Meteorology has been defined as the science of the earth's atmosphere; it deals with its continuously occurring global changes and the daily variations in the conditions of the air and their effects on the earth. The analyses and prediction of weather is probably the most important aspect of meteorology.

Variations in weather are caused by the uneven heating of the earth's surface; the result is that the atmosphere is in a constant state of imbalance. As a result, the weather elements that vary are temperature, humidity, visibility, clouds, kind and amount of precipitation, atmospheric pressure, and winds. Heat from the sun and the gravitation pull of the sun and moon combined with the earth's rotation keep the atmosphere in constant motion.

Weather conditions are a result of the atmosphere's attempt to gain equilibrium, which never occurs as heat is continuously redistributed within the atmosphere-earth system. The movement of cold and warm air masses, cloud formations, and storms serves to counter heat balances. Meteorologists use a continuous flow of weather reports or mathematical models (based on the physics of the atmosphere) to predict the future conditions of the atmosphere. Individual air masses have their own uniform temperature and humidity characteristics, and when they meet they often clash along the narrow boundary between them, the front, producing severe weather conditions.

Changes in temperature and pressure are important parameters. A change in pressure usually means that a change in the weather is approaching; rising pressure indicates fair weather, falling pressure a storm. Rising temperature usually indicates that winds are approaching from the south and dropping temperature from the north. In the Northern Hemisphere rising clouds signal that the weather is clearing, and clouds that get thicker and lower usually forecast precipitation. People learn these characteristics by observing the weather patterns where they live, and they usually can predict the daily weather— as probably did ancient people.

However, there are weather patterns and phenomena, such as tornadoes and severe storms, that are hard to predict locally and yet are important to people's security and their property. Today, people who are threatened by severe weather rely on reports from the National Weather Service or from television and radio broadcasts. Beyond the standard instruments—such as the barometer, which measures pressure; the thermometer, which measures temperature; and the hygrometer, which measures moisture—there are modern instruments such as Doppler radar that continuously measure wind, moisture, and temperature in the upper atmosphere. Other radar systems detect and track hurricanes. Many weather stations are fully automated, transmitting data to a central office; several earth-orbiting satellite systems as well as weather balloons also continuously sense the world's weather systems.

In addition, meteorologists are concerned with the changing quality of the air. Although rain is naturally mildly acidic, the term acid rain refers to the production of sulfuric acid in the atmosphere from gaseous sulfur compounds emitted by industrial processes. Acidic precipitation often alters the chemistry of water supplies as well as natural lakes; it can kill plants and animals, and it damage structures by corrosion. In places where limestone bedrock occurs, however, the effect of acid rain is reduced, as the acids react with limestone and are neutralized. But in areas such as the Adirondacks in New York state, where the bedrock is granitic and the water already acidic, acid rain increases the acidity.

Thermal inversions occur where warm air overlies cold air. Under these conditions pollutants become trapped and continue to accumulate, creating potential health hazards for people with respiratory ailments. Aside from reducing the sources of air pollution, there is not much to be done but wait until the inversion disappears. In Germany, along the industrial Ruhr Valley one episode of an inversion was so severe and prolonged that schools were closed and private automobile use was banned.

Since weather plays such an important role in our daily lives, everyone is interested in forecasts: about conditions at sea, flood warnings, hail damage to crops, driving conditions, and deciding whether or not to take an umbrella to work. Although there were many

Meteorologists use satellites to study weather events such as this von Karman vortex, a turbulent atmospheric flow pattern, that formed around the Canary Islands. (NASA)

attempts in the past to predict the weather, it was not until the seventeenth century, when the thermometer and barometer were perfected, that accurate measurements could begin. But people were not able to communicate their observations quickly over long distances until the telegraph was invented and first used in 1849 by Joseph Henry of the

Smithsonian Institution to make weather maps.

"Everybody complains about the weather, but no one does anything about it" is a common statement. But people have tried to control the weather. Orchard owners and farmers use smudge pots to prevent frosts from killing their crops. People seed supercooled clouds with dry ice pellets or silver iodide dust to produce rain during drought. Seeding has been used to prevent rain, to control flooding, and to disperse fog. In reality, however, it is difficult to tell what the net effects of these attempts are, because it is difficult to compare what would have happened it there had been no seeding.

—Sidney Horenstein

See also: Atmosphere; Atmospheric Cycles; Climatology; Global Climate Change; Hydrologic Cycle


Griffin, Dale W. et al. 2002. "The Global Transport of Dust." American Scientist 90:228-235; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Ledley, Tamara et al. 1999. "Climate Change and Greenhouse Gases." EOS 80:453-458; Lutgens, Frederick K., and Edward J. Tarbuck. 2001. The Atmosphere: An Introduction To Meteorology, 8th ed. Upper Saddle River, NJ: Prentice Hall.

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