud formation. Because the upthrust is delivered along a boundary between the two air masses, the cumulonimbus form a well-defined line in contrast to the well-spaced clouds forming during thermal convection. More rain may fall in a few minutes as the cold front passes than during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5°C or more within the first hour.

• On synoptic (weather) charts a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.

Condensation• Condensation is the process whereby water vapour in the atmosphere is

returned to its original liquid state. In the atmosphere, condensation may appear as clouds, fog, mist, dew or frost, depending upon the physical conditions of the atmosphere. Condensation is not a matter of one particular temperature but of a difference between two. Condensation of water vapour occurs when the temperature of air is lowered to its dew point.

• All air contains water vapour of varying quantities. The lower the air temperature, the smaller the maximum possible capacity for vapour. When air is cooled, relative humidity increases, until at a particular temperature, called the dew point, the air becomes saturated. Further cooling below the dew point will induce condensation of the excess water vapour.

• The temperature of the dew point will depend upon the absolute content of water vapour, that is the absolute humidity, measured in g/m3 (grams per cubic metre). The dew point of humid air will be higher than the dew point of dry air. Both air temperature and absolute humidity will determine what type of condensation will occur when the air is cooled. If air in contact with the ground is cooled to its dew point, dew or frost will form, dew if the point is above 0°C, or frost if it is below 0°C. Cooling of a larger layer of air near to the ground may produce mist of fog, which freezes if the dew point is below 0°C. Air that is cooled to its dew point by rising and expansion will condense to form clouds. Above 0°C, small droplets of water are formed.

Cooling Air• Many of the common phenomena of weather - clouds, frost, fog and rain - are

due to the cooling of air and the consequent condensation of excess water vapour. Air is cooled by two main processes: cooling by contact (usually with the Earth's surface); and cooling by uplift.

• A common form of contact cooling is radiation cooling. When the Sun sets, the Earth and any other radiating body upon it continue to radiate heat from their supplies. With no heat from the Sun to replenish their stocks, their temperature falls. Maximum loss of heat occurs under clear skies. As surface temperature falls the air in closest contact with it begins to cool. Eventually, the surface air will cool below its dew point temperature, and begin to condense water vapour out as dew. When the dew point of the cooling air is below 0°C, hoarfrost results from radiation cooling instead. Condensation then occurs directly as a crust of white crystals. Sometimes a much thicker layer of moist air may be cooled. Condensation then occurs throughout, giving rise to fog.

• Usually wind serves to prevent or restrict the formation of dew, frost or fog. This is because a steady flow of air over the cooling surface does not remain in contact with the ground long enough to cool below its dew point for condensation to occur. The one occasion when wind does not deter condensation is when warm air from different sources passes over a much colder surface. Advection cooling, as it is known, is a common source of sea fog in coastal areas, when warmer sea air comes inland passing over colder land.

Cumulonimbus Clouds

• Cumulonimbus clouds are much larger and more vertically developed than cumulus clouds which form in a more stable atmosphere. They can exist as individual towers or form a line of towers called a squall line often present at cold fronts. Underneath they are dark. At a distance they rise up like huge white mountains when the Sun shines on them. Fuelled by vigorous convection of air in an unstable atmosphere the tops of cumulonimbus clouds can easily reach 12 km or higher. Lower levels of cumulonimbus clouds consist mostly of water droplets while at higher elevations, where temperatures are well below 0°C, ice crystals dominate.

Cumulus Clouds• Cumulus clouds look like white fluffy balls of

cotton wool and mark the vertical extent of convection or thermal uplift of air taking place in the in the atmosphere. The level at which condensation and cloud formation begins is defined by the flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base.

Depressions• Depressions, sometimes called mid-latitude cyclones, are areas of low pressure

located between 30° and 60° latitude. Depressions develop when warm air from the sub-tropics meets cold air from the polar regions. There is a favourite meeting place in the mid-Atlantic for cold polar air and warm sub-tropical air. Depressions usually have well defined warm and cold fronts, as the warm air is forced to rise above the cold air. Fronts and depressions have a birth, lifetime and death; and according to the stage at which they are encountered, so does the weather intensity vary.

• A depression appears on a synoptic (weather) chart as a set of closed curved isobars with winds circulating anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the rotation of the Earth. The warm and cold fronts associated with depressions bring with them characteristically unsettled weather. Depressions vary from between 200 and 2,000 miles in diameter; they may be deep when pressure at their centre is very low and the isobars are tightly packed, or shallow when less well developed.

• A depression develops like the propagation of a wave in water. Initially, a uniform boundary or front exists between cold air pushing southwards and warm air pushing northwards (in the Northern Hemisphere). A wave-shaped distortion may appear on the front, and a small low-pressure centre develops at the crest of the wave. In the immediately surrounding area the pressure begins to fall. A disturbance of this kind is called a wave depression. As the "wave" develops, a warm sector of air forms bounded by the warm and cold fronts, which begins to tie over the engulfing cold air. Both the warm and cold fronts originate from the centre of the depression.

Dew• When air is cooled the amount of water vapour that it can

hold decreases. At the dew point temperature, air is saturated. A further fall in temperature will result in condensation of excess water vapour in the form of water droplets. On the ground this is known as dew.

• When air close to the ground cools at night through a loss of radiation the temperature often falls below the dew point temperature and dew may form. Dew forms most easily over grass because the thin layer of air next to it is always moist due to water transpiration by the grass blades. Wind sometimes limits or prevents the formation of dew because the cooling surface air is readily mixed with air above it. The wind also increases the rate of evaporation, precluding the formation of water droplets.

• When dew freezes it is known as frost.

Dew Point

• All air contains water vapour of varying quantities. The dew point indicates the amount of moisture in the air. The higher the dew point, the higher the moisture content of the air at a given temperature. Conversely, the dew point of humid air will be higher than the dew point of dry air.

• Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapour content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapour possible at the existing temperature and pressure.

• Condensation of water vapour begins when the temperature of air is lowered to its dew point and beyond. The dew point, like other measures of humidity, can be calculated from readings taken by a hygrometer.

Energy• Weather is caused by the movement or transfer of energy.

Energy is transferred wherever there is a temperature difference between two objects. There are three main ways energy can be transferred: radiation, conduction and convection.

• The Earth receives and absorbs energy from the Sun in the form of electromagnetic radiation (mostly light and ultraviolet energy). The Earth also re-radiates a lot of heat back to the atmosphere and into space (as infrared radiation). Within the atmosphere however, a lot of energy is transferred by convection, which drives much of the world's weather. Convection involves the movement of air.

Evaporation• Evaporation of water from the Earth’s surface forms one part of the

water cycle. At 100°C, the boiling point, all water will rapidly be turned to vapour, for the energy supplied to the water is enough to break apart all the molecular bonds in water. At temperatures between 100°C and 0°C, only some of the molecules in the water have enough energy to escape to the atmosphere and the rate at which water is converted to vapour is much slower.

• The rate of evaporation will depend upon a number of factors. Rates increase when temperatures are higher. An increase of 10°C will approximately double the rate of evaporation. The humidity of the surrounding air will also influence evaporation. Drier air has a greater "thirst" for water vapour than humid, moist air. It follows therefore, that the presence of wind will also increase evaporation. On still days, water evaporating to the air remains close to its source, increasing the local humidity.

Fog

• When air is cooled the amount of water vapour that it can hold decreases. At the dew point temperature, air is saturated. A further fall in temperature will result in condensation of excess water vapour in the form of water droplets. If a sufficiently thick layer of air is moist, condensation can occur throughout giving rise to fog. Visibility is usually reduced to below 1,000 metres.

• With no wind at all, fog will form first as shallow streaks near the ground. More usually there is a little prevailing wind serving to spread the fog evenly within one or two hundred metres of the ground. The moister the air, the greater the likelihood of fog forming under clear skies at night when radiation cooling is greatest. As with dew and frost, fog formation is most likely in low-lying grounds and hollows into which colder air sinks, and least likely on hilltops.

Forecasting• Weather forecasts provide critical information about the weather to

come. There are many different techniques involved in weather forecasting, from relatively simple observation of the sky to highly complex mathematical models run on computers. Weather prediction can be for the next day, next week, or next few months. The accuracy of weather forecasts however, falls significantly beyond about 10 days. Weather forecasting remains a complex business, because the weather can be so chaotic and unpredictable.

• If weather patterns are relatively stable, the persistence method of forecasting provides a relatively useful technique to predict the weather for the next day. If it is hot and sunny on one day, it is likely to be hot and sunny the next. Unfortunately, in many areas of the world the weather is more unpredictable and changeable than that, particularly in the mid-latitudes where depressions influence much of the weather.

• With an understanding of how the air moves and how clouds and rain form, some prediction can be made by simply observing the sky overhead, observing wind direction and noting the temperature and humidity of the air.

Fronts• Fronts occur at the boundaries of converging air masses which come

together from different parts of the world. Since air masses usually have different temperatures, they cannot mix together immediately owing to their different densities. Instead, the lighter, warmer air mass begins to rise above the cooler, denser one.

• Fronts are usually associated with depressions, regions of low pressure centred on the rising air which develop as a result of the Earth's rotation. As the sector of warm air is forced to rise, the cold air begins to engulf it. The leading edge of the warm air is marked by the warm front. The cold front marks the rear edge of the warm air and the leading edge of the ensuing cold air. When the warm air is completely uplifted off the ground, this may be marked on a synoptic chart by an occluded front.

• Fronts are accompanied by clouds of all types, and very often by precipitation. Precipitation is usually heavier although less prolonged at cold fronts than at warm fronts, since the uplift of warm air there is more vigorous due to the undercutting of cold air, resulting in increased atmospheric instability.

Humidity• Some water in the form of invisible vapour is intermixed with the air

throughout the atmosphere. It is the condensation of this vapour which gives rise to most weather phenomena: clouds, rain, snow, dew, frost and fog. There is a limit to how much water vapour the air can hold and this limit varies with temperature. When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Warm air can hold more vapour than cold air. In general the air is not saturated, containing only a fraction of the possible water vapour.

• The amount of vapour in the air can be measured in a number of ways. The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. This is the absolute humidity of air. Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10°C contains 9.4 g/m3 (grams per cubic metre) of water vapour when saturated. If air at this temperature contains only 4.7 g/m3 of water vapour, then the relative humidity is 50%.

• When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Relative humidity is 100%. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point.

Isobars• Isobars on a synoptic (weather) chart are

lines along which the atmospheric pressure is the same. They are of the same nature as height contours on a geographical map. Usually they are drawn at intervals of 2 or 4 millibars. By definition, isobars can never cross each other.

Measuring Weather• In everyday language, weather means such qualities as wet or fine, warm or

cold. For most people, such descriptive terms are adequate. However, many industries today require more quantitative assessments of the weather, with the use of standardised terms measured by suitably designed instruments. The science of the study of weather is called meteorology. The meteorologist measures temperature, rainfall, pressure, humidity, sunshine and cloudiness, and makes predictions and forecasts about what the weather will do in the future.

• Meteorologists still use simple ground-based instruments to measure the various elements of the weather, including thermometers, rain gauges and barometers. However, to make really accurate weather forecasts it is useful to know what the current weather is like over a large geographical area. Weather radar and satellite photography can offer the meteorologist a snapshot of the weather in a single image across an entire continent. Radar uses microwaves to scan for raindrops. Wherever it is raining the raindrops bounce the signal and by listening to the returning pulse, the radar can compute the location and intensity of the rain. Satellites allow meteorologists to track the path and development of weather systems. Satellites don't just "look" in the visible part of the spectrum. They can also measure the temperature of the ground and the clouds by "seeing" in infrared. Some satellites even measure the amount of water vapour in the atmosphere.

Meteorology• The science of the study of weather is called meteorology.

Meteorology is the study of the changes in temperature, air pressure, moisture, and wind direction in the lowest part of the atmosphere in which most of the observed weather phenomena occur. Meteorologists investigate these day-by-day variations in the weather.

• Weather phenomena are governed by a set of physical and chemical processes which are determined by simple mathematical relationships. The way these processes interact however, creates a much more complex system, which is why the weather can be so unpredictable and hard to forecast. Meteorology combines the disciplines of mathematics, physics, chemistry and geography to try to simplify the understanding of an inherently complex atmospheric system.

Movement of Air• Movement of air is caused by temperature or pressure differences and

is eperienced as wind. Where there are differences of pressure between two places, a pressure gradient exists, across which air moves: from the high-pressure region to the low-pressure region. This movement of air however, does not follow the quickest straight-line path. In fact, the air moving from high to low pressure follows a spiralling route, outwards from high pressure and inwards towards low pressure. This is due to the rotation of the Earth beneath the moving air. Consequently, air blows anticlockwise around a low-pressure centre (depression) and clockwise around a high-pressure centre (anticyclone) in the Northern Hemisphere. This situation is reversed in the Southern Hemisphere.

• Air temperature is generally higher at ground level due to heating by the Sun, and decreases with increasing altitude. This vertical temperature difference creates a significant uplift of air, since warmer air nearer the surface is lighter than colder air above it. This vertical uplift of air can generate clouds and rain. Sometimes air from warmer regions of the world collides with air from colder regions. This air mass convergence occurs in the mid-latitudes, where the warm air is forced to rise above the colder air, generating fronts and depressions.

Occluded Fronts• Mid-latitude depressions are usually associated with warm

and cold fronts separating warm and cold sectors of air. The lighter warm air rises above the heavier cold air, more gently at a warm front but more vigorously at the cold front following behind. Cold fronts usually travel faster than warm fronts, and therefore at some stage of depression development, the cold front catches up with the warm front. In cross section, the warm air is lifted right off the ground, so that the observer on the surface misses out the warm sector stage. This is known as an occlusion or occluded front.

• On synoptic (weather) charts an occluded front is represented by a solid line with alternating triangles and circles pointing the direction the front is moving. On colored weather maps, an occluded front is drawn with a solid purple line.

Precipitation• All the forms of water that fall from the air to the Earth's surface are

called precipitation. Whether the precipitation is snow, rain, sleet or hail depends on the temperature of the air that the water falls through. If the air is above freezing, the precipitation will most likely be rain. If the air is below freezing, the precipitation will most likely be snow. When air temperature is only a few degrees above freezing, precipitation may fall as sleet.

• Hail is most commonly formed within the cumulonimbus clouds of thunderstorms. Large updrafts of air can throw rain droplets high up into the tops of the cloud. Here, the temperature is well below freezing, and the droplets freeze. The droplets then fall and can become caught in further updrafts, adding a second coating of ice to make the hailstones larger. This cycle continues until the hailstones are too heavy to be lifted again. They then falls as hail.

• The amount of rain, sleet, snow or hail which falls in a specified time is expressed as the depth of water it would produce on a large, level impermeable surface.

Pressure• Pressure is a force, or weight, exerted on a surface per unit area, and

is measured in Pascals (Pa). Usually, atmospheric pressure is quoted in millibars (mb). Because pressure decreases with altitude, pressure observed at various stations must be adjusted to the same level, usually sea level.

• Atmospheric pressure is measured by a barometer. • Air blows from regions of high atmosphere pressure ("highs" or

anticyclones) to regions of low atmospheric pressure. In a high-pressure system, air pressure is greater than the surrounding areas. This difference in air pressure results in wind, or moving air. In a high-pressure area, air is denser than in areas of lower pressure. The result is that air will move from the high-pressure area to an area of lower density, or lower pressure..

• Air moving from high to low pressure does not however, follow a straight-line path. In fact, the air moving from high to low pressure follows a spiralling route due to the rotation of the Earth beneath the moving air.

Sunshine• The Sun is the Earth's only source of

radiative energy, heating the surface by daytime. Greatest heating occurs under cloudless skies, but even when the sky is overcast, heating is usually sufficient to raise the surface temperature above the night-time minimum.

Synoptic Charts• With an understanding of how the air moves and

how clouds and rain form, much prediction can be made by simply observing the sky overhead, observing wind direction and noting the temperature and humidity of the air. But to be able to predict and forecast weather it is necessary to understand the development of weather systems such as depressions and anticyclones by means of isobar plots. Meteorologists plot isobaric patterns on synoptic charts.

Temperature• The hotness or coldness of a substance is called

its temperature and is measured with a thermometer.

• Most temperature scales today are expressed in degrees Celsius (°C), although one will sometime see Fahrenheit (°F) in use, particularly in the United States. The Celsius scale is fixed by two points, the freezing and boiling point of water, which at normal atmospheric pressure are 0°C and 100°C respectively. The scale is then divided into 100 units. 0°C is equivalent to 32°F and 100°C to 212°F.

Warm Fronts• A warm front exists when warm air is rising over

cold air. In vertical cross-section, the boundary takes the form of a gradual slope (roughly 1:100) and lifting is slow but persistent. As the air lifts into regions of lower pressure, it expands, cools and condenses water vapour as flat sheet cloud (altostratus), from which rain can start to fall once cloud has thickened to about 2,500 metres from the ground. On synoptic (weather) charts a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.

Water Cycle• Water covers 70% of the Earth's surface. Almost all of this is stored in

the oceans (97.5%) and in freshwater lakes, rivers and streams on land (2%). The atmosphere holds less than .001% in the form of water vapour. If all this water vapour was precipitated completely and evenly over the whole Earth, it would yield only about 25mm or 1 inch of rainfall. Water vapour in the atmosphere plays a very important role in the weather.

• There is always water vapour present in the atmosphere. When the air becomes saturated, excess water vapour is released as condensation. This condensation is the source of all clouds and rain. Water vapour enters the atmosphere by evaporation from surface bodies of water. These include puddles, ponds, streams, rivers, lakes and oceans. Water also enters the atmosphere by evapotranspiration from plants and trees. The water vapour is returned to the Earth's surface as precipitation (rain, hail, sleet or snow), and is received by soil, vegetation, surface streams, rivers and lakes and ultimately the sea. This cycle of evaporation, condensation and precipitation is called the water cycle of the Earth and atmosphere.

Weather Symbols• Weather symbols are plotted on a synoptic chart used for

weather forecasting, and show the position of weather stations collecting information about the weather, and details of the various weather elements. The position of a meteorological station is marked by a small circle. The weather report for each station is then plotted in and around the circle. Elements like temperature and pressure are entered as plain figures. Others, like the occurrence of rain, snow, cloud and fog are plotted as internationally agreed symbols.

• Temperature is measured in degrees Celsius to the nearest whole degree. Pressure at a station is standardised to sea-level pressure measured in millibars. Sometimes the hundreds figure for the pressure is omitted as being understood since the pressure is almost always between 950 and 1050 millibars. For example, a pressure of 987.8 millibars would be written as 878; 1014.3 millibars as 143.

Typical weather symbols

Wind• The air is nearly always in motion, and this is felt as wind.

Two factors are necessary to specify wind, its speed and direction. The direction of wind is expressed as the point of the compass from where the wind is blowing.

• Wind develops as a result of pressure or temperature differences between two locations on the Earth's surface. Sea breezes for example, develop due to the differential heating of land and sea at the coast during warm sunny days. Winds also blow out from high-pressure regions or anticyclones and into low-pressure regions, for example depressions. The wind however, does not blow in a straight line, but follows a spiralling path.

• http://www.ace.mmu.ac.uk/eae/Weather/Older/Weather_Introduction.html

• http://sixthsense.osfc.ac.uk/geography/pages/general_links/environment.asp