The state of the atmosphere, as determined by the simultaneous occurrence of several meteorological phenomena at a geographical locality or over broad areas of the Earth. When such a collection of weather elements is part of an interrelated physical structure of the atmosphere, it is termed a weather system and includes phenomena at all elevations above the ground. More popularly, weather refers to a certain state of the atmosphere as it affects humans' activities on the Earth's surface. In this sense, it is often taken to include such related phenomena as waves at sea and floods on land.
An orderly association of weather elements accompanying a typical weather system of the Northern Hemisphere may be illustrated by a large anticyclone, or high-pressure region. In such a “high,” extending over an area of hundreds of square miles, the usually gentle winds circulate clockwise around the high-pressure center. This system often brings fair weather locally, which implies a bright sunny day with few clouds. The temperature may vary widely depending on season and time of day. However, a cyclone or low-pressure region is frequently associated with a dark cloudy sky with driving rain (or snow) and strong winds which circulate counterclockwise about a low-pressure center of the Northern Hemisphere.
A weather element is any individual physical feature of the atmosphere. At a given locality, at least seven such elements may be observed at any one time. These are clouds, precipitation, temperature, humidity, wind, pressure and visibility. Each principal element is divided into many subtypes. For a discussion of a characteristic local combination of several elements, as they might be observed at a U.S. Weather Bureau station, See also: Weather map
The various forms of precipitation are included by international agreement among the hydrometeors, which comprise all the visible features in the atmosphere, besides clouds, that are due to water in its various forms. For convenience in processing weather data and information, this definition is made to include some phenomena not due to water, such as dust and smoke. Some of the more common hydrometeors include rain, snow, fog, hail, dew and frost.
Both a physical (or genetic) and a descriptive classification of clouds and hydrometeors have been devised. The World Meteorological Organization, which among many other activities coordinates the taking of weather observations among the nations of the world, recognizes at least 36 cloud types and 100 classes of hydrometeors.
Certain optical and electrical phenomena have long been observed among weather elements, including lightning, aurora, solar or lunar corona and halo. See also: Air mass; Atmosphere; Atmospheric general circulation; Cloud; Front; Meteorology; Precipitation (meteorology); Storm; Weather observations; Wind