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Anticyclone	Depressions	Dew	Dew Point	Energy	Fog
Fronts	Frost	Humidity	Hurricanes	Isobars	Measuring Weather
Monsoons	Movement of Air	Occluded Fronts	Precipitation	Pressure	Stability of Air
Sunshine	Synoptic Charts	Thunderstorms	Tornadoes	Warm Fronts	Meterology

Meteorology

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. 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.

Anticyclone

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 subsidence takes place throughout a depth of the atmosphere up to 12km. Such subsidence 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. Such debilitating weather is common in winter when the Sun’s radiation is too weak to burn off the cloud layer. 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.

Depressions

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. 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. On the ground, sudden changes in the wind direction may be experienced when fronts pass by. Wave depressions can grow off the tail ends of primary cold fronts. The depression so formed is then called a secondary depression. New centres may also develop from occluded fronts within the primary depression. The secondary system can then become the main system, and the primary occluded front becomes caught up in the developing circulation, effectively becoming a third front.

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. If the dew point temperature is below freezing, any condensation of water vapour takes place directly as ice crystals known as hoarfrost.

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. Greenhouse gases in the atmosphere keep the Earth's surface much warmer than it should be. Greenhouse gases let in sunlight energy but trap infrared heat energy trying to escape back to space. This process is called the natural greenhouse effect. Man-made pollution may be enhancing the natural greenhouse effect and causing global warming.

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. Fogs formed as a result of radiation cooling are termed radiation fogs. Advection fogs, in contrast, form when warm humid air from different sources passes over a much colder surface causing condensation. Sea fog in coastal areas is a form of advection fog, formed when warmer sea air comes inland passing over colder land.

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.

Frost

When air is cooled the amount of water vapour that it can hold decreases. At the dew point temperature, air becomes saturated. A further fall in temperature will result in condensation of excess water vapour in the form of water droplets. If the dew point temperature of air close to the ground is below freezing, condensation will form not as dew but as hoarfrost. Hoarfrost is made up of white crystals. Usually, air is too moist for hoarfrost to form directly. More usually dew forms first, which if ground temperature falls below 0°C will freeze. Both frost and hoarfrost can be particularly damaging to outdoor crops and plants. Since cooling air will always drain downhill if possible, valleys and hollows suffer the greatest risk from frost. Coastal areas benefit from the moderating influence of the warmer sea. If the air is particularly dry, its dew point may be well below freezing. Whilst an air frost may occur, hoarfrost will be absent if the air temperature does not fall below the dew point.

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. The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer. A hygrometer has two thermometers, one dry bulb or standard air temperature thermometer, and one wet bulb thermometer. The wet bulb thermometer is an ordinary thermometer which has the bulb covered with a muslin bag, kept moist via an absorbent wick dipped into water. Evaporation of water from the muslin lowers the temperature of the thermometer. The difference between wet and dry bulb temperatures is used to calculate the various measures of humidity.

Hurricanes

Frontal depressions in the mid-latitudes are often poorly defined features. Their tropical counterparts, in contrast, have very distinct shapes and can be easily identified on images provided by satellites. Tropical storms have different names in different parts of the world. Known as cyclones in the Indian Ocean, and typhoons in Southeast Asia, Hurricanes (the name used in the Caribbean) can be some of the most damaging weather phenomena on Earth. The word hurricane originates from West Indian language, meaning "big wind." The Caribbean and Gulf of Mexico experience the most organised annual season of hurricanes from about August through to October. These hurricanes begin life as disorganised storms which form over warm, tropical waters in the Atlantic. Usually surface seawater must be at least 27°C for there to be sufficient heat and moisture input into the atmosphere to generate the more violent storms. When wind speeds reach 75 mph (120 kph), the storm is classified as a hurricane. On a synoptic chart, a hurricane appears as a set of almost circular, tightly packed isobars. Hurricanes are usually about 300 to 400 miles wide. The winds of a hurricane are structured around a central "eye", which is an area that is often free of clouds and relatively calm. Inside the eye of the hurricane, atmospheric pressure is very low, sometimes falling below 900 millibars. Around this "eye" area, storm clouds wrap in an anticlockwise manner (in the Northern Hemisphere) following the wind direction. This "eyewall" of clouds, wind and rain, is the most destructive part of the storm. In fact, it is the eyewall that creates the eye, since the rapid spinning clouds in the wall reduce the pressure in the eye and suck out any clouds that are present there.

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. The completed isobars usually reveal a few standard patterns. A set of curved isobars surrounding an area of low pressure reveals a depression, with the wind in the Northern Hemisphere blowing anticlockwise around its centre. A set of curved isobars surrounding a high pressure reveals an anticyclone, with the winds in the Northern Hemisphere blowing clockwise around its centre. Open V-shaped isobars with low pressure inside mark a trough of low pressure. In contrast N-shaped isobars with high pressure inside mark a ridge of high pressure. A col. is the indefinite isobar configuration between two highs and two lows arranged alternately, and has no particular type of weather associated with it other than light winds. When isobaric patterns are plotted with information gathered at weather stations in the form of weather symbols, the weather forecaster can use his skill to predict the weather over the next few hours or days.

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.

Monsoons

The "Monsoon", from the Arabic "mauism" meaning "season", defines a wind that changes direction with the seasons. Monsoons develop as a result of changing patterns of atmospheric pressure caused by the varied heating and cooling rates of continental landmasses and oceans. The strongest and most well known monsoons are those which affect India and Southeast Asia during June to September. The summer monsoon, which blows southwesterly across the Indian Ocean, is extremely wet. The winter monsoon, in contrast, blows northeasterly and is generally dry.

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, which causes an apparent deflection of the wind to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. The deflection of air is caused by the Coriolis force. 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. Wind caused by differences in temperature is known as convection or advection. In the atmosphere, convection and advection transfer heat energy from warmer regions to colder regions, either at the Earth surface or higher up in the atmosphere. Small-scale air movement of this nature is observed during the formation of sea and land breezes, due to temperature differences between seawater and land. At a much larger scale, temperature differences across the Earth generate the development of the major wind belts. Such wind belts, to some degree, define the climate zones of the world. 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 stronger the updrafts in the cloud, the longer the hail develops, and the larger the hailstones are when they falls. 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. Usually it is expressed in millimetres although inches may sometimes be used. Precipitation is measured daily (24 hours) by means of a rain gauge. Today’s rain gauges are simple to use with pre-calibrated scales on their sides. When measuring precipitation, certain precautions have to be taken against the effects of obstructions, wind, splashing and evaporation.

Pressure

Like all fluids, the air exerts a pressure on everything within and around it, although we are not aware of it. Pressure is a force, or weight, exerted on a surface per unit area, and is measured in Pascals (Pa). The pressure exerted by a kilogram mass on a surface equals 9.8 Pa. The pressure exerted by the whole atmosphere on the Earth’s surface is approximately 100,000 Pa. Usually, atmospheric pressure is quoted in millibars (mb). 1 mb is equal to 100 Pa, so standard atmospheric pressure is about 1000mb. In fact, actual values of atmospheric pressure vary from place to place and from hour to hour. At sea level, commonly observed values range between 970 mb and 1040 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. A mercury barometer measures the pressure by noting the length of mercury which is supported by the weight of the atmosphere. One centimetre of mercury is equal to 13.33 mb, so normal atmospheric pressure can support a column of mercury about 75 cm (or 30 inches) high. An aneroid barometer is a more compact instrument for measuring pressure. It consists of a box of partially exhausted air which expands and contracts as the pressure falls and rises. The box is connected through a system of levers to a pointer which, in conjunction with a dial, indicates the pressure. 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. Conversely, winds tend to blow into low-pressure areas because air moves from areas of higher pressure into areas of lower pressure. As winds blow into a low, the air can be uplifted. This uplift of air can lead to the development of a depression with clouds and rain. 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, which causes an apparent deflection of the wind to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere.

Stability of Air

The stability of air in the atmosphere depends on the temperature of rising air relative to the temperature of the stationary surrounding air that it passes through, which varies from place to place and with changing atmospheric conditions. Air stability determines whether clouds form when air is uplifted, and the type of cloud. When a packet of air near the Earth’s surface is heated it rises, being lighter than the surrounding air. Whether or not this air packet continues to rise will depend upon how the temperature in the surrounding air changes with altitude. The rising packet of air will lose heat because it expands as atmospheric pressure falls, and its temperature drops. If the temperature of the surrounding air does not fall as quickly with increasing altitude, the air packet will quickly become colder than the surrounding air, lose its buoyancy, and sink back to its original position. In this case the atmosphere is said to be stable. If the temperature of the surrounding air falls more quickly with increasing altitude, the packet of air will continue to rise. The atmosphere in this circumstance is said to be unstable. As uplifted air cools, it condenses excess vapour out as cloud. The more unstable the atmosphere the more prolonged the uplift. Small cumulus clouds are evidence of a fairly stable atmosphere. Large cumulonimbus clouds are evidence of a highly unstable atmosphere, conducive to the formation of thunderstorms. Within depressions, atmospheric pressure is low and there is considerable atmospheric uplift and cooling at altitude, increasing atmospheric instability. Low-pressure systems are usually associated with an abundance of cloud and precipitation. In high-pressure systems or anticyclones, air may be descending, compressing and gaining energy, such that temperature at altitude rises, thereby increasing atmospheric stability. Anticyclones are often associated with cloudless skies.

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. Many weather stations record the amount of bright sunshine during daylight hours. A typical measuring instrument is the Campbell-Stokes sunshine recorder. This consists of a glass ball which focuses the Sun’s rays, burning a hole in an index card. As the Sun moves around the sky, so the burnt hole extends along the index, which is marked in hours to record the amount of direct sunshine

Synoptic Chart

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. The first stage in preparing a synoptic chart is to chart the position of each meteorological station. These are marked by a small circle. The weather report for each station is then plotted in and around the circle, documenting the station's recorded temperature, rainfall, pressure, wind speed and direction, and cloud coverage. The station circle and various elements of the weather make up weather symbols. When plotting of the meteorological observations is completed, the forecaster then uses the values of pressure at all the stations to identify isobars - lines of equal pressure. The completed synoptic chart with symbols and isobars usually reveal a few standard weather patterns. The positions of fronts and regions of low and high pressure can also be plotted on the synoptic chart, which show the weather conditions of different areas at a particular time. With skill and experience the meteorologist can use the synoptic chart to forecast the weather up to 24 to 48 hours ahead. Synoptic charts are updated at least every six hours, plotting new weather symbols and isobars, in order that the weather forecast can remain as accurate as possible.

Thunderstorms

A thunderstorm is rain or hail accompanied by thunder and lightning and gusty winds. Thunderstorms usually develop when there is sufficient heating of air near the Earth's surface which rises in a very unstable atmosphere. Thunderstorms are a violent example of atmospheric convection, with uplift and cooling of air, and subsequent cloud formation. As the cloud forms, water vapour changes to liquid and/or frozen cloud particles. This results in a large release of heat that takes over as the principal source of energy for the developing cloud. Once the cloud starts to form by other forces, this release of heat helps keep it growing. The cloud particles grow by colliding and combining with each other, forming rain, snow, and hail. When the droplets become heavy enough to fall against the updraft, precipitation begins, which may be short-lived but very heavy. Having reached its final stage of growth, the towering cumulonimbus cloud may be several miles wide and often 10,000 metres or more in height. High level winds shear the cloud top into the familiar anvil shape. When the Sun illuminates these cloud towers, they appear as huge white mountains. When moving several abreast they may form a squall line. Once precipitation begins the updraft which initiated the cloud's growth weakens and is joined by a downdraft generated by the precipitation. This updraft-downdraft couplet constitutes a single storm "cell". On the ground the updrafts and downdrafts of air are felt as rapid gusts of wind. Most storms are composed of several cells that form, survive for about half an hour, and then die. New cells may replace old ones, and it is possible for some storms to continue for several hours. Lightning always accompanies the thunderstorm. Lightning arises from a discharge of electrical energy which has built up within the cumulonimbus cloud as a result of repeated separation and splitting of water and ice particles in the turbulent conditions that prevail. Although air is a fairly good insulator, eventually the separation of electric charge becomes so great that the insulation breaks down and a lightning strike results. Lightning discharge may occur entirely within the cumulonimbus cloud or between the cloud and the ground. The lightning strike causes a rapid heating of the surrounding air, resulting in a sudden expansion and contraction of air that is heard as thunder. Close to the lightning strike the thunder may be heard as a short loud crack. Further away, the thunder rumbles or echoes, because sound from different parts of the lightning strike are not all heard at the same time. One can work out how far away the lightning strike was by counting the time taken for the thunder to arrive. A 5 seconds difference is roughly equal to a distance of 1 mile. In warmer regions of the world thunderstorms can be particularly violent because they contain so much energy, made available from the strong surface heating by the Sun. Such thunderstorms may also be accompanied by tornadoes, rapidly spinning columns or spouts of air. Tornadoes are particularly common in the central United States. It is estimated at any given moment nearly 2,000 thunderstorms are in progress over the Earth's surface, and lightning strikes the Earth 100 times each second. There are about 45,000 thunderstorms daily and 16 million annually around the world.

Tornadoes

A tornado is defined as a violently rotating column of air in contact with the ground that originated from the base of a cumulonimbus cloud. They can be highly destructive but fortunately are relatively small in comparison to other extreme weather phenomena such as thunderstorms and hurricanes. The tornado can be seen by the rapid rotation of cloud caught up within it. Tornadoes are also known popularly as "twisters". Tornadoes are particularly common in the central United States. Tornadoes can be categorised as "weak", "strong", and "violent". Weak tornadoes, making up about 70% of all tornadoes, often have a thin, rope-like appearance. Rotating wind speeds are no greater than about 110 mph. "Strong" tornadoes, making up nearly 30% of all tornadoes, often exhibit the more "classic" funnel-shaped cloud associated with the whirling air column. Rotating wind speeds vary from 110 to 200 mph. The most violent tornadoes are relatively rare, making up only 2% of all tornado cases. Rotating wind speeds in "violent" tornadoes can approach speeds close to 300 mph. Most tornadoes form in the presence of thunderstorms. All thunderstorms are characterised by rising or convecting air, called updrafts. These updrafts supply the warm, humid air that fuels thunderstorms. In some cases, the column of rising air becomes a vortex - a funnel cloud or tornado. Often, a tornado is located on the edge of the updraft, adjacent to the thunderstorm downdraft that accompanies the falling rain or precipitation. Meteorologists however, still do not fully understand why updrafts in some thunderstorms become tornadoes while those in apparently similar thunderstorms do not. The center of the tornado's vortex is a low-pressure area. Inside the vortex, atmospheric pressure may fall as low as 500 millibars, or half the normal value. The depressurisation of the vortex cools the air which is rushing in, allowing the condensation of water vapour to form the tornado's familiar funnel-shaped cloud. As the swirling winds pick up dust, dirt, and debris from the ground, the funnel may become even darker. Although the air is rising in a tornado, the funnel itself grows from the cloud above downwards toward the ground as the tornado is forming. The term "funnel cloud'' refers to a tornado-like vortex that doesn't reach the ground. When a funnel cloud touches the ground, it becomes a tornado.

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. Cloud continues to lower towards the boundary at ground level, known as the surface front. This lower level cloud is called stratus or nimbostratus, from which appreciable amounts of rain may fall. Sometimes, nimbostratus cloud may be only a few hundred feet above the ground, and can completely cover hilltops and mountains. Because frontal systems have a velocity of there own, an observer on the ground will witness a succession of cloud types with cloud gradually thickening before rain arrives. These telltale signs can be used by the observer to predict the onset of bad weather within a few hours. When the surface warm front arrives, there may be a burst of rather heavier rain, and this offers a hopeful sign that a drier interlude is on the way. Clouds will break, rain cease, and there may be a noticeable rise in temperature as the warm air engulfs the observer. 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.

Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Absolute humidity The mass of water vapor in a given volume of air. It represents the density of water vapor in the air.

Absolute zero A temperature of -273ºC, -460ºF, or 0ºK. Theoretically, there is no molecular motion at this temperature.

Absorptivity The efficiency of radiation absorption.

Acclimatization The gradual adjustment of the body to new climatic or other environmental conditions, for example, the adjustment to low levels of oxygen at high altitudes.

Accretion The growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.

Actual evapotranspiraton The rate of water lost from vegetation and soil, ordinarily at a slower rate than the potential rate.

Actual vapor pressure See vapor pressure.

Adiabatic process A process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. In an adiabatic process compression always results in warming, and expansion results in cooling.

Advection The horizontal transfer of any atmospheric property by the wind.

Advection fog Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point.

Aerovane A device that resembles a wind vane with a propeller at one end. Used to indicate wind speed and direction.

Air density Mass per unit volume of air; about 1.275 km per cubic meter at 0ºC and 1000 millibars.

Air mass A large expanse of air having similar temperature and humidity at any given height.

Air pressure The cumulative force exerted on any surface by the molecules composing air.

Albedo The percent of radiation returning from a surface compared to that which strikes it.

Altimeter An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.

Altocumulus A middle cloud, usually white or gray. Often occurs in layers or patches with wavy, rounded masses or rolls.

Altocumulus castellanus An altocumulus showing vertical development, individual cloud elements have towerlike tops, often in the shape of tiny castles.

Altocumulus lenticularis A lens-shaped altocumulus cloud; a mountain-wave cloud generated by the disturbance of horizontal airflow caused by a prominent mountain range.

Altostratus A middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sun or moon usually appears dimly visible.

Ambient air The air surrounding a cloud, or the air surrounding rising or sinking air parcels.

Ambient temperature Temperature of the surrounding (ambient) air.

Anemometer An instrument designed to measure wind speed.

Aneroid barometer An instrument designed to measure atmospheric pressure. It contains no liquid.

Annual range of temperature The difference between the warmest and coldest months at any given location.

Anomalies Departures of temperature, precipitation, or other weather elements from long-term averages.

Arctic air A very cold and dry air mass that forms primarily in winter and the northern interior of North America.

Atmospheric window A region of the electromagnetic spectrum from 8 to 12 µm where the atmosphere is transparent to radiation.

Autumnal equinox The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23.

Barograph A recording instrument that provides a continuous trace of air pressure variation with time.

Barometer An instrument that measures atmospheric pressure. The two most common barometers are the mercury barometer and the aneroid barometer.

Beaufort scale A scale of wind strength based on visual assessment of the effects of wind on seas and vegetation.

Black body A hypothetical object that absorbs all of the radiation that strikes it. It also emits radiation at a maximum rate for its given temperature.

Blizzard A severe weather condition characterized by low temperatures and strong winds (greater than 32 mi/hr) bearing a great amount of snow. When these conditions continue after the falling snow has ended, it is termed a ground blizzard.

Bora A cold katabatic wind that originates in Yugoslavia and flows onto the coastal plain of the Adriatic Sea.

Bowen ratio The ratio of energy available for sensible heating to energy available for latent heating.

Boyle's law When the temperature is held constant, the pressure and density of an ideal gas are directly proportional.

Ceilometer An instrument that automatically records cloud height.

Centrifugal force A force directed outward, away from the center of a rotating object; equal in magnitude to the centripetal force but in the opposite direction.

Centripetal force An inward-directed force that confines an object to a circular path; equal in magnitude to the centrifugal force but in the opposite direction.

Charles's law With constant pressure, the temperature of an ideal gas is inversely proportional to the density of the gas.

Chinook A warm, dry wind on the eastern side of the Rocky Mountains. In the Alps, the wind is called a Foehn.

Cirrocumulus A high cloud that appears as a white patch of cloud without shadows. It consists of very small elements in the form of grains or ripples.

Cirrostratus A high cloud appearing as a whitish veil that may totally cover the sky. Often produces halo phenomena.

Cirrus A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.

Climate The accumulation of daily and seasonal weather events over a long period of time. A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Cloud base The lowest portion of a cloud.

Cloudburst Any sudden and heavy rain shower.

Cloud cover The amount of the sky obscured by clouds when observed at a particular location.

Cloud deck The top of a cloud layer, usually viewed from an aircraft.

Cloud seeding The introduction of artificial substances (usually silver iodide or dry ice) into a cloud for the purpose of either modifying its development or increasing its precipitation.

Coalescence The merging of cloud droplets into a single larger droplet.

Cold fog See Supercooled cloud.

Cold front The leading edge of a cold air mass.

Condensation Process by which water changes phase from a vapor to a liquid.

Condensation nuclei Small particles in the atmosphere that serve as the core of tiny condensing cloud droplets. These may be dust, salt, or other material.

Conduction The transfer of heat by molecular activity from one substance to another, or through a substance. Transfer is always from warmer to colder regions.

Continental air mass An air mass that forms over land; it is normally relatively dry.

Continental Climate A climate lacking marine influence and characterized by more extreme temperatures than in marine climates: therefore, it has a relatively high annual temperature range for its latitude.

Continental polar air Relatively dry air mass that develops over the northern interior of North America; very cold in winter and mild in summer.

Continental tropical air Warm, dry air mass that forms over the subtropical deserts of the south-western United States.

Contrail (condensation trail) A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air.

Convection Motions in a fluid that result in the transport and mixing of the fluid's properties. In meteorology, convection usually refers to atmospheric motions that are predominantly vertical, such as rising air currents due to surface heating. The rising of heated surface air and the sinking of cooler air aloft is often called free convection. (Compare with forced convection.)

Convective condensation level (CCL) The level above the surface marking the base of a cumiliform cloud that is forming due to surface heating and rising thermals.

Convergence An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region.

Cooling degree-day A form of degree-day used in estimating the amount of energy necessary to reduce the effective temperature of warm air. A cooling degree-day is a day on which the average temperature is one degree above a desired base temperature.

Coriolis effect A deflective force arising from the rotation of the earth on its axis; affects principally synoptic-scale and global-scale winds. Winds are deflected to the right of the initial direction in the Northern Hemisphere, and to the left in the Southern Hemisphere.

Crepuscular rays Alternating light and dark bands of light that appear to fan out from the sun's position, usually at twilight.

Cumulonimbus An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail. It is also known as a thunderstorm cloud.

Cumulus A cloud in the form of individual, detached domes or towers that are usually dense and well defined. It has a flat base with a bulging upper part that often resembles cauliflower. Cumulus clouds of fair weather are called cumulus humilis. Those that exhibit much vertical growth are called cumulus congestur or towering cumulus.

Cumulus Congestus An upward building convective cloud with vertical development between that of a cumulus cloud and a cumulonimbus.

Cup anemometer An instrument used to monitor wind-speed. Wind rotation of cups generates and electric current calibrated in wind speed.

Cutoff high Anticyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Cutoff low Cyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Daily range of temperature The difference between the maximum and minimum temperatures for any given day.

Degree days Computed from each day's mean temperature (max+min/2). For each degree that a day's mean temperature is below or above a reference temperature is counted as one degree day.

Density The ratio of the mass of a substance to the volume occupied by it.

Deposition A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first. (Also called sublimation in meteorology.)

Deposition nuclei Tiny particles in the atmosphere that serve as the core of tiny ice crystals as water vapor changes to the solid form. These are also called ice nuclei.

Desert One of two types of dry climate-the driest of the dry climates.

Dew Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.

Dew point (dew-point temperature) The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur. When the dew point falls below freezing it is called the frost point.

Diffraction The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.

Diffuse insolation Solar radiation that is scattered or reflected by atmospheric components (clouds, for example) to the earth's surface.

Direct insolation Solar radiation that is transmitted directly through the atmosphere to the earth's surface without interacting with atmospheric components.

Divergence An atmospheric condition that exists when the winds cause a horizontal net outflow of air from a specific region.

Downbursts A severe localized downdraft that can be experienced beneath a severe thunderstorm. (Compare Microburst)

Downdraft Downward moving air, usually within a thunderstorm cell.

Drainage basin A fixed geographical region from which a river and its tributaries drain water.

Drizzle Small drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain.

Drought A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.

Dry adiabatic rate The rate of change of temperature in a rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is 10ºC per 1000 m (5.5ºF per 1000 ft).

Dry climate A climate in which yearly precipitation is not as great as the potential loss of water by evaporation.

Dust devil (or whirlwind) A small but rapidly rotating wind made visible by the dust, sand, and debris it picks up from the surface. It develops best on clear, dry, hot afternoons.

Eddy A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Effective emissivity A correction factor, dependent on the radiational characteristics of the earth -atmosphere system, that permits application of black body radiation laws to the earth-atmosphere system

Emissivity The fractional amount of radiation emitted by a given object or substance in comparison to the amount emitted by a perfect emitter.

Emittance The rate at which a black body radiates energy across all wave-lengths.

Entrainment The mixing of environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud.

Environmental lapse rate The rate of decrease of temperature with elevation. It is most often measured with a radiosonde.

Equilibrium vapor pressure The necessary vapor pressure around liquid water that allows the water to remain in equilibrium with its environment. Also called saturation vapor pressure.

Equinox The time when the sun crosses the plane of the earth's equator occurring about March 21 and September 22.

Evaporation The process by which a liquid changes into a gas.

Evapotranspiration Vaporization of water through direct evaporation from wet surfaces and the release of water vapor by vegetation.

Evaporation fog Fog produced when sufficient water vapor is added to the air by evaporation. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass.

Exosphere The outermost portion of the atmosphere.

Fall Freeze date The date of occurrence in the fall of the first minimum at or below a temperature threshold.

Fall streaks Falling ice crystals that evaporate before reaching the ground.

Foehn See Chinook.

Fog A cloud with its base at the earth's surface. It reduces visibility to below 1 km.

Forced convection On a small scale, a form of mechanical stirring taking place when twisting eddies of air are able to mix.

Free convection Convection triggered by intense solar heating of the earth's surface.

Freeze A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost.

Freeze free season The number of days between the last spring freeze date and the first fall freeze date.

Freezing rain and freezing drizzle Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called glaze.

Front The transition zone between two distinct air masses.

Frontal fog See Evaporation fog.

Frost (also called hoarfrost) A covering of ice produced by deposition (sublimation) on exposed surfaces when the air temperature falls below the frost point (the dew point is below freezing).

Frost point See Dew point.

Frozen dew The transformation of liquid dew into tiny beads of ice when the air temperature drops below freezing.

Funnel cloud A rotating conelike cloud that extends down-ward from the base of a thunderstorm. When it reaches the surface it is called a tornado.

Geostrophic wind A theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.

Glaciation The conversion of all the supercooled liquid water in a cloud into ice crystals, thus reducing the growth rate of ice crystals and hail.

Glaciated cloud A cloud or portion of a cloud where only ice crystals exist.

Glaze A coating of ice on objects formed when supercooled rain freezes on contact. A storm that produces glaze is called an icing storm.

Glory Colored rings that appear around the shadow of an object.

Graupel See Snow pellets

Green flash A small, green color that occasionally appears on the upper part of the sun as it rises or sets.

Ground fog See Radiation fog.

Growing degree-day A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates.

Growing season The number of days between the last spring freeze date and the first fall freeze date.

Haboob A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.

Hail Solid precipitation in the form of chunks or balls of ice with diameters greater than 5 mm. The stones fall from cumulonimbus clouds.

Hailstones Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.

Hair hygrometer An instrument used to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.

Halos Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Haze Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

Heat A form of energy transferred between systems by virtue of their temperature differences.

Heat capacity The ratio of the heat absorbed (or released) by a system to the corresponding temperature rise (or fall).

Heat index (HI) An index that combines air temperature and relative humidity to determine an apparent temperature-how hot it actually feels.

Heat of fusion Heat released when water changes phase from liquid to solid; 80 calories per gram

Heat of melting Heat required to change the phase of water from solid to liquid; 80 calories per gram.

Heating degree-day A form of the degree-day used as an index for fuel consumption. Needed on days when average air temperature falls below 69 ºF (18 ºC); computed by subtracting the day's average temperature from 65 ºF.

Heat lightning Distant lightning that illuminates the sky but is too far away for its thunder to be heard.

Heiligenschein A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.

Heterosphere The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.

High inversion fog A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland climate Complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Hoarfrost Fernlike crystals of ice that form by deposition of water vapor on twigs, tree branches, and other vegetation.

Homosphere The atmosphere up to 80 km (50 mi) in which the proportionality of principal gaseous constituents, such as oxygen and nitrogen, is constant.

Humid continental climate A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40 and 50º north latitude. This climate is not found in the southern hemisphere, where the middle latitudes are dominated by the oceans.

Humid Subtropical Climate A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

Hurricane A severe tropical cyclone having winds in excess of 64 knots (74 mi/hr).

Hydrograph An instrument that provides a continuous trace of relative humidity with time.

Hygrometer An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

Hypothermia The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.

Ice Cap Climate A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice fog A type of fog composed of tiny suspended ice particles that forms at very low temperatures.

Ice nuclei Particles that act as nuclei for the formation of ice crystals in the atmosphere.

Ice pellets See Sleet

Indian summer An unseasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather.

Infrared radiation Electromagnetic radiation with wavelengths between about 0.7 and 1000 µm. This radiation is longer than visible radiation but shorter than microwave radiation.

Insolation The incoming solar radiation that reaches the earth and the atmosphere.

Intertropical convergence zone (ITCZ) The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.

Inversion An increase in air temperature with height.

Ion An electrically charged atom, molecule, or particle.

Ionosphere An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist.

Iridescence Brilliant spots or borders of colors, most often red and green, observed in clouds up to about 30º from the sun.

Isobar A line connecting points of equal pressure

Isotach A line connecting points of equal wind speed.

Isotherm A line connecting points of equal wind temperature.

January thaw A period of relatively mild weather around January 20 to 23 that occurs primarily in New England; an example of a singularity in the climatic record.

Jet stream Relatively strong winds concentrated within a narrow band in the atmosphere.

Katabatic wind Any wind blowing downslope. Usually cold.

Lake breeze A wind blowing onshore from the surface of a lake.

Lake-effect snows Localized snowstorms that form on the downwind side of a lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water.

Land breeze A coastal breeze that blows from land to sea, usually at night.

Lapse rate The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental lapse rate.)

Latent heat The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.

Lenticular cloud A cloud in the shape of a lens.

Lightning A visible electrical discharge produced by thunderstorms.

Longwave radiation A term most often used to describe the infrared energy emitted by the earth and the atmosphere.

Magnetosphere The region around the earth in which the earth's magnetic field plays a dominant part in controlling the physical processes that take place.

Mammatus clouds Clouds that look like pouches hanging from the underside of a cloud.

Marine climate A climate dominated by the ocean, because of the moderating effect of water, sites having this climate are considered relatively mild.

Maritime air mass An air mass that originates over the ocean. These air masses are relatively humid.

Maritime polar air Cool, humid air mass that forms over the cold ocean waters of the North Pacific and North Atlantic.

Maritime tropical air Warm, humid air mass that forms over tropical and subtropical oceans.

Mean annual temperature The average temperature at any given location for the entire year.

Mesoscale The scale of meteorological phenomena that ranges in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes.

Mesosphere The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 km above the earth's surface.

Meteorology The study of the atmosphere and atmospheric phenomena as well as the atmosphere's interaction with the earth's surface, oceans, and life in general.

Microburst A strong localized downdraft less than 4 km wide that occurs beneath severe thunderstorms. A strong downdraft greater than 4 km across is called a downburst.

Microclimate The climate structure of the air space near the surface of the earth.

Microscale The smallest scale of atmospheric motions.

Millibar(mb) A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 mb.

Mirage A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher than it actually is, it is called a superior image. When an object appears lower than it actually is, it is an inferior mirage.

Mist Very thin fog in which visibility is greater than 1.0 km (0.62 mi).

Mistral A katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Mixing ratio The ratio of the mass of water vapor in a given volume of air to the mass of dry air.

Moist adiabatic rate The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6ºC per 1000 m (3.3ºF per 1000 ft) is used.

Molecular viscosity The small-scale internal fluid friction that is due to the random motion of the molecules within a smooth-flowing fluid, such as air.

Mountain and valley breeze A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.

Nacreous clouds Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the earth's surface. They are also called mother-of-pearl clouds.

Nimbostratus A dark, gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning, thunder, or hail.

Noctilucent clouds Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.

Nocturnal inversion See Radiation inversion.

Offshore breeze A breeze that blows from the land out over the water. Opposite of an onshore breeze.

Onshore breeze A breeze that blows from the water onto the land. Opposite of an offshore breeze.

Orographic uplift The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds.

Orographic precipitation Rainfall or snowfall from clouds, induced by topographic uplift.

Permafrost A layer of soil beneath the earth's surface that remains frozen throughout the year.

Photodissociation The splitting of a molecule by a photon.

Photon A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light.

Pileus cloud A smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud.

Polar air mass A cold air mass that forms in a high-latitude source region.

Polar climates Climates in which the mean temperature of the warmest month is below 10ºC; climates that are too cold to support the growth of trees.

Potential energy The energy that a body possesses by virtue of its position with respect to other bodies in the field of gravity.

Potential evapotranspiration (PE) The amount of moisture that, if it were available, would be removed from a given land area by evaporation and transpiration.

Potential temperature The temperature that a parcel of dry air would have if it were brought dry adiabatically from its original position to a pressure of 1000 mb.

Precipitable water vapor The depth of water that would result if all the vapor in the atmosphere above a location were condensed into liquid water.

Precipitation Any form of water particles-liquid or solid-that falls from the atmosphere and reaches the ground.

Prevailing wind The wind direction most frequently observed during a given period.

Probability forecast A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather conditions.

Psychrometer An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

Pyranometer An instrument that measures the amount of radiation.

Q R

Radar An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by such reflecting objects as raindrops within clouds.

Radiant energy (radiation) Energy propagated in the form of electromagnetic waves. These waves do not need molecules to propagate them, and in a vacuum they travel at nearly 300,000 km per sec.

Radiation fog Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as ground fog and valley fog.

Radiation inversion An increase in temperature with height due to radiational cooling of the earth's surface. Also called a nocturnal inversion.

Radiosonde A balloon-borne instrument that measures and transmits pressure, temperature, and humidity to a ground-based receiving station.

Rain Precipitation in the form of liquid water drops that have diameters greater than that of drizzle.

Rain gage A device-usually a cylindrical container-for measuring rain-fall.

Rain Shadow The region on the leeside of a mountain where the precipitation is noticeable less than on the windward side.

Rawinsonde An instrument carried by weather balloons to measure the temperature, humidity, pressure, and winds of the atmosphere.

Reflection The process whereby a surface turns back a portion of the radiation that strikes it.

Refraction The bending of light as it passes from one medium to another

Refractive index The ratio of the speed of light in a vacuum to its speed in a transparent medium.

Relative humidity The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at the particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure.

Rime ice A white, granular deposit of ice formed by the freezing of water drops when they come in contact with an object.

Santa Ana The local name given a foehn wind in southern California.

Saturation vapor pressure The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that the air can hold at any given temperature and pressure. (See Equilibrium vapor pressure.)

Scattering The process by which small particles in the atmosphere deflect radiation from its path into different directions.

Scintillation The apparent twinkling of a star due to its light passing through regions of differing air densities in the atmosphere.

Sea breeze A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is termed a sea breeze front.

Sea level pressure The atmospheric pressure at mean sea level.

Semiarid See Steppe.

Sensible heat transfer Movement of heat from one place to another as a consequence of conduction or convection or both.

Sensible temperature The sensation of temperature that the human body feels in contrast to the actual temperature of the environment as measured with a thermometer.

Shear See wind shear.

Sheet lightning A fairly bright lightning flash from distant thunderstorms that illuminates a portion of the cloud.

Shortwave radiation A term most often used to describe the radiant energy emitted from the sun, in the visible and near ultraviolet wavelengths.

Shower Intermittent precipitation from a cumuliform cloud, usually of short duration but often heavy.

Sleet A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets.

Smog Originally smog meant a mixture of smoke and fog. Today, smog means air that has restricted visibility due to pollution, or pollution formed in the presence of sunlight-photochemical smog.

Snow Solid precipitation in the form of minute ice flakes that occur below 0ºC.

Snowflake An aggregate of ice crystals that falls from a cloud

Snow flurries Light showers of snow that fall intermittently.

Snow grains Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.

Snow pellets White, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.

Snow rollers A cylindrical spiral of snow shaped somewhat like a child's muff and produced by the wind.

Snow squall (shower) An intermittent heavy shower of snow that greatly reduces visibility.

Solstice Either of the two times of the year when the sun is the greatest distance from the celestial equator, occurring about June 22 and December 22. See winter solstice and summer solstice.

Southern oscillation The reversal of surface air pressure at opposite ends of the tropical Pacific Ocean that occur during El Nino events.

Specific heat The ratio of the heat absorbed (or released) by the unit mass of the system to the corresponding temperature rise (or fall).

Specific humidity The ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.

Spontaneous nucleation (freezing) The freezing of pure water without the benefit of any nuclei.

Spring freeze date The date of occurrence in the spring of the last minimum at or below a temperature threshold.

Squall line Any nonfrontal line or band of active thunderstorms.

Station pressure The actual air pressure computed at the observing station.

Steam fog See Evaporation fog.

Steppe One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates. Steppe also refers to the short-grass vegetation associated with this semiarid climate.

Storm surge An abnormal rise of the sea along a shore. Primarily due to the winds of a storm, especially a hurricane.

Stratocumulus A low cloud, predominantly stratiform with low, lumpy, rounded masses, often with blue sky between them.

Stratopause The boundary between the stratosphere and the mesosphere.

Stratosphere The layer of the atmosphere above the troposphere and below the mesosphere (between 10 km and 50 km), generally characterized by an increase in temperature with height.

Stratus A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

Subarctic climate A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short cool summers. Places within this climatic realm experience the highest annual temperature ranges on earth.

Sublimation The process whereby ice changes directly into water vapor without melting. In meteorology, sublimation can also mean the transformation of water vapor into ice. (See Deposition.)

Subsidence The slow sinking of air, usually associated wit high-pressure areas.

Subsidence inversion A temperature inversion produced by the adiabatic warming of a layer of sinking air.

Summer solstice Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23.5º N, the Tropic of Cancer.

Sundog A colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. Also called parhelion.

Sun pillar A vertical streak of light extending above (or below) the sun. It is produced by the reflection of sunlight of ice crystals.

Supersaturated air A condition that occurs in the atmosphere when the relative humidity is greater that 100 percent.

Surface inversion See Radiation inversion

Synoptic scale The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning a continent. Also called the cyclonic scale.

Taiga The northern coniferous forest; also a name applied to the subarctic climate.

Temperature The degree of hotness or coldness of a substance as measured by a thermometer. It is also a measure of the average speed or kinetic energy of the atoms and molecules in a substance.

Temperature inversion An extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the troposphere.

Terminal velocity The constant speed obtained by a falling object when the upward drag on the object balances the downward force of gravity.

Thermal A small, rising parcel of warm air produced when the earth's surface is heated unevenly.

Thermograph A recording instrument that gives a continuous trace of temperature with time.

Thermometer An instrument used to measure temperature.

Thermosphere The atmospheric layer above the mesosphere. It extends from 90 km to outer space.

Thunder The sound due to rapidly expanding gases along the channel of a lightning discharge.

Tipping bucket rain gage A device that accumulates rainfall in increments of 0.01 in. by containers that alternately fill and empty (tip).

Tornado An intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope and touches the ground. (See Funnel cloud.)

Trade winds The winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low.

Transpiration The release of water vapor to the atmosphere by plants.

Tropical air mass A warm-to-hot air mass that forms in the subtropics.

Tropical depression A mass of thunderstorms and clouds generally with a cyclonic wind circulation of between 20 and 34 knots

Tropical disturbance An organized mass of thunderstorms with a slight cyclonic wind circulation of less than 20 knots.

Tropical storm Organized thunderstorms with a cyclonic wind circulation between 35 and 64 knots.

Tropopause The boundary between the troposphere and the stratosphere.

Troposphere The layer of the atmosphere extending from the earth's surface up to the tropopause (about 10 km above the ground).

Tundra Climate Found almost exclusively in the northern hemisphere or at high altitudes in many mountainous regions. A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by a long, bitterly cold winter.

Turbulence Any irregular or disturbed flow in the atmosphere that produces gusts and eddies.

Twilight The time immediately before sunrise and after sunset when the sky remain illuminated.

Typhoon A hurricane that forms in the western Pacific Ocean.

Ultraviolet radiation Electromagnetic radiation with wave-lengths longer than X-rays but shorter than visible light.

Upslope fog Fog formed as moist, stable air flows upward over a topographic barrier.

Upslope precipitation Precipitation that forms due to moist, stable air gradually rising along an elevated plain. Upslope precipitation is common over the western Great Plains, especially east of the Rock Mountains.

Upwelling The rising of water (usually cold) toward the surface from the deeper regions of a body of water.

Urban heat island The increased air temperatures in urban areas as contrasted to the cooler surrounding rural areas.

Valley breeze See Mountain breeze.

Valley fog See Radiation fog.

Vapor pressure The pressure exerted by the water vapor molecules in a given volume of air.

Vernal equinox The equinox at which the sun approaches the Northern Hemisphere and passes directly over the equator. Occurs around March 20.

Virga Precipitation that falls from a cloud but evaporates before reaching the ground. (See Fall streaks.)

Virtual temperature An adjustment applied to the real air temperature to account for a reduction in air density due to the presence of water vapor.

Viscosity The resistance of fluid flow.

Visibility The greatest distance an observer can see and identify prominent objects.

Visible light That portion of the electromagnetic spectrum from 0.4 to 0.7 µm wavelengths that is visible.

Vorticity A measure of the spin of a fluid, usually small air parcels. Absolute vorticity is the combined vorticity due to the earth's rotation and the vorticity due to the air's circulation relative to the earth. Relative vorticity is due to the curving of the air flow and wind shear.

Warm front The leading edge of a warm air mass.

Water balance The comparison of actual and potential evapotranspiration with the amount of precipitation, usually on a monthly basis.

Water budget Balance sheet for the inputs and outputs of water to and from the various global water reservoirs.

Water equivalent The depth of water that would result from the melting of a snow sample. Typically about 10 inches of snow will melt to 1 inch of water, producing a water equivalent of 10 to 1.

Weather The state of the atmosphere in terms of such variables as temperature, cloudiness, precipitation, and radiation.

Weighing bucket rain gage A device that is calibrated so that the weight of rainfall is recorded directly in terms of rainfall in millimeters or in inches.

Wet-bulb depression The difference in degrees between the air temperature (dry-bulb temperature) and the wet-bulb temperature.

Wet-bulb temperature The lowest temperature that can be obtained by evaporating water into the air.

White frost Ice crystals that form on surfaces instead of dew when the dew point is below freezing.

Wind chill equivalent temperature A theoretical air temperature at which the heat loss from exposed skin under calm conditions is equivalent to the heat loss at the actual air temperature and under the actual wind speeds.

Wind-chill factor The cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also called wind-chill index.

Wind shear A difference in wind speed or direction between two wind currents in the atmosphere.

Wind Vane An instrument used to determine wind direction.

Windsock A large, conical, open bag designed to indicate wind direction and relative speed; usually used at small airports.

Winter solstice Approximately December 22 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 23.5ºS, the Tropic of Capricorn.

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References:

C. Donald Ahrens, 1985. Meteorology Today. West Publishing Company, 497-509.

Frederick K. Lutgens, and Edward J. Tarbuck, 1995. The Atmosphere. Prentice-Hall Inc., 443-456.

Joe R. Eagleman, 1980. Meteorology The Atmosphere in Action. Litton Educational Publishing Inc., 353-364.

Joseph M. Moran, and Michael D. Morgan, 1986. Meteorology. Burgess Publishing, 486-496.

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