World Book at NASA
Earth is not the only planet with a variety of weather conditions. Every planet in the solar system except Mercury and perhaps Pluto has enough of an atmosphere to support weather systems. In addition, Titan, a moon of the planet Saturn, has such an atmosphere. Pluto is so far away that little is known about its atmosphere. The remainder of this article discusses the weather on Earth.
The weather affects our lives every day. For example, it can have an impact on what type of clothing we wear and how we spend our free time. Weather also affects agriculture, transportation, and industry. Freezing temperatures can damage citrus crops in Florida or Spain, causing a rise in the price of oranges at the grocery store. Winter snows often create hazardous driving conditions. Thick fog may slow traffic on the roads and cause delays at airports. Our use of air conditioning during heat waves and heating during cold weather means that utility companies must supply more power at those times. Severe weather, such as tornadoes, hurricanes, and blizzards, can damage property and take lives.
Because of weather's importance, meteorologists (scientists who study the atmosphere and the weather) have developed ways to forecast weather conditions. Forecasts for the next 12 to 24 hours are correct more than 80 percent of the time. Long-range forecasts for the next week or month are less accurate. These forecasts indicate general trends, such as whether or not temperatures are expected to be warmer or colder than normal.
Closely related to weather is climate. Climate is the weather of a place averaged over a length of time. Scientists determine a region's climate by examining its vegetation, average monthly and annual temperature, and average monthly and annual precipitation. Earth's surface is a patchwork of climate zones. For example, in various parts of Earth, we find deserts; tropical rain forests; prairies; forests of cone-bearing trees; frozen, treeless plains; and coverings of glacial ice. Unlike changes in the weather, which can occur in minutes, climate changes generally take many years.
This article discusses Weather (What causes weather) (Weather systems) (Measuring the weather) (Weather forecasting) (Reporting the weather) (How people affect the weather) (Careers in weather).
What causes weather
Weather takes place in the atmosphere, the layer of air that surrounds Earth. Air is a mixture of gases and tiny suspended particles.
The most plentiful gas is nitrogen, which accounts for about 78 percent of the air we breathe. The second most plentiful gas is oxygen, at 21 percent. The remaining 1 percent consists of a variety of gases. In spite of their low concentrations, some of these gases play vital roles. For example, the atmosphere contains little water vapor -- an invisible gas produced when water evaporates. Yet, without water vapor there would be no clouds, no rain, no snow, and no plants or animals.
Most of the tiny particles floating in the atmosphere are too small to be visible. They are solid or liquid and come from a number of sources, such as the wind erosion of soil, volcanic eruptions, and the release of pollutants by smokestacks and automobile tailpipes.
Most weather occurs in the lowest portion of the atmosphere, called the troposphere. The troposphere extends from Earth's surface up to an altitude of about 6 to 12 miles (10 to 19 kilometers). Three key factors that determine the weather in the troposphere are air temperature, air pressure, and humidity.
Air temperature is a measure of the energy of motion of the air's gas molecules. The factors most responsible for the heating and cooling of the atmosphere are radiation arriving from the sun and radiation flowing from Earth.
The sun continually sends energy into space as electromagnetic radiation. One kind of solar radiation is visible light. The other forms of solar electromagnetic radiation are invisible to human beings. They include infrared (heat) rays and ultraviolet rays. About 30 percent of the solar electromagnetic radiation that reaches the atmosphere is reflected back into space, mostly by clouds. The atmosphere and Earth's surface absorb the remaining 70 percent, becoming warmer.
The warmed Earth cools by radiating infrared rays. Some of this radiation travels directly into space. The atmosphere absorbs almost all the remainder as it streams off the surface of the planet. This absorption of radiation, known as the greenhouse effect, makes the air near Earth's surface about 59 Fahrenheit degrees (33 Celsius degrees) warmer than it would be otherwise.
The atmosphere also sheds heat energy by radiating infrared rays. Some of this infrared radiation flows down to the surface, while the remainder travels out into space.
Air temperature generally varies from day to night and from season to season because of changes in the amount of radiation heating Earth's atmosphere. For example, days usually are warmer than nights because Earth receives the heating rays of the sun only during the day. At night, infrared radiation from the planet streams off into space, and the air temperature drops.
Air temperature also changes with the seasons. Except near the equator, where temperatures remain fairly constant the year around, summers are warmer than winters. In the summer, the sun is higher in the sky, and days are longer. When the sun is higher above the horizon, the intensity of the sunlight striking Earth's surface increases. More hours of sunlight in summer also mean more solar heating.
Altitude also affects air temperature. Within the troposphere, the air temperature generally drops 3.5 Fahrenheit degrees per 1,000 feet of elevation (6.5 Celsius degrees per 1,000 meters of elevation). Thus, it is usually colder on top of a mountain than in the surrounding lowlands.
Air pressure is the weight per unit of area of a column of air that reaches to the top of the atmosphere. Air pressure always decreases with increasing altitude because as you move higher there is less and less air above you. Air pressure is, on average, highest at sea level and drops to about half its sea-level value at an average altitude of about 18,000 feet (5,500 meters).
Air pressure also changes from place to place across Earth's surface. Part of this change is due to differences in land elevation. Most of the remainder is caused by changes in air temperature. Cold air is relatively dense -- that is, it has more air molecules per unit volume -- and so it exerts relatively high pressure. Warm air is less dense and exerts relatively low pressure.
Regions where air pressure is relatively high usually experience fair weather, while regions where air pressure is relatively low experience cloudy, stormy weather. Generally, the weather stays fair or improves if air pressure rises. If the air pressure falls steadily, however, the weather may turn cloudy and rainy or snowy.
Air moves from areas where the air pressure is relatively high toward areas where the air pressure is relatively low. This movement of air is what we call wind.
Humidity is a measure of the amount of water vapor in the air. There is an upper limit to this amount. Air that contains its maximum amount of water vapor is described as saturated. The amount of water vapor the air can hold increases as the air temperature rises and decreases as the temperature falls. Thus, saturated warm air has more water vapor than saturated cold air.
Weather reports commonly describe the amount of water vapor in the air in terms of the relative humidity. Relative humidity compares the amount of water vapor in the air with the amount of water vapor at saturation. It is expressed as a percentage. If the relative humidity is 50 percent, the amount of water vapor in the air is half of what it would be if the air were saturated. Lowering the air temperature increases the relative humidity.
If the relative humidity is 100 percent, the air is saturated. When air becomes saturated, water vapor begins to condense into droplets of water. Condensation is the opposite of evaporation. It is a change from a gas to a liquid. If the air is cold enough, at saturation the water vapor develops into tiny ice crystals.
If condensation occurs on a cold surface such as the surface of an automobile window at night, dew or frost forms. Dew and frost do not fall from the sky like rain or snow. Rather, they form when air in contact with a relatively cold surface is chilled to saturation. The same process occurs when small drops of water appear on the outside of a cold soft-drink can on a hot summer day. The temperature to which air must be cooled to reach saturation and produce dew is known as the dew point.
When saturation occurs within the atmosphere, water vapor condenses into droplets (or develops into tiny ice crystals) that form clouds. A cloud that is in contact with Earth's surface is known as fog. Most clouds occur within the troposphere. Because air temperature drops with increasing altitude within the troposphere, high clouds are extremely cold and consist mostly of ice crystals. The crystals give these clouds a fuzzy appearance. Low clouds are warmer and generally are composed of droplets. These clouds appear to have sharper edges.
Clouds usually form where air moves upward. As air ascends, it encounters lower and lower pressure. Air responds to lower pressure by expanding. Whenever gases expand, they cool. As air cools, its relative humidity increases until it reaches saturation and clouds form.
Where air moves downward, clouds usually do not develop. Descending air is compressed, it warms up, and its relative humidity decreases. Saturation is not possible, and so clouds do not form. Weather systems
Meteorologists classify weather systems according to their size and how long they last. The two largest and longest-lasting types of systems are planetary-scale systems and synoptic-scale systems. Planetary-scale systems are the belts of winds that circle the globe and may blow in the same direction for weeks at a time. Synoptic-scale systems cover a portion of a continent or ocean and last up to a week or so. The term synoptic comes from a Greek word meaning a general view.
 |
|
A supercell thunderstorm is a violent storm dominated by a single gigantic cell -- a weather system made up of storm clouds and the winds associated with them. Rain and hail may fall for hours. Image credit: National Center for Atmospheric Research/University Corporation for Atmospheric Research/National Science Foundation |
Planetary-scale systems
Suppose that Earth did not rotate and that the noon sun was always directly above the equator. Air temperatures would be highest at the equator and decrease toward the poles. Cold air is denser than warm air. Thus, air pressure would be higher at the poles and lower at the equator. Because air moves from areas of high pressure to areas of low pressure, cold air would sweep toward the equator, where it would push the warm air upward. In the upper atmosphere, the warm air would move toward the poles, cool, and sink over the poles. Thus, the planetary-scale circulation of wind would consist of two huge cells, one in each hemisphere.
The real Earth rotates
Rotation of Earth on its axis causes winds that blow long distances -- thousands of miles or kilometers -- to shift direction gradually. This shift is known as the Coriolis effect, which causes winds in the Northern Hemisphere to shift to the right and winds in the Southern Hemisphere to shift to the left. In the Northern Hemisphere, for example, winds blowing southward shift to the west. Winds blowing northward shift to the east.
Rotation of the planet also causes winds near Earth's surface to split into three belts in each hemisphere. These three belts are (1) the trade winds, which blow near the equator, between 30 degrees north latitude and 30 degrees south latitude; (2) the westerlies (winds from the west), which blow in the middle latitudes between 30 degrees and 60 degrees north and south of the equator; and (3) the polar winds, which blow in the Arctic and Antarctic, from 60 degrees latitude toward the poles.
Trade winds north of the equator blow from the northeast. South of the equator, they blow from the southeast. The trade winds of the two hemispheres meet near the equator, causing air to rise. Rising air cools, and its relative humidity therefore increases. Thus, a band of cloudy, rainy weather circles the globe near the equator.
Westerlies blow from the southwest in the Northern Hemisphere and from the northwest in the Southern Hemisphere. Westerlies and trade winds blow away from the 30 degrees latitude belt. Over broad regions centered at 30 degrees latitude, surface winds are light or calm, and air slowly descends. Air warms as it descends, and its relative humidity decreases, making clouds and precipitation unlikely. As a result, fair, dry weather characterizes much of the 30 degrees latitude belt.
Polar winds are easterlies (winds from the east). They blow from the northeast in the Arctic and from the southeast in the Antarctic. In the Northern Hemisphere, the boundary between the cold polar easterlies and the mild westerlies is known as the polar front. A front is a narrow zone of transition, usually between a mass of cold air and a mass of warm air. Storms develop and move along the polar front, bringing cloudiness, rain, or snow.
Important seasonal changes take place in Earth's wind patterns. Wind belts shift toward the poles in spring and toward the equator in fall. For example, during the fall, the polar front in the Northern Hemisphere often moves from Canada down to the continental United States.
Planetary-scale winds control the direction of movement of smaller-scale weather systems. For example, in the tropics, trade winds generally steer hurricanes and other weather systems from east to west. In middle latitudes, westerlies move weather systems from west to east. The westerlies are particularly vigorous near the top of the troposphere and just over the polar front. This corridor of exceptionally strong winds is known as a jet stream. The jet stream of the polar front supplies energy to developing storms and then moves them rapidly along the front.
Synoptic-scale systems include air masses, fronts, lows, and highs. The movement of these systems causes the day-to-day changes in the weather of Europe, the continental United States, and other regions in the middle latitudes.
Air masses
An air mass is a huge volume of air covering thousands of square miles or kilometers that is relatively uniform in temperature and humidity. The properties of an air mass depend on where it forms. Air masses that develop at high latitudes are colder than air masses that form over low latitudes. Air masses that form over the ocean are relatively humid, and those that form over land are relatively dry. The four basic types of air masses are (1) cold and dry, (2) cold and humid, (3) warm and dry, and (4) warm and humid.
Across North America, warm air masses move north and northeastward, and cold air masses move south and southeastward. Maritime polar air, which is cool and humid, forms over the North Pacific and North Atlantic. This air mass brings low clouds and precipitation to the Pacific Northwest, New England, and eastern Canada. Continental polar air, which is dry and cold in winter and dry and mild in summer, forms in north-central Canada. Arctic air, which is dry and much colder than continental polar air, forms over the snow-covered regions north of about 60ยก latitude in the Northern Hemisphere. The movement of Arctic air to the south causes the bone-numbing cold waves that sweep across the Great Plains and Northeast in winter.
Most of the maritime tropical air that invades North America originates over the Gulf of Mexico and the tropical Atlantic. This warm, humid air mass brings oppressive summer heat waves to areas east of the Rocky Mountains. Continental tropical air forms over the deserts of Mexico and the southwest United States. In summer, this hot, dry air mass surges over Texas and other parts of the American Southwest.
As an air mass travels from place to place, its temperature and humidity can change. For example, air over the Pacific Ocean west of North America is mild and humid. If that air mass moves eastward, it is forced up the slopes of the coastal mountain ranges. Air temperature drops, the relative humidity increases to saturation, clouds form, and rain or snow develops. As the air travels down the opposite slopes of the mountain ranges, the air temperature rises, the relative humidity decreases, and clouds thin out or vanish.
This process repeats with each mountain range the air mass encounters as it moves eastward. By the time it reaches the Western Plains, the air mass has become considerably drier and milder. This modified air mass, known as Pacific air, brings mild, dry weather to much of the central and eastern regions of the United States and Canada.
Fronts form where air masses meet. A front is a narrow zone of transition between air masses that differ in temperature or humidity. In most cases, the air masses differ in temperature, so that the fronts are either warm or cold.
A warm front is the leading edge of an advancing warm air mass. Warm air is less dense than cold air, so warm air advances by riding up and over the retreating cold air. As the warm air ascends, its temperature drops, relative humidity increases, and clouds and perhaps precipitation form. In North America, clouds can extend hundreds of miles or kilometers to the north and northwest of a warm front. Rain or snow is usually light to moderate and may last 12 to 24 hours or longer.
A cold front is the leading edge of an advancing cold air mass. Cold air is denser than warm air, so that cold air advances by moving under and pushing up the retreating warm air. As warm air ascends, its temperature falls, relative humidity rises, and clouds and often precipitation develop. Clouds associated with a cold front typically form a narrow band along the front. Rain or snow falls in brief showers. If the cold front is fast-moving and well-defined by considerable temperature contrast across the front, thunderstorms are likely. Some of these thunderstorms could become severe and produce hail, torrential rains, or strong winds. Tornadoes also may develop from severe thunderstorms.
A front that stalls is known as a stationary front. The weather along a stationary front often consists of considerable cloudiness and light rain or snow.
Cold fronts move faster than warm fronts, so a cold front may catch up to and merge with a warm front. The warm air is lifted off Earth's surface, and the merged front is known as an occluded front. An occluded front sometimes moves very slowly and causes several days of considerable cloudiness and light precipitation.
Lows are areas of relatively low air pressure. The winds in a low pressure system bring contrasting air masses together to form fronts. For this reason, lows are sometimes described as the chief weather-makers of regions in the middle latitudes. Scientists use the term cyclone to refer to a synoptic-scale low-pressure area. They also use the term to mean a hurricane in some parts of the world.
Viewed from above in the Northern Hemisphere, surface winds in a low-pressure area blow in a counterclockwise and inward direction. Surface winds converging in the low cause air to rise, cool, and reach saturation. Clouds and precipitation usually develop. Air ascends mostly along fronts that develop as winds in the low bring cold and warm air masses together.
Lows generally travel from southwest to northeast across North America and may complete a journey from Colorado to New England in three or four days. As a rule, temperatures are lower to the left (north) of the path followed by the low-pressure area and higher to the right (south). In winter, the heaviest snows usually fall about 90 to 150 miles (150 to 250 kilometers) to the north and west of the moving low-pressure area.
Highs, also known as anticyclones, are areas of relatively high air pressure. A high, which brings fair weather, often follows in the wake of a low. Viewed from above in the Northern Hemisphere, surface winds in a high blow in a clockwise and outward direction. As winds blow out and away from a high, air descends near the center of the system. Descending air warms, and the relative humidity decreases.
Highs are either warm or cold. Warm highs form south of the polar front and are characterized by high temperatures and low relative humidities. Such highs are massive weather systems that extend from Earth's surface to the top of the troposphere. In the summer, a warm high sometimes stalls over North America. If the high remains stationary for several weeks, it creates a drought.
Cold highs form north of the polar front. They are shallow masses of cold, dry air that develop mostly in the winter over the snow-covered regions of northern Canada, Alaska, and Siberia. As cold highs move southeastward over Canada and into the continental United States, they bring fair but cold weather.
Mesoscale and microscale systems result from the development and movement of synoptic and planetary-scale systems. Mesoscale systems, which may last an hour or less and affect only part of a city, include thunderstorms and sea breezes. A tornado is an example of a microscale system, the smallest and briefest of significant weather systems.
Measuring the weather
Because no single country can constantly measure and report on conditions in every part of the atmosphere, the world's nations must cooperate to monitor the weather effectively. To this end, they founded the International Meteorological Organization in 1873, renaming it the World Meteorological Organization (WMO) in 1950. Members of the WMO participate in the worldwide observation of the atmosphere and in the exchange of weather data and forecasts. Weather observations come from many different sources, including land-based observation stations, radar systems, weather balloons, airplanes, ships, and satellites.
Land-based observation stations. About 10,000 land-based weather stations -- also known as surface stations -- monitor the weather worldwide. In the United States, the National Weather Service coordinates weather observations at about 1,000 land-based stations with information obtained from about 10,000 volunteer weather observers. The Atmospheric Environment Service of Canada operates a similar observation network.
Observation stations use a variety of instruments to monitor the state of the atmosphere. An electronic thermometer checks air temperature and registers the highest and lowest temperature of the day. A hygrometer measures the amount of water vapor in the air. A barometer shows the air pressure. A weather vane indicates the direction of the wind, and an anemometer monitors wind speed. Rain gauges measure rainfall or snowfall. For more information, see the separate articles on weather instruments in World Book.
Weather radar
Some observation stations use radar to track storms. Weather radar can operate in either a reflectivity mode or a velocity mode.
In the reflectivity mode, weather radar locates areas of rain or snow. The system sends out a radar signal, which consists of microwave energy pulses. If the radar signal encounters rain, snow, or hail, the falling precipitation reflects some of that signal back to the radar. The reflected radar signal, called a radar echo, appears as a blotch on a television-type screen. As the radar rotates, it generates a map of radar echoes that represents the pattern of precipitation surrounding the radar. In the reflectivity mode, weather radar can detect precipitation more than 250 miles (400 kilometers) away.
In the velocity mode, weather radar determines the circulation of air from the motion of raindrops, snowflakes, or dust particles. This radar is also known as Doppler radar because it uses the Doppler effect to calculate how the air in a weather system is moving.
The Doppler effect is the change in frequency of sound or radiation waves caused by the motion of the source of the waves relative to their observer. For example, the pitch (frequency) of a train whistle seems higher as a train approaches and lower as the train moves away. Similarly, as raindrops, snowflakes, or dust particles move through the atmosphere, the radar signals they reflect change in frequency. The radar monitors these frequency changes and then uses them to calculate the speed at which the drops, flakes, or particles are advancing or receding.
 |
|
A Doppler radar image is color-coded to indicate the speed and direction at which rain clouds and other masses of air are moving. Doppler radar can provide warning of severe weather. Image credit: National Center for Atmospheric Research/University Corporation for Atmospheric Research/National Science Foundation |
Weather balloons, airplanes, and ships
To obtain information on the state of the atmosphere above Earth's surface, meteorologists routinely use an instrument package called a radiosonde. The radiosonde, which is carried aloft by a weather balloon, measures changes in temperature, pressure, and humidity. A small radio transmitter beams these data back to a weather station, where they are recorded by computer. At an altitude of about 19 miles (30 kilometers), the balloon bursts, and a parachute carries the instrument package back to Earth's surface.
Meteorologists use a special antenna to track a radiosonde and thereby measure wind speed and direction at different altitudes within the atmosphere. Such an observation is known as a rawinsonde. Radiosonde and rawinsonde observations are made every 12 hours.
Meteorologists sometimes use a dropwindsonde to obtain atmospheric measurements over the oceans. A dropwindsonde is a radiosonde attached to a parachute and dropped from an aircraft. As the instrument package falls toward the sea, it radios back to the aircraft measurements of temperature, pressure, and humidity.
Ships also report on weather conditions at sea. Some launch weather balloons, and others release special ocean buoys that record and transmit information about weather at sea level.
Weather satellites play a major role in worldwide weather observation. They monitor clouds associated with weather systems, track hurricanes and other severe weather systems, measure winds in the upper atmosphere, and obtain temperature measurements.
 |
|
A hurricane approaches the Bahamas and Florida in this image taken from a weather satellite in orbit about 22,300 miles (35,900 kilometers) above the earth. Only satellites can provide images of the weather over vast expanses of the earth's surface, making satellites an essential part of modern storm detection. Image credit: National Oceanic and Atmospheric Administration |
Sensors aboard weather satellites detect two types of radiation signals coming from the planet. One signal consists of reflected sunlight. These satellite images, which resemble black-and-white photographs of the planet, reveal cloud patterns.
The second signal recorded by weather satellites is infrared radiation (IR). The intensity of the infrared radiation emitted by an object depends on the object's temperature. For example, low clouds and fog, which are relatively warm objects, give off more intense infrared radiation than do high clouds, which are relatively cool. Thus, an IR image reveals not only cloud patterns but also cloud temperatures. Weather satellites can record IR images at any time -- day or night -- because objects continually emit infrared radiation.
There are two main types of weather satellites -- geostationary and polar-orbiting. A geostationary satellite orbits about 22,300 miles (35,900 kilometers) above the equator and travels eastward at the same rate as Earth rotates. Thus, a geostationary satellite remains above the same point on the equator. Because geostationary satellites orbit at such a high altitude, they can record images that cover a wide area. For example, two of them cover most of the United States and Canada. Most satellite images shown on televised weather reports come from this type of satellite.
A polar-orbiting satellite follows a north-south path that takes it over the polar regions. Because the satellite does not rotate east as Earth does, the rotation of Earth causes the satellite to pass over different areas of Earth each orbit. Polar-orbiting satellites travel at a much lower orbit than geostationary satellites, and so they record more detailed images. Weather forecasting
After meteorologists have gathered weather data from around the world, they can try to predict the development and movement of future weather systems. To forecast the weather, meteorologists use such tools as weather maps and mathematical models.
Weather maps
Meteorologists use weather observation data to create weather maps that represent the state of the atmosphere at a particular time. To capture the three-dimensional nature of weather, they draw maps for conditions at Earth's surface and at various levels within the atmosphere. By examining a sequence of weather maps, forecasters can determine how the weather changes through time and then make predictions about the future state of the atmosphere.
Mathematical models
Since the mid-1950's, weather forecasters have used mathematical models of the atmosphere, processed by computers, to improve the accuracy of their predictions. A mathematical model of the atmosphere consists of a set of equations intended to approximate the atmospheric processes that drive the development and movement of weather systems. Mathematical models are based on scientific laws, and through the years scientists have developed increasingly sophisticated models. Improvements in computer technology have greatly enhanced meteorologists' ability to use mathematical models effectively because computers can process enormous amounts of observation data and perform a multitude of calculations extremely rapidly.
A mathematical model begins with the current state of the atmosphere, as determined by the most recent weather observations. The model uses these data to predict the state of the atmosphere for a specific time interval -- for example, the next 10 minutes. Using this predicted state as a new starting point, the model then forecasts the state of the atmosphere for another 10-minute period. This process repeats over and over again until the model produces short-range weather forecasts for the next 12, 24, 36, and 48 hours.
The accuracy of weather forecasts generated by mathematical models declines steadily over time for two main reasons. First, the weather observation data initially fed into the model can never provide a complete picture of the present state of the atmosphere. Not all the data are reliable, due to both technical and human error, and data are missing from vast stretches of the atmosphere over the oceans. Second, mathematical models of the atmosphere are only approximations of the way the atmosphere actually works, and errors in the models tend to grow with each repetition.
Meteorologists understand the limitations of mathematical models. They base their forecasts on observations of how the weather has changed over the past several days and on their understanding of atmospheric processes. Experience and even intuition play important roles, along with the cautious interpretation of the output of mathematical models.
Meteorologists also use mathematical models to produce long-range weather forecasts, such as 6- to 10-day forecasts and monthly (30-day) and seasonal (90-day) outlooks. Long-range forecasts and outlooks typically are less accurate than short-range forecasts, but they can provide an indication of general trends, such as whether conditions will be wetter or drier than normal. Reporting the weather
The National Weather Service of the United States and the Atmospheric Environment Service of Canada issue weather forecasts, watches, warnings, and advisories to the public through regional forecast offices. The public accesses this information through radio and television broadcasts, newspapers, and the Internet.
When hazardous weather threatens, forecasters issue outlooks, watches, warnings, and advisories. An outlook provides advance notice of a general weather trend. For example, the outlook for spring flooding due to expected snowmelt is usually available many weeks in advance. A weather watch is issued when hazardous weather is possible based on current or predicted atmospheric conditions. A weather warning applies when hazardous weather is taking place nearby. Watches and warnings are issued for severe thunderstorms, tornadoes, floods, hurricanes, and winter storms, such as blizzards and ice storms.
Weather advisories refer to expected weather hazards that are less serious than those covered by a warning. An example is a winter weather advisory.
Weather advisories are also issued for low wind chill temperatures and for a high heat index. Wind chill is a measure of the cooling power of a combination of low air temperature and strong winds. Even if the air temperature remains constant, the human body loses increasing amounts of heat to the environment as wind speed increases. At low wind chill temperatures, people need to take special precautions to prevent frostbite (the freezing of skin tissue) and hypothermia (a dangerous drop in body temperature).
Heat index is a measure of the stress produced by a combination of high air temperature and high relative humidity. During excessively hot and muggy weather, the human body may not be able to release sufficient heat to prevent hyperthermia (a dangerous rise in body temperature). High humidity reduces the rate at which perspiration evaporates from the skin's surface. The cooling that accompanies this evaporation represents one of the body's main ways to release heat.
Private weather forecasters provide weather information tailored to a special need. For example, prior to pouring concrete, a building contractor may consult a private forecaster to find out the probability of rainfall during specific hours of the day. Department stores may hire a private forecaster to advise them of prospects for hot summer weather to ensure the stores have enough air conditioners and fans in stock.
How people affect the weather
Human activities affect the weather both intentionally and unintentionally. For example, the construction of cities creates areas that are drier and warmer than the surrounding countryside. Cities are drier because they have storm sewer systems that quickly carry off rainwater and snowmelt.
Cities are warmer for several reasons. The use of storm drainage systems means that less solar radiation is used to evaporate water and more is used to heat the city surfaces and air. The brick, asphalt, and concrete surfaces of city buildings, sidewalks, and streets readily transmit the heat they absorb and so raise urban air temperatures even more. In addition, cities themselves generate heat from a number of sources, including motor vehicles and heating and air conditioning systems.
Large urban areas also affect the weather in the areas downwind of them. Smokestacks and automobile tailpipes in cities emit water vapor and tiny particles that stimulate the formation of clouds. Heat energy rising from a city also spurs the growth of clouds. Thus, the weather downwind from many large urban areas is cloudier and rainier than the weather upwind from those same areas.
Urban and industrial areas also produce air pollutants, such as carbon monoxide, nitrogen oxides, and hydrocarbons. Although improved controls on factories and motor vehicles have reduced considerably the amount of these gases released into the atmosphere, air quality problems persist. For example, many large cities still have problems with smog -- a mixture of gases and tiny particles that reduces visibility and poses serious health hazards.
Smog and other air pollution problems are particularly serious in areas where winds are light and a temperature inversion occurs in the lower atmosphere. In a temperature inversion, warm air overlies cold air, so that the air temperature rises with increasing altitude, which is the opposite of the usual situation in the troposphere. Under such circumstances, smokestack and tailpipe emissions do not rise and disperse, and so emissions may build up to unhealthy concentrations.
From time to time, scientists have tried to alter the weather. Most of these efforts -- including attempts to modify hurricanes, suppress hailstorms, and clear fog -- have not worked. Today, scientists focus their weather modification efforts primarily on cloud seeding, an attempt to stimulate the natural precipitation-forming process.
Most clouds do not produce rain or snow. This is because few clouds have just the right combination of tiny ice crystals and supercooled water droplets (droplets that remain liquid even at subfreezing temperatures). In such clouds, ice crystals grow at the expense of water droplets and eventually become snow crystals. If the temperature is below freezing all the way to Earth's surface, precipitation falls as snow. If the air temperature is above freezing, the snow crystals melt into raindrops.
Careers in weather
High school students interested in meteorology as a career should take college-preparatory classes, including courses in mathematics, computer science, physics, and chemistry. Most entry-level jobs in meteorology require at least a bachelor's degree, and many require a master's degree. A limited number of U.S. and Canadian colleges and universities offer degrees in meteorology, and the U.S. armed forces offer meteorological training as well.
Meteorologists specialize in a number of different areas. For example, research meteorologists study some subfield of meteorology, such as tropical weather systems. Regional forecasters prepare weather forecasts for portions of one or more states or provinces. Consulting meteorologists provide weather information tailored for specific industrial, business, or government needs. Broadcast meteorologists have skills in both meteorology and television or radio broadcasting. This type of meteorologist informs the public of current and expected weather conditions. Specialists called synoptic meteorologists analyze weather observations, interpret the output of computer models, and monitor weather systems. Physical meteorologists study the physical and chemical properties of the atmosphere. Dynamic meteorologists focus on creating models of atmospheric circulation.
Contributor: Joseph M. Moran, Ph.D., Professor Emeritus, Department of Earth Science, University of Wisconsin, Green Bay; Associate Director, Education Program, American Meteorological Society.
How to cite this article:
To cite this article, World Book recommends the following format:
Moran, Joseph M. "Weather." World Book Online Reference Center. 2005. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar596160.
| › Return to Topics | › Back to Top |