Meteorology

IINTRODUCTION

Tornado Watching

A meteorologist tracks a tornado as part of ongoing weather observations to better understand the earths atmosphere. Since the 19th century, scientific forecasting has greatly improved. Weather radar can detect and track tornados, hurricanes, and other severe storms.

Howard Bluestein/Science Source/Photo Researchers, Inc.

Meteorology,studyoftheearths atmosphere and especially the study of weather. A meteorologist is a person who studies the atmosphere. Meteorology is divided into a number of specialized sciences. Physical meteorology deals with the physical aspects of the atmosphere, such as the formation of clouds, rain, thunderstorms, and lightning. Physical meteorology also includes the study of visual events such as mirages, rainbows, and halos. The study of the winds and the laws that govern atmospheric motion is called dynamic meteorology. Synoptic meteorology is the study and analysis of large weather systems that exist for more than one day. Weather forecasting is part of synoptic meteorology. Agricultural meteorology deals with weather and its relationship to crops and vegetation. The study of atmospheric conditions over an area smaller than 1 sq km (0.4 sq mi) is called micrometeorology. Climate describes the average weather of a region. Climatology, a division of meteorology, is the study of a regions average daily and seasonal weather events over a long period.

IIPHYSICAL CHARACTERISTICS OF AIR

Inordertostudy,describe, and understand the events that occur within the atmosphere, meteorologists measure the physical characteristics of the air within which these events take place. Meteorologists describe the air primarily in terms of its composition, temperature, pressure, wind speed, wind direction, precipitation, and humidity.

AAir Composition

Theearthsatmosphere is a mixture of gases, mainly nitrogen (N2) and oxygen (O2), that are held to the earth by gravity. Near the earths surface, air is composed of about 78 percent nitrogen and about 21 percent oxygen. Small amounts of other gases, such as carbon dioxide (CO2), argon (Ar), methane (CH4), nitrous oxide (N2O), and water vapor (H2O), are also present. The concentration of the invisible water vapor varies from place to place and from time to time. Near the ground in warm tropical locations, the concentration of water vapor may reach 4 percent, while in polar areas its concentrations may be only a small fraction of a percent. Clouds are comprised of billions of small droplets of condensed water or tiny ice crystals.

BAir Temperature

Satellite Meteorology

Most weather services use satellite information in their forecasts. Pictures such as this one of Hurricane Gloria reveal patterns and movement that provide clues about what the storm might do next. The weather is continuously monitored and photographed by satellites worldwide, but it is still difficult to predict weather because of the many variables involved. Infrared sensors help meteorologists to interpret the pictures by determining the height and temperature of the clouds.

Phototake NYC

Airmoleculesareinconstant motion. The speed of air molecules corresponds to their kinetic energy, which in turn corresponds to the amount of heat energy in the air. Air temperature is a measure of the average speed at which air molecules are moving; high speeds correspond to higher temperatures. The temperature of a substance is measured by a thermometer.

CAir Pressure

Mercury Barometer

A mercury barometer is an accurate and relatively simple way to measure changes in atmospheric pressure. At sea level, the weight of the atmosphere forces mercury 760 mm (29.9 in) up a calibrated glass tube. Higher elevations yield lower readings because the atmosphere is less dense there, and the thinner air exerts less pressure on the mercury.

Microsoft Corporation. All Rights Reserved.

Airisheldtotheearth by gravity. This strong invisible force pulls the air downward, giving air molecules weight. The weight of the air molecules exerts a force upon the earth and everything on it. The amount of force exerted on a unit surface area (a surface that is one unit in length and one unit in width) is called atmospheric pressure or air pressure. The air pressure at any level in the atmosphere can be expressed as the total weight of air above a unit surface area at that level in the atmosphere. Higher in the atmosphere, there are fewer air molecules pressing down from above. Consequently, air pressure always decreases with increasing height above the ground. Because air can be compressed, the density of the air (the mass of the air molecules in a given volume) normally is greatest at the ground and decreases at higher altitudes.

Acolumnofair1sqcm (.16 sq in) in area, extending from the ocean surface (sea level) up to the top of the atmosphere would contain slightly more than 1 kg (about 2.35 lb) of air. If more air molecules are packed into the column, the total weight of air at the bottom of the column would increase, and the air pressure there would increase. If air is removed from the column, the total weight of the air at the bottom of the column would decrease, and the air pressure would decrease. The most common unit of pressure found on surface weather maps is the millibar (1 millibar equals 100 newtons/sq m, where newtons are the metric unit of force). Inches of mercury is a pressure unit commonly used in television and radio weather broadcasts. On average, at sea level, the standard value of the atmospheric pressure is 1013.25 millibars, 29.92 inches of mercury, and 14.7 lbs/sq in. Barometers are instruments that measure air pressure.

DWind

Windisairinmotion. It is caused by horizontal variations in air pressure. The greater the difference in air pressure between any two places at the same altitude, the stronger the wind will be. The wind direction is the direction from which the wind is blowing. A north wind blows from the north and a south wind blows from the south. The prevailing wind is the wind direction most often observed during a given time period. Wind speed is the rate at which the air moves past a stationary object.

Avarietyofinstruments measure wind. A wind vane measures wind direction. Most wind vanes consist of a long arrow with a tail that moves freely on a vertical shaft. The arrow points into the wind and gives the wind direction. Anemometers measure wind speed. Most anemometers consist of three or more cups that spin horizontally on a vertical post. The rate at which the cups rotate is related to the speed of the wind.

EPrecipitation

Precipitationisanyform of water (either liquid or solid) that falls from the atmosphere and reaches the ground, such as rain, snow, or hail. Rain gauges are instruments that measure rainfall. The standard rain gauge consists of a funnel-shaped collector that is attached to a long measuring tube.

FHumidity

Humidityreferstothe airs water vapor content. Hygrometers are instruments that measures humidity. The maximum amount of water vapor that the air can hold depends on the air temperature; warm air is capable of holding more water vapor than cold air. Relative humidity is the ratio of the amount of water vapor in the air compared to the maximum amount of water vapor that the air could hold at that particular temperature. When the air is holding all of the moisture possible at a particular temperature, the air is said to be saturated. Relative humidity and dew-point temperature (the temperature to which air would have to be cooled for saturation to occur) are often obtained with a device called a psychrometer. The most common type of psychrometer is a sling psychrometer. This instrument consists of two thermometers mounted side by side and attached to a handle that allows the thermometers to be whirled. A cloth wick covers one thermometer bulb. The wick-covered thermometer bulb (called the wet bulb) is dipped in water, while the other thermometer bulb (the dry bulb) is kept dry. Whirling both thermometers allows water to evaporate from the wick, which cools the wet bulb. By looking up the dry and wet bulb temperatures in a set of tables, known as humidity tables, it is possible to find the corresponding relative humidity and dew-point temperature.

IIISPECIAL METEOROLOGICAL INSTRUMENTS

Meteorologistshavedeveloped several sophisticated instruments that measure multiple physical characteristics of the air simultaneously and at more than one location. The most important of these special instruments are radiosondes, Doppler radar, and weather satellites.

ARadiosonde

Weather Balloon

Preparing for a launch in Antarctica, scientists plan to gather weather information from a radiosonde transmitter attached to an expandable balloon. The radiosonde will continuously measure temperature, humidity, and pressure, as well as signal its location. The balloon is filled with a light gas such as helium. As the balloon rises, the gas will expand. At some point, the balloon will burst and a parachute will bring the weather instrument back down to earth.

Robert Harding Picture Library

Aradiosondemeasures air temperature, air pressure, and humidity from the earths surface up to an altitude of about 30,000 m (about 100,000 ft). The radiosonde consists of a small box attached to a gas-filled balloon. As the balloon rises, a barometer measures air pressure, a thermometer measures temperature, and a hygrometer measures humidity. All of this information is transmitted by radio back to the ground. Special tracking equipment monitors the movement of the radiosonde, and this tracking information is then converted into wind speed and wind direction. When the balloon bursts, the radiosonde descends to the earth by parachute.

BDoppler Radar

Doppler Radar Image

Doppler radar measures the speed and direction of the movement of clouds, in addition to cloud density. In this image of a thunderstorm over Oklahoma, Doppler radar shows a mesocyclone, a rotating mass of air that may signal that the formation of a tornado is imminent.

Phil Degginger/Bruce Coleman, Inc.

Radarprovidesmeteorologists with information about precipitation and storms. A radar unit sends out a pulse of microwaves. When the microwaves strike objects, such as falling precipitation, some of the microwaves are reflected back to the radar unit, where they are detected by an antenna and displayed on a screen. The elapsed time between transmission and return indicates how far away the precipitation is. Doppler radar can determine wind speed by measuring the speed at which precipitation is moving horizontally toward or away from the radar antenna. It does this by measuring the change in frequency of the returning microwavesthe frequency of the returning waves decreases if the rain is moving away from the radar unit and increases if the rain is moving toward it. This change in frequency is called the Doppler effect. Meteorologists also use Doppler radar to peer into severe thunderstorms and locate tornadoes. Presently, there is a network of 135 Doppler radar units at selected sites within the continental United States.

CWeather Satellites

GOES Weather Satellite

Broadcasters use data from meteorological satellites to predict weather and to broadcast storm warnings when necessary. Satellites such as the Geostationary Operational Environmental Satellite (GOES) collect meteorological and infrared information about the atmosphere and the ocean. A camera on the GOES is continuously pointed at Earth, broadcasting satellite images of cloud patterns both day and night. Here, the GOES-C satellite is being encapsulated inside its payload fairing aboard a Delta rocket.

NASA

Aweathersatelliteis a cloud-observing platform in space. Satellites provide cloud observations day and night over vast regions. There are two main types of weather satellites: geostationary satellites and polar orbiting satellites. Geostationary satellites orbit the earth at the same rate that the earth spins. Hence, they remain about 36,000 km (about 22,000 mi) above a fixed spot on the equator and constantly monitor a specific region below them. Successive cloud photographs from geostationary satellites provide meteorologists with valuable information about the development, movement, and dissipation of weather fronts, storms, and clouds. Polar orbiting satellites, situated about 850 km (about 530 mi) above the earths surface, pass over the North and South poles on each orbit photographing the clouds directly beneath them. Because the earth rotates beneath the satellite, each orbit enables the satellite to monitor an area that is west of its previous pass. Thus, the satellite photographs the entire surface of the earth every 12 hours. Since polar orbiting satellites observe clouds at a much lower altitude than geostationary satellites, they provide more photographic detail of cloud systems.

IVSTRUCTURE OF THE ATMOSPHERE

Divisions of the Atmosphere

The atmosphere can be divided into a series of layers, based on the air temperature at different altitudes. In two of the layers, the troposphere and the mesosphere, air gets colder as altitude increases. In the other two layers, the stratosphere and the thermosphere, air gets warmer as altitude increases. This second pattern is called a temperature inversion, and it hinders the mixing of air that accompanies vertical air currents. Consequently, air in the troposphere is well mixed, but air in the stratosphere is not.

Microsoft Corporation. All Rights Reserved.

Bystudyingtheatmosphere, meteorologists have discovered that it can be divided into a series of layers. Based on a vertical profile of temperature, the layers consist of the troposphere, stratosphere, mesosphere, and thermosphere.

Thelowestlayer,the troposphere, is warmed by the earth. Sunlight warms the earths surface, and the surface warms the air. Therefore, the warmest air is next to the ground and air temperature normally decreases with height. This pattern of decreasing air temperature with altitude occurs usually up to an altitude of between about 8000 m (about 26,000 ft) at the poles and 16,000 m (about 52,000 ft) at the equator. This region of the lower atmosphere where air temperature normally decreases with height is called the troposphere. The troposphere is kept well stirred by rising and descending air currents.

Thelayerofatmosphere above the troposphere is called the stratosphere. In the stratosphere, air temperature begins to increase with height, mainly because ozone (a type of oxygen) in the stratosphere absorbs energy from the sun, principally ultraviolet radiation. Although the amount of ozone in the stratosphere is quite small, it is important because it protects living things on the earth by absorbing the suns harmful ultraviolet radiation. However, chemicals emitted near the earth, such as chlorofluorocarbons (CFCs), can be injected into the stratosphere by the updrafts in thunderstorms. In the stratosphere these chemicals can help to destroy ozone. Occasionally, concentrations of CFCs in the stratosphere are high enough to destroy nearly all of the ozone over large regions, producing a hole in the ozone layer. In recent years, such holes have occurred every spring over Antarctica. Many regions in the temperate latitudes have occasionally experienced a significant thinning of the ozone layer.

Abovetheozone-richstratosphere lies the mesosphere, where air temperature, again, decreases with height. The mesosphere is the coldest layer of the atmosphere and extends from an altitude of about 50 km to about 85 km (about 30 mi to 50 mi). Above the mesosphere lies the hot thermosphere, where air temperatures can exceed 1000 C (1800 F), primarily due to oxygen absorbing the suns energetic rays.

VENERGY FLOW AND GLOBAL CIRCULATION

Tropical Weather Cell

The intertropical convergence zone (ITCZ) is a dynamic weather area located near the equator. Sunlight at the equator warms the air, making it rise and thereby creating low pressure at the equator, along with rain-producing cumulonimbus clouds. The rising air eventually cools and sinks back to the surface at latitudes roughly 30 north and south of the equator. The sinking cool air creates high-pressure systems that keep those latitudes relatively dry. The sinking air is drawn back to the low pressure near the equator, creating a flow of air known as the trade winds. Eventually, the trade winds converge near the equator and create the ITCZ.

Microsoft Corporation. All Rights Reserved.

Theaverageamountof energy the earth absorbs from the sun each year is equal to the average amount of energy the earth loses to space. This energy balance, however, is not maintained for each latitude. Annually, tropical regions gain more energy from the sun than they lose, while polar regions lose more energy to space than they gain. The tropics do not continuously grow warmer and the polar regions do not continuously grow colder, however, because the atmosphere transports warm air toward the poles and cold air toward the equator. The oceans do the same with water. The wind pattern generated by the unequal heating of the earths surface produces the major wind belts over the globe. This average wind flow is called the general circulation of the atmosphere.

Theaveragewindflow of the general circulation is much less complex than the actual global circulation at any given instant. Winds tend to develop into rotating eddies, called high and low pressure areas, that move across the middle latitudes and complicate the wind flow pattern.

Modelsmakesimplifying assumptions about the earth, its atmosphere, and its winds, and meteorologists have developed a simple model, called the three-cell model, to describe the average wind flow of the general circulation. The three-cell model assumes that the earth is covered with water and that the sun is always over the equator. With these assumptions, the model makes the following predictions, each of which matches the average surface wind patterns observed, but not the winds aloft.

Inthetropics,theintense sunlight heats the surface, which warms the air, causing it to rise. This reduces the air pressure at the surface, forming a broad region of low pressure. As the warm, humid air rises, it often condenses into huge thunderstorms that provide the tropics with ample rainfall.

World Precipitation and Average Rainfall

Different regions of the earth receive widely differing amounts of rainfall. A band of high precipitation runs along the equator, while bands of low precipitation occur in each of the polar regions and on each side of the tropics, around 30 degrees latitude.

Microsoft Corporation. All Rights Reserved.

Nearthetopofthetroposphere, the rising air branches and moves toward the North and South poles. As the air aloft moves toward the poles, it gradually cools. At the same time, the air slowly squeezes together and becomes more dense.

Near30latitude,the air aloft becomes dense enough to produce high-pressure areas at the surface, called the subtropical highs. As the surface air moves outward from the surface highs, the air aloft sinks to replace it and warms by compression, which tends to evaporate any clouds in it. Cloudless skies and little rainfall characterize the region around 30 latitude. Many of the worlds deserts are found near this latitude.

Atthesurface,someof the sinking air moves back toward the lower pressure at the equator. This flow of air toward the equator is known as the trade winds. Due to the Coriolis force, a force that results from the rotation of the earth, the trade winds are deflected to the west. In the northern hemisphere, the trade winds blow from the northeast, and in the southern hemisphere, they blow from the southeast. The trade winds complete a thermally driven convection cell that begins with the sun warming the tropics, air rising above the equator, flowing toward the poles, then sinking near 30 latitude and returning to the equator. At the equator, the trade winds from the northern hemisphere meet the trade winds from the southern hemisphere forming a boundary called the intertropical convergence zone (ITCZ).

Prevailing Winds

The prevailing winds include the doldrums near the equator, the northeast (or southeast) trade winds somewhat above (or below) the equator, the prevailing westerlies near the middle latitudes, and the polar easterlies near the poles. The winds are named with respect to the direction from which they blow. For example, the northeast trade wind blows in the northern hemisphere from the east toward the west.

Microsoft Corporation. All Rights Reserved.

Anothercelloccursin the polar latitudes. At the poles, cold air sinks into an area of surface high pressure. As the cold surface air flows toward the equator, it meets milder middle latitude air near 50 to 60 latitude. Here, the converging air rises for the return trip to the poles. The rising air also cools and often condenses into clouds. Hence, plentiful rainfall characterizes the region between 50 and 60 latitude. In this region, where the rising air moves toward the poles, regions of surface low pressure often form.

Thethirdcellofthe three-cell model occupies the mid-latitudes between the other two cells. Some of the rising air between 50 and 60 latitude begins the journey aloft back toward the equator. At about 30 latitude, this air begins to sink in the vicinity of the subtropical highs. The surface winds tend to blow from the high-pressure region at about 30 latitude toward the low-pressure region between 50 and 60 latitude. The Coriolis force deflects these surface winds, producing the prevailing westerlies of the middle latitudes.

Meanwhile,polewardof the prevailing westerlies, cold, polar air moves toward the equator, and the Coriolis force deflects this air, producing a polar wind belt called the polar easterlies. Near 50 to 60 latitude, the polar easterlies meet the prevailing westerlies. Here the winds are blowing in opposite directions along a boundary called the polar front. It is along the polar front that middle latitude storms often develop.

Thethree-cellmodelof the general circulation assumes that the sun is always above the equator. In the real world, the zone of maximum surface heating shifts seasonally. Because the sun is overhead in the northern hemisphere in July and overhead in the southern hemisphere in January, the major surface wind belts and pressure systems shift northward in July and southward in January.

VICLOUDS

Clouds

Microsoft Corporation. All Rights Reserved.

Cloudsaremadeupof tiny water droplets or ice crystals. Clouds form as water vapor either condenses or freezes onto minute floating particles such as dust or tiny salt particles from the sea. These cloud droplets and ice crystals are so small (with an average diameter of 0.002 cm / 0.001 in) that they stay suspended in the air. Raindrops, which typically have a million times more water in them, are heavy enough to fall from clouds.

Asystemofidentifying clouds was proposed by French botanist and zoologist Jean-Baptiste Lamarck in 1802, and a better system was proposed by English naturalist Luke Howard in 1803. With slight modification, Howards system is still in use. Howards system uses Latin words to describe clouds as they appear to an observer on the ground. High wispy clouds are called cirrus (from the Latin word for curl of hair); sheetlike clouds are called stratus (from the Latin word for layer); billowing, puffy clouds are called cumulus (from the Latin word for heap); and rain-producing clouds are called nimbus (from the Latin word for rain).

Cloudsaredividedinto four main groups based on their height above the ground: high clouds, middle clouds, low clouds, and clouds with vertical development. High clouds have bases generally above 6000 m (20,000 ft). Because at high altitudes the air is thin and cold, high clouds are thin, and their names often include the prefix cirro (from cirrus). They are almost entirely composed of ice crystals. Middle clouds, which have names with a prefix of alto, (from the Latin word for high) typically have bases between 2000 m and 6000 m (between 6500 ft and 20,000 ft) above the ground. They are usually composed of a mixture of water droplets and ice crystals. Low clouds have bases lying below 2000 m (6500 ft). Low clouds are almost always composed of water droplets. Clouds of vertical development are taller than they are wide. Their bases are below 2000 m (6500 ft) while their tops may extend above the top of the troposphere.

Meteorologistsdivide these four main groups of clouds into ten principal cloud types. The high cloud group consists of cirrus clouds, which are thin and wispy; cirrostratus clouds, which are thin and sheetlike; and cirrocumulus clouds, which are small, white, and puffy. The middle cloud group consists of altostratus clouds, which are gray and sheetlike; and altocumulus clouds, which are gray and puffy. Low clouds consist of stratus clouds, which are low, gray, and sheetlike; nimbostratus clouds, which are sheetlike and dark gray from which rain or snow is falling; and stratocumulus clouds, which are dark, low, and lumpy. Clouds of vertical development consist of cumulus clouds, which are small and puffy; and the giant cumulonimbus clouds, which are thunderstorm clouds with a top that may extend more than 15,000 m (50,000 ft) above the ground.

VIIPRECIPITATION

Thunder and Lightning

Thunderstorms are violent rain storms that produce thunder, lightning, and frequently hail. They form when violent updrafts push hot, humid air high into the cold, upper atmosphere. The subsequent rapid cooling of the air produces an intense, but short-lived, storm.

BBC Worldwide Americas, Inc./Hot Shots Cool Cuts Inc.

Precipitationisanyform of water, either solid or liquid, that falls from the atmosphere and reaches the ground. There are several types of precipitation, including rain, snow, sleet, and hail. One of the important triumphs of 20th-century meteorology was discovering how precipitation forms in clouds.

AFormation of Precipitation

Precipitationbeginsin a cloud when cloud droplets or ice crystals grow large and heavy enough to fall toward the ground. Cloud droplets may grow bigger as large droplets collide and merge with smaller drops. This process is called coalescence.

Icecrystalsgrowlarger through a process called the ice crystal process, or Bergeron process, after the Swedish meteorologist Tor Bergeron, who proposed that raindrops begin as ice crystals. If the temperatures inside a cloud are below freezing, then liquid cloud droplets and ice crystals may coexist. Liquid water droplets existing at below freezing temperatures are called supercooled droplets (see Supercooling). If supercooled droplets and ice crystals are close together, then water vapor may leave the liquid droplets and freeze onto the ice crystals. In this manner, the ice crystals grow larger at the expense of the surrounding supercooled droplets. As ice crystals grow larger by the Bergeron process, they may become heavy enough to fall. Falling ice crystals may collide and stick to other ice crystals, forming a snowflake. Ice crystals may also collide with supercooled cloud droplets, changing the liquid droplets into ice on contact. These ice particles may even stick together producing a chunk of icy matter called graupel.

BTypes of Precipitation

Precipitationcantake several different forms. Rain is falling drops of liquid water with diameters that are 0.5 mm (0.02 in) or greater. Drizzle is falling drops of water smaller than rain. Some raindrops are cloud droplets that grew by coalescence and fell. However, the majority of raindrops that fall over the middle and higher latitudes begin as snowflakes or graupel. As they fall, they enter warmer layers of air and melt, forming raindrops. If the falling rain evaporates before reaching the ground, it forms streaks in the sky called virga. In the cold air of winter, falling snowflakes and graupel may reach the ground without melting and accumulate as snow. Graupel that reaches the ground is called snow pellets. If rain falls into a deep, subfreezing layer of air near the ground, some of the rain may freeze into tiny ice pellets called sleet. When rain falls into a shallow, subfreezing layer of air near the ground, it may remain as a supercooled liquid and freeze upon striking a cold surface, forming freezing rain. Freezing rain can coat everything with glistening ice, the weight of which can break tree branches and snap power lines.

Hailisthelargestform of precipitation, varying in size from peas to golf balls or larger. Hail forms as graupel grows in size by colliding with and sticking to supercooled liquid droplets, all while suspended in violent updrafts in a thunderstorm. When the ice particles become large and heavy enough to overcome the updrafts, they begin to fall as hailstones. Hail damage in the United States alone amounts to hundreds of millions of dollars annually.

Dewandfrostarenot actually forms of precipitation because they do not fall from the atmosphere. Dew consists of tiny beads of water that form as water vapor condenses onto surfaces near the ground (such as blades of grass) when the surfaces temperature drops to below the airs dew-point temperature. When the dew-point temperature is below freezing, water vapor changes directly into ice without becoming a liquid first. The white, delicate ice crystals that form in this manner are called frost.

VIIILARGE-SCALE (SYNOPTIC) PHENOMENA

Weather Map, United States

This national weather map shows the conditions expected across the United States on a specific day and time. Forecasters compile weather maps after collecting data from a variety of sources, including radar maps and satellite images. As indicated by the coded information, this map calls for snow across most of the northern continental United States, rain in the upper-middle tier of the country, and progressively warmer temperatures to the south.

Microsoft Corporation. All Rights Reserved.

Large-scale,orsynoptic, weather phenomena are weather patterns that persist for more than a day and cover thousands of square kilometers. Air masses, large bodies of air of uniform temperature and humidity, may provide several days or weeks of persistent weather. Middle latitude cyclones and tropical cyclones are huge traveling storm systems that cover large areas and may persist for a week or more.

AAir Masses

Anairmassisabody of air that extends over a large area and has nearly uniform temperature and humidity in any horizontal direction. Places where air masses form are called source regions, and they are generally flat with light winds. Ideal source regions are those dominated by large high-pressure areas, such as the arctic plains in winter and the subtropical oceans and desert regions in summer.

Airmassesareclassified according to their source region. Polar air masses originate over cold regions. Tropical air masses originate over the warm tropics. Continental air masses originate over land, and maritime air masses originate over water. Consequently, a cold, dry air mass that forms over land is called a continental polar air mass and a hot, moist air mass that forms over water is called a maritime tropical air mass.

Generally,theupper-level winds move air masses from one region to another. For example, arctic air masses that form over northern Canada move into the United States when strong upper-level winds, called jet streams, direct these frigid masses of air southward.

BFronts

Afrontisaboundary where air masses with sharply contrasting temperature and humidity meet. Many kinds of storms occur along fronts.

Acoldfrontmarksthe boundary where cold air is replacing warm air. On a weather map, cold fronts are drawn as a solid blue line with triangles. The triangles point in the direction of movement. Typically, warmer, more humid air is found in advance of a cold front, while colder, drier air is behind it. Along the front, the warm, humid air often rises and condenses into towering cumulus clouds that may develop into thunderstorms. A narrow band of heavy precipitation, often in the form of rain showers, usually accompanies the front. As a cold front approaches, atmospheric pressure normally drops. As the cold front moves on by, atmospheric pressure rises and the winds shift direction. The passage of the front is often accompanied by the heaviest precipitation and the strongest and gustiest winds. Occasionally, however, a line of thunderstorms may develop, out ahead of a cold front. This line is called a squall line and it produces heavy rain and strong, gusty winds.

Awarmfrontmarksthe region where warm air is replacing cold air. On a weather map, warm fronts are drawn as a solid red line with half circles. The half circles point in the direction of movement. An average warm front has a more gentle slope than that of a typical cold front. As a warm front advances, warmer air glides up and over the colder, denser surface air. This process, called overrunning, produces widespread cloudiness and precipitation well in advance of the fronts surface position.

Warmfrontsarebestdeveloped in winter. As a typical warm front approaches, the atmospheric pressure drops and high, wispy cirrus clouds form 12 to 24 hours ahead of the front. These clouds give way to thicker and lower clouds (cirrostratus and altostratus). As the warm front moves closer, cloud level descends and steady rain, snow, sleet, or freezing rain may fall from nimbostratus clouds into the cold air ahead of the front. Just before the front passes, there may be low stratus clouds and fog. As the warm front passes, the air temperature and humidity rise, the atmospheric pressure stops falling, the winds shift, the rain ends, and the fog dissipates. However, these weather changes are less noticeable than those of a typical cold front.

Coldfrontsusuallymove faster than warm fronts. Consequently, when a cold front overtakes a warm front, a new front, called an occluded front, forms. Occluded fronts appear on weather maps as a solid purple line with alternating triangles and half circles, both pointing in the direction toward which the front is moving. Generally, the air behind an occluded front is colder than the air ahead of it. The weather and clouds preceding an occluded front are often similar to that of a warm front.

Astationaryfrontis a cold front or warm front that shows little or no movement. On a weather map, stationary fronts are represented as alternating red and blue lines with half circles pointing toward the colder air and triangles pointing toward the warmer air.

CMiddle Latitude Cyclones

Blizzard Aftermath

A snow blizzard that struck the East Coast of the United States in January, 1996 left the Boston area with about 46 cm (about 18 in) of snow. Most blizzards that strike the United States are middle latitude cyclones.

Lisa Quinones/Black Star

Middlelatitudecyclones are huge low-pressure storm systems that consist of a cold front and a warm front, and, usually for part of their lifecycle, an occluded front as well. Middle latitude cyclones usually develop along a slow-moving or stationary front. Such fronts are common at the boundary between the midlatitude cell and the polar cell of the three-cell model. The boundary is a trough of low pressure, with (in the northern hemisphere) warm air to the south and cold air to the north. When a jet stream moves over a stationary front, the front may bend, as a cold front pushes southward and, to its east, a warm front moves northward. The junction of the two fronts is the center of the developing storm and has the lowest atmospheric pressure.

Windsattheground(in the northern hemisphere) blow counterclockwise and inward around the area of low pressure. As the surface winds converge toward the center of the storm, the air gradually rises, often condensing into clouds. The heat released during condensation supplies some of the energy for the storms development (see latent heat). Additional energy is derived as the air masses struggle to obtain equilibrium. Warm air rises along the warm front and cold air sinks behind the cold front. The rising and sinking air transforms potential energy into kinetic energy (energy of motion).

Thestormsdevelopment and movement depend upon the winds aloft. Strong winds above the storm quickly sweep the rising air downwind. If the winds aloft remove the air above the storm more quickly than the surface air converges, the surface pressure drops and the storm system intensifies. Conversely, if the converging surface air is greater than the removal of air aloft, the surface pressure rises and the storm system weakens. Because the winds above the surface storm typically blow from the southwest (in the northern hemisphere), the center of the surface low normally moves northeastward.

Asthestormsystemmoves northeastward, the faster-moving cold front catches up to the slower-moving warm front. Eventually the cold front overtakes the warm front and the storm system becomes occluded. With cold surface air on both sides of the occluded front, warm air is no longer rising and the cold air is no longer sinking. The storm is now without its primary source of energy (the conversion of potential energy into kinetic energy during the forceful lifting of warm air) and the storm system dies out and dissipates.

DTropical Cyclones

Hurricane

Archive Films/CBS News/Microsoft Corporation

Tropicalcyclones,also known as hurricanes and typhoons, are storms with sustained winds in excess of 120 km/h (74 mph). They form over warm, tropical waters between 5 and 20 latitude, where the winds are light and the humidity is high.

Hurricane Elena

On September 2, 1985, Hurricane Elena was photographed with a 70-millimeter lens from the space shuttle Discovery. Because the hurricane is in the northern hemisphere, the air circles in a counterclockwise motion toward the low pressure center or eye.

NASA/Science Source/Photo Researchers, Inc.

Tropicalcyclonesform over water when a mass of thunderstorms becomes organized and spirals in toward the storms center, or eye. The storms highest winds, strongest thunderstorms, and heaviest rain, occur just outside the eye in the region called the eye wall. In the eye itself, winds are usually light and skies are partly cloudy.

Infrared Image of Hurricane

Meteorologists use satellite images taken in infrared light to determine the heat of areas of the atmosphere. The data is translated into a visible image, which is often enhanced, or colored with shades representing temperatures. In enhanced images, red usually represents the hottest temperatures and violet usually represents the coolest temperatures.

NOAA/Science/Photo Researchers, Inc.

Tropicalcyclonestend to form along a weak area of low pressure, called a disturbance or wave, in the intertropical convergence zone (ITCZ). As the disturbance becomes more organized, it first becomes a tropical depression, then a tropical storm, and finally a tropical cyclone. For the tropical cyclone to intensify, the outflow of air above the storm must exceed the inflow of air at the bottom. Tropical cyclones derive their energy from the transfer of heat from the warm water and from the latent heat given up to the system during condensation. Tropical cyclones dissipate when they are cut off from their energy sources either by moving over cold water or a large land mass. Although tropical cyclones often take erratic paths, the prevailing easterly winds in the tropics tend to steer tropical cyclones westward or northwestward until they leave the tropics, then the prevailing westerlies tend to sweep them northeastward.

IXWEATHER PREDICTION

Weatherforecastingentails predicting how the present state of the atmosphere will change. Present weather conditions are obtained by ground observations, observations from ships and aircraft, radiosondes, Doppler radar, and satellites. This information is sent to meteorological centers where the data are collected, analyzed, and made into a variety of charts, maps, and graphs. These charts, maps, and graphs are then sent electronically to forecast offices where local and regional weather forecasts are made. In addition, these offices prepare weather advisories and warnings of impending severe weather.

Modernhigh-speedcomputers transfer the many thousands of observations onto surface and upper-air maps. Computers draw the lines on the maps with help from meteorologists, who correct for any errors. A final map is called an analysis. Computers not only draw the maps but predict how the maps will look sometime in the future. The forecasting of weather by computer is known as numerical weather prediction.

Topredicttheweather by numerical means, meteorologists have developed atmospheric models that approximate the atmosphere by using mathematical equations to describe how atmospheric temperature, pressure, and moisture will change over time. The equations are programmed into a computer and data on the present atmospheric conditions are fed into the computer. The computer solves the equations to determine how the different atmospheric variables will change over the next few minutes. The computer repeats this procedure again and again using the output from one cycle as the input for the next cycle. For some desired time in the future (12, 24, 36, 48, 72 or 120 hours), the computer prints its calculated information. It then analyzes the data, drawing the lines for the projected position of the various pressure systems. The final computer-drawn forecast chart is called a prognostic chart, or prog.

Aforecasterusesthe progs as a guide to predicting the weather. There are many atmospheric models that represent the atmosphere, with each one interpreting the atmosphere in a slightly different way. The forecaster learns the idiosyncrasies of each model and places more emphasis on the ones that do the best job of predicting a particular aspect of the weather.

Weatherforecastsmade for 12 and 24 hours are typically quite accurate. Forecasts made for two and three days are usually good. Beyond about five days, forecast accuracy falls off rapidly.

XWEATHER MODIFICATION

Althoughweatherconditions may sometimes be inconvenient for people, or even dangerous, most people accept weather as an unchangeable force of nature. Some meteorologists, however, have been involved in a variety of experimental techniques designed to modify the weather.

ACloud Seeding

Onemethodofweather modification is to seed clouds with tiny particles to try to coax more precipitation from them. There are two primary ways to seed clouds. The first method uses the coalescence process of rain formation. Small water drops or other particles are injected into the base of a cloud. As updrafts carry these particles up through the cloud, the particles grow in size by colliding and merging (coalescing) with drops in their path. Eventually, the drops grow large and heavy enough to fall. On their way down, the drops continue to grow in size and may even fragment into many new drops.

Thesecondmethodofseeding clouds employs the ice-crystal (Bergeron) process of rain formation. Small particles of silver iodide (AgI) are injected into a cloud that contains both ice crystals and water droplets at below freezing temperatures. Inside the cloud, the silver iodide particles act like ice crystals. Water vapor from the surrounding liquid droplets evaporates and freezes onto the silver iodide particles, which grow larger at the expense of the surrounding liquid droplets. The growing crystals eventually become heavy enough to fall as precipitation.

Theeffectivenessofthese methods of cloud seeding is disputed because it is difficult to determine how much precipitation would have fallen had the cloud not been seeded. Some studies indicate that seeding under optimum conditions will enhance precipitation by as much as 15 percent. On the other hand, some attempts to seed clouds have reduced the amount of precipitation. It is thought that in some cases the clouds were overseeded, which produced so many small ice particles that there was not enough water droplets and water vapor left to allow the ice crystals to grow large enough to fall. (See Cloud Seeding)

BHail Suppression

Hailformsinthunderstorms when supercooled liquid droplets accumulate on small clumps of graupel. In an attempt to reduce the destructiveness of hail, large quantities of silver iodide are injected into the thunderstorm. The idea is to overseed the cloud so that many smaller hailstones form, preventing them from growing into large destructive hailstones. Results of hail-suppression experiments have been inconclusive.

CFog Dispersal

Fogisacloudonthe ground. Fog-clearing operations have mainly been attempted at airports to improve runway visibility. An early attempt at fog dispersal burned large quantities of fuel oil along runways, so that the air would warm enough to evaporate the fog. This expensive technique proved to be ineffective and very smoky. Another method employs helicopters that hover above the fog layer. The turbulence created by the blades mixes the drier, warmer air above the fog with the cooler, saturated air below. The mixing of the drier air into the fog evaporates the fog. This method works well when the fog is shallow, winds are light, and the air temperature is above freezing.

Foghasalsobeenseeded in an attempt to dissipate it. The seeding usually involves salt particles or dry ice (frozen carbon dioxide). Tiny salt particles cause the fog droplets to grow in size and fall out as drizzle. Dry ice only works in fog at below freezing temperatures. As small pieces of cold dry ice descend, they freeze the liquid fog droplets into ice crystals. The ice crystals grow in size and fall to the ground. The remaining fog droplets evaporate, leaving a clear area in the fog for aircraft operations. No matter how successful the fog-clearing operation, it must be applied continuously or the fog will reform as it moves in from the surrounding area.

DHurricane Modification

Hurricaneshavebeenseeded with silver iodide in an attempt to reduce their destructive winds. During the 1960s, project STORMFURY, a joint effort of the National Oceanic and Atmospheric Administration (NOAA) and the United States Navy, seeded several hurricanes just outside their centers so that clouds would develop farther away from the main area of severe thunderstorms and high winds. The hope was that this would reduce the air pressure locally and thereby reduce the hurricanes winds. Although some of the results were encouraging, some uncertainty remains as to the effectiveness of seeding hurricanes.

XIHUMAN INDUCED GLOBAL WARMING

Greenhouse Effect

Microsoft Corporation. All Rights Reserved.

In1988,theUnitedNations Environment Program and the World Meteorological Organization established the Intergovernmental Panel on Climate Change (IPCC) to assess the environmental, social, economic, and scientific information available on climate change. The IPCC consists of more than 200 leading earth scientists. Their Second Assessment Report, published in 1995, concluded that the earths average surface air temperature has increased by between 0.3 and 0.6 Celsius degrees (between 0.5 and 1.1 Fahrenheit degrees) in the past 100 years. Their report states that this warming should continue and that global average surface temperature will increase by between 1.0 and 3.5 Celsius degrees (between 1.8 and 6.3 Fahrenheit degrees) by the year 2100 (see Global Warming). If such a warming should occur, sea level should rise by between 15 cm and 95 cm (6 in and 37 in) by the year 2100, with the most likely rise being 50 cm (20 in). Such a rise in sea level might have a damaging effect on coastal ecosystems. Other changes brought on by this warming might include a shift in the worlds wind and rainfall patterns, which might put added stress on important agricultural areas, especially those in the western United States that depend on irrigation water from reservoirs and streams.

Manyclimatescientists believe that human activity is responsible for global warming. They attribute the main cause of global warming to the burning of fossil fuels, which increases the concentration of carbon dioxide (CO2) gas in the atmosphere. Carbon dioxide levels, presently about 360 parts per million (ppm), have increased 28 percent in the past century. The IPCC estimates that the concentration of CO2 in the atmosphere will surpass 500 ppm, an increase of another 40 percent, before the end of the 21st century.

Carbondioxidewarmsthe atmosphere through a process known as the atmospheric greenhouse effect. The atmospheric greenhouse effect is caused by certain gases in our atmosphere, called greenhouse gases, selectively absorbing and emitting infrared radiation, or heat energy. The two most plentiful greenhouse gases are water vapor (H2O) and carbon dioxide (CO2). Other less plentiful (and hence less important) greenhouse gases include nitrous oxide (N2O), methane (CH4), and chlorofluorocarbons (CFCs).

Agreenhousegasislike a filter; it allows the shorter wavelengths of radiant energy (such as visible light) to pass through it, but it absorbs some of the longer wavelengths of radiant energy (such as infrared radiation). Visible sunlight readily passes through the greenhouse gases to reach the earths surface, where it warms the surface. The earths surface, which is much cooler than the sun, emits radiant energy in the form of longer infrared waves. The greenhouse gases absorb some of these infrared waves emitted by the earths surface. When greenhouse gases absorb infrared energy, they share this energy with other gases and the atmosphere warms. The greenhouse gases also emit infrared radiation. Some of the emitted radiation travels back to the earths surface, where it warms the earth again. By preventing the rapid escape of infrared energy to space, greenhouse gases act as an insulating layer around the earth, keeping its surface much warmer than it would be if these gases were not present.

Theatmosphericgreenhouse effect is a natural effect that has been occurring for billions of years. Indeed, without it, the earth would be a frozen planet with an average temperature of about -18 C (about 0 F). Due to the greenhouse effect, the earths average surface temperature is a comfortable 15 C (about 59 F).

Itisnotthegreenhouse effect that concerns scientists, but the enhancement of the greenhouse effect by human induced increases in the levels of greenhouse gases. Climate models predict that the worlds average surface temperature should rise by between 1 and 3.5 Celsius degrees (1.8 and 6.3 Fahrenheit degrees) by the year 2100. However, these models show that increasing the concentration of carbon dioxide to 500 ppm and keeping everything else constant only accounts for a global warming of less than 1 Celsius degree (1.8 Fahrenheit degrees). This slight warming, however, would increase the airs capacity for holding water vapor. The added water vapor, the most plentiful greenhouse gas, would enhance the atmospheric greenhouse effect by producing a positive feedback on the climate system. A positive feedback occurs when an initial change is reinforced by another process. In this situation, the increase in temperature causes an increase in water vapor, which absorbs more of the earths infrared energy, thus accounting for the rest of the warming.

Theinteractionsbetween the earth and its atmosphere are complex. There are many uncertainties in the climate system, especially with regard to clouds (which tend to cool the earth by reflecting sunlight) and the oceans (which act as a huge storehouse of heat energy). It is difficult to prove that increasing concentrations of greenhouse gases are responsible for the recent global warming. Most climate scientists contend, however, that at least part of the warming is due to human induced greenhouse gases.

XIIATMOSPHERIC OPTICS

Atmosphericopticsis the study of how light interacts with the atmosphere and objects in it. It explains, for example, why a mirage occurs, how a rainbow forms, why sunsets are red, and why the sky is blue.

AMirages

Amirageoccurswhenan object appears displaced from its true position. Atmospheric mirages are created when light is bent, or refracted, as it travels through layers of air with differing densities (see Optics). Changes in air density are usually caused by changes in air temperature. If the air near the ground is much warmer than the air above, light from the sky will bend up into an observers eyes so that an observer looking down at the distant ground sees light from the sky. The image of sky where the distant ground should be produces the mirage of a watery pavement, or water resting on hot desert sand. When the light from an object is bent, making the object appear higher than it actually is, a superior mirage occurs. When an object appears lower than it actually is, the mirage is called an inferior mirage.

BRainbows

Primary Rainbow

A rainbow forms when raindrops separate white sunlight into a spectrum. As sunlight passes through a drop of water, it is first bent and then reflected from the back surface of the drop toward the viewers eye. The amount of bending, known as refraction, differs for light of different colors. Red light bends the least and violet light bends the most. Here, a primary rainbow arcs through the sky after a rainstorm. Primary rainbows have red on the outside arc; the colors of secondary bows are reversed .

Phil Jude/Science Source/Photo Researchers, Inc.

Arainbowisanarcof concentric colored bands that spans a section of the sky. For a rainbow to form, rain must be falling in one part of the sky and the sun must be shining from behind the observer. Rainbows form when sunlight enters a raindrop and the various wavelengths of visible light, representing the different colors, begin to slow and bend. Violet light bends the most and red light bends the least. Most of the light passes through the raindrop. But the refracted light that hits the back of the drop at a certain angle (called the critical angle) is reflected off the back of the drop. The light is then refracted, or bent, a second time as it emerges from the drop. Because each color bends differently, each color emerges from the drop at a slightly different angle, producing a spectrum of colors. Because only a single color from each drop reaches an observer, it takes many raindrops, each one reflecting light back to an observer at slightly different angles, to produce the colors of a primary rainbow.

Rainbows

Rainbows form when sunlight is split into many colors and bounced back by raindrops. Click to learn more about rainbow formation and how best to view them.

Microsoft Corporation. All Rights Reserved.

Fainter,secondaryrainbows often form above the primary rainbow. Secondary rainbows form when sunlight enters a raindrop at such an angle that two reflections occur inside the raindrop. The second reflection weakens the light intensity and causes a reversal of colors. The weakened light that emerges produces a dimmer rainbow.

XIIIHISTORY OF METEOROLOGY

Thescholarsofancient Greece were interested in the atmosphere and its related phenomena. About 340 bc Greek philosopher Aristotle wrote Meteorologica, a treatise on natural philosophy. His works, although speculative, represented the sum of knowledge about the natural science, including weather and climate. At that time, anything that fell from the sky (including rain and snow) and anything that was in the sky (including clouds) were called meteors, from the Greek word meteoros, meaning high in the sky. From meteoros comes the term meteorology. Several years later, Theophrastus, a pupil of Aristotle, compiled a book on weather forecasting, called the Book of Signs. His work consisted of ways to foretell the weather by noticing various weather-related indicators, such as a ring around the moon, which is often followed by rain. The work of Aristotle and Theophrastus remained a dominant influence in the study of weather and in weather forecasting for nearly 2000 years.

Althoughweatherrecords were kept for different locations as early as the 14th century, meteorology did not become a genuine natural science until the invention of weather instruments. These instruments gave scientists data, so that the physical laws could be tested. Italian physicist and astronomer Galileo invented a crude thermometer in the late 1500s. Italian mathematician and physicist Evangelista Torricelli, a student of Galileo, invented the barometer in 1643. A few years later, French mathematician-philosophers Blaise Pascal and Ren Descartes, using a barometer, demonstrated that atmospheric pressure decreases with increasing altitude. In 1667 Robert Hooke, an English scientist, invented an anemometer for measuring wind speed. In 1714 German physicist Gabriel Daniel Fahrenheit worked on the boiling and freezing of water, and from that work he developed a temperature scale. In 1780 Horace de Saussure, a Swiss geologist and meteorologist, invented the hair hygrometer for measuring humidity.

Thescienceofmeteorology benefited from advances in other sciences, technology, and mathematics. In 1660 Irish-born English scientist Robert Boyle discovered the relationship between pressure and volume of a gas. English meteorologist George Hadley, in 1735, used physics and mathematics to explain how the earths rotation influences the trade winds in the tropics. By flying a kite in a thunderstorm in 1752, American statesman and scientist Benjamin Franklin demonstrated the electrical nature of lightning. French chemist Jacques Charles, in 1787, discovered the relationship between temperature and volume in a gas. In 1835 French physicist Gaspard de Coriolis mathematically demonstrated the effect that the earths rotation has on atmospheric motions.

Thefirstsystemofclassifying clouds was formulated by French botanist and zoologist Jean-Baptiste Lamarck in 1802. In 1803 Luke Howard, an English naturalist, devised a better system of classifying clouds. In 1806 British Admiral and hydrographer Francis Beaufort invented a wind scale for mariners. Enough weather information was available in 1821 that William Redfield, an American saddle maker and amateur meteorologist, was able to draw a crude weather map. By the 1840s ideas about winds and storms were partially understood. Meteorology got a giant boost in 1843 with the invention of the telegraph. Weather observations and information could now be rapidly disseminated.

Asignificantmilestone in meteorology took place about 1920 when a group of Norwegian scientists, led by Vilhelm Bjerknes, and including Tor Bergeron, developed a model explaining the life cycle of a middle latitude storm system. These ideas were expanded as upper air observations became available from aircraft and radiosondes. By 1940 upper-level measurements of temperature, pressure, humidity, and wind gave atmospheric scientists a three-dimensional view of the atmosphere.

Weatherradarbecameavailable to scientists during the early 1940s. At the same time, high-flying military aircraft discovered the existence of jet streamsswiftly flowing air currents that girdle the earth. In 1946 American chemist and Nobel laureate Irving Langmuir and American atmospheric physicist Vincent Schaefer found that tiny pellets of dry ice could induce supercooled liquid water droplets to crystallize. During the same year, Bernard Vonnegut, an American chemist, discovered that silver iodide crystals could cause these same water droplets to freeze. These events ushered in an active period of cloud seeding.

Theatmosphericsciences advanced again in the 1950s when high-speed computers were able to solve the mathematical equations that describe the behavior of the atmosphere. Today, computers not only plot the observations and draw the lines on surface and upper-level maps, but they also predict the state of the atmosphere up to five days into the future.

In1960theNationalAeronautics and Space Administration (NASA) launched Tiros 1, the first weather satellite. Subsequent satellites have been more sophisticated and have been capable of monitoring more aspects of the atmosphere. In the mid-1990s, the National Weather Service upgraded its conventional radar with a network of 135 Doppler radar units that are capable of peering into severe thunderstorms, unveiling hail, tornadoes, and strong winds.

Contributed By:C. Donald Ahrens

Microsoft Encarta 2009. 1993-2008 Microsoft Corporation. All rights reserved.