weather, state of the atmosphere at a given time and place with regard to temperature, air pressure (see barometer), wind, humidity, cloudiness, and precipitation. The term weather is restricted to conditions over short periods of time; conditions over long periods, generally at least 30–50 years, are referred to as climate.
The earliest evidence of scientific activity in the field of meteorology, the study of the earth's atmosphere, especially as it relates to weather forecasting, is from the 4th cent. BC; Aristotle wrote what is probably the first treatise on the subject. The first attempt to chart weather from reports over a considerable area was made (1820) in Europe by H. W. Brandes, but it was not until after the invention of the telegraph that the rapid collection of weather data from remote stations became possible.
In the United States, a government weather service was established (1870) under the army Signal Corps. In 1891 the weather service was transferred to the U.S. Weather Bureau under the Dept. of Agriculture, and it later came (1940) under the jurisdiction of the Dept. of Commerce. The U.S. Weather Bureau has since been renamed the U.S. National Weather Service and transferred to the National Oceanic and Atmospheric Administration. The central forecast office is the National Meteorological Center (NMC), in Suitland, Md.; first-order stations are located chiefly in the larger cities, and numerous substations for special purposes (e.g., observing river stages, measuring depth of snow, and maintaining records of climate) are distributed throughout the country.
Devices used for meteorological observations include rockets, weather satellites, radiosondes, barometers, anemometers, weather vanes, psychrometers, thermometers, and radar. By means of high-speed telecommunications, information from all over the world is sent to the NMC, where the data is decoded and plotted. These data are used to create weather maps based on simultaneous weather observations at different atmospheric levels over any desired geographic region. On a typical map the various weather elements are shown by figures and symbols; isobars are drawn to show areas of low pressure (cyclones) and high pressure (anticyclones); fronts (boundaries between air masses) and areas of precipitation are indicated.
By using computer models based on mathematical formulations of the dynamics of the atmosphere, weather charts are also produced as prognostics of future weather patterns. The many simplifying assumptions required in these formulations, as well as the incompleteness of weather data, limit the accuracy of the computer predictions; though as advances in computer systems occur, these models are becoming more complete and, hence, more accurate. Meteorologists interpret and modify such prognostics according to their knowledge of the prognostics' reliability and their familiarity with local influences, such as topography and proximity to large bodies of water, in order to derive the best possible weather forecasts.
Forecasts are disseminated by television, radio, telephone, newspapers, and the Internet. Detailed forecasts can usually be made only for a short future period (generally 48 hr or less). Forecasts for up to five days can usually predict departures from normal temperature and precipitation fairly well; longer-range predictions are more general and less accurate, being based on the known normal weather of the area. Mathematical models, particularly those run on supercomputers, have helped to understand weather changes, including general global circulation patterns, and how perturbations in the atmosphere and oceans effect the weather.
See J. R. Eagleman, Weather Concepts and Terminology (1989); J. Farrand, Jr., Weather (1990); H. M. Conway and L. L. Liston, Weather Handbook (1990); R. C. McNeill, Understanding the Weather (1991); S. H. Schneider, Encyclopedia of Climate and Weather (2 vol., 1996); J. L. Fry et al., The Encyclopedia of Weather and Climate Change (2010).