weather vane or wind vane, instrument used to indicate wind direction. It consists of an asymmetrically shaped object, e.g., an arrow or a rooster, mounted at its center of gravity so it can move freely about a vertical axis. Regardless of the design, the portion of the object with greater surface area (usually the tail) offers greater resistance to the wind and thus positions the vane so that the forward part points in the direction from which the wind is blowing. The compass direction of the wind may then be determined by reference to an attached compass rose; alternatively, the orientation of the vane may be relayed to a remote calibrated dial. The wind vane must be mounted at a distance from the nearest obstacle equal to at least twice the height of the obstacle above the vane if the observed wind direction is to be representative of meteorologically significant wind patterns; for this reason, the vane is often mounted on a pole or tower that is in turn mounted on the roof of a tall building.

weather satellite, artificial satellite used to gather data on a global basis for improvement of weather forecasting. Information includes cloud cover, storm location, temperature, and heat balance in the earth's atmosphere. The first weather satellites in the United States were those of the Tiros series, which began in 1960; the Nimbus series, which moved in a polar orbit, was next; the Environmental Science Services Administration (ESSA) started in 1966 and launched weather satellites; and in 1972, the Earth Resources Technology Satellite (ERTS) provided photographs to help forecasting. Other meteorological satellites include a series of Geostationary Operational Environmental Satellites (GOES), which send weather data and pictures that cover a section of the United States; China, Japan, India, and the European Space Agency (ESA) have similar craft. The National Oceanic and Atmospheric Administration's satellite series relay meteorological data to stations on the surface, including information on possible changes in various weather parameters that may signal climate change.

weather map: see weather.

weather balloon, balloon used in the measurement and evaluation of mostly upper atmospheric conditions (see atmosphere). Information may be gathered during the vertical ascent of the balloon through the atmosphere or during its motions once it has reached a predetermined maximum altitude. Today, atmospheric information is most often gathered by height-finding radar, remote sensing by earth-orbiting or stationary satellites, and aircraft instruments, with weather balloons augmenting the data. Helium, which is less dense than air (see buoyancy), is usually used to inflate weather balloons. A pilot balloon is a small balloon (diameter c.1 m/39 in.) whose ascent is followed visually to obtain data for the computation of the speed and direction of winds at different altitudes. A smaller ceiling balloon is used to determine the altitude of cloud bases. A much larger, teardrop-shaped balloon is used to carry a radiosonde aloft. The balloon expands as it rises, usually reaching an altitude of at least 90,000 ft (27,400 m) before it bursts. A small parachute lowers the instruments to the ground. Teardrop-shaped balloons are also used for horizontal sounding of the atmosphere. Atmospheric pressure, temperature, and humidity information may be sent by radio from a balloon; monitoring of its movement provides information about winds at its flight level. Techniques also have been developed whereby many horizontal sounding balloons can be monitored by earth-orbiting satellites that relay information to earth-based stations. The tetroon is a tetrahedral balloon used for horizontal sounding. It was developed to withstand the extremely low pressures of high-altitude flight; the straight seals joining its four triangular faces are stronger than the curved seals of the more traditionally shaped balloons. Tetroons have been used extensively in tracing low-level atmospheric currents by following their movement with radar; they have thus increased the meteorologist's understanding of atmospheric turbulence, low-level vertical motions, and air pollution dispersion.

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. B.C.; 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.

Deliberate or inadvertent alteration of atmospheric conditions by human activity, sufficient to modify the weather on a local or regional scale. Deliberate alterations include covering plants to keep them warm at night, seeding clouds to induce or augment precipitation, and firing silver-iodide particles into clouds to suppress or mitigate hail and to reduce fog at airports. Inadvertent alterations are the result of industrialization and urbanization, which have added billions of tons of carbon dioxide and other gases to the atmosphere (see acid rain, global warming, greenhouse effect).

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Prediction of the weather through application of the principles of physics and meteorology. Weather forecasting predicts atmospheric phenomena and changes on the Earth's surface caused by atmospheric conditions (snow and ice cover, storm tides, floods, etc.). Scientific weather forecasting relies on empirical and statistical techniques, such as measurements of temperature, humidity, atmospheric pressure, wind speed and direction, and precipitation, and computer-controlled mathematical models.

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State of the atmosphere at a particular place during a short period of time. It involves day-to-day changes in such atmospheric phenomena as temperature, humidity, precipitation (type and amount), air pressure, wind, and cloud cover. Most weather occurs in the troposphere, but phenomena of the higher regions of the atmosphere, such as jet streams, and geographic features, most notably mountains and large bodies of water, also affect it. Seealso climate.

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