Weather, is the state of the atmosphere at a particular place during a short period of time.

It involves such atmospheric phenomena as temperature, humidity, precipitation (type and amount), air pressure, wind, and cloud cover. Weather differs from climate in that the latter is the synthesis of weather conditions that have prevailed over a given area during a long time period--generally 30 years.

Weather occurs in the troposphere, the lowest region of the atmosphere that extends from the Earth's surface to 6-8 km (4-5 miles) at the poles and to about 17 km (11 miles) at the Equator.

Although weather is largely confined to the troposphere, phenomena of the higher regions of the atmosphere such as jet streams and upper-air waves significantly affect sea-level atmospheric-pressure patterns--the so-called highs and lows--and thereby the weather conditions at the terrestrial surface.

Geographic features, most notably mountains and large bodies of water (e.g., lakes and oceans), also affect weather.

Recent research, for example, has revealed that ocean-surface temperature anomalies are a potential cause of atmospheric temperature anomalies in successive seasons and at distant locations.

One manifestation of such weather-affecting interactions between the ocean and the atmosphere is what scientists call the El Nino/Southern Oscillation (ENSO).

It is believed that ENSO is responsible not only for unusual weather events in the equatorial Pacific region (e.g., the exceedingly severe drought in Australia and the torrential rains in western South America in 1982-83) but also for those that periodically occur in the mid-latitudes (as, for example, the record-high summer temperatures in western Europe and unusually heavy spring rains in the central United States in 1982-83).

The ENSO phenomenon appears to influence mid-latitude weather conditions by modulating the position and intensity of the polar-front jet stream (see jet stream).

Generally speaking, the changeability of weather varies widely in different parts of the world.

It is most pronounced in the mid-latitude belts of the westerly winds, where a continuous procession of traveling high- and low-pressure centres produces a constantly shifting weather pattern.

In tropical regions, by contrast, weather varies little from day to day or from month to month.

Weather has a tremendous influence on human settlement patterns, food production, and personal comfort.

Extremes of temperature and humidity cause discomfort and may lead to the transmission of disease; heavy rain can cause flooding, displacing people and interrupting economic activities; thunderstorms, tornadoes, hail, and sleet storms may damage or destroy crops, buildings, and transportation routes and vehicles.

Storms may even kill or injure people and livestock.

At sea and along adjacent coastal areas, tropical cyclones (hurricanes, typhoons, and willy-willies) can cause great damage through excessive rainfall and flooding, winds, and wave action to ships, buildings, trees, crops, roads, and railways, and they may interrupt air service and communications.

Heavy snowfall and icy conditions can impede transportation and increase the frequency of accidents.

The long absence of rainfall, by contrast, can cause droughts and severe dust storms when winds blow over parched farmland, as with the "dustbowl" conditions of the U.S. plains states in the 1930s.

The variability of weather phenomena has resulted in a long-standing human concern with forecasts and predictions of future weather conditions. In early historical times, severe weather was ascribed to annoyed or malevolent divinities.

Since the mid-19th century, scientific weather forecasting has evolved, using the precise measurement of air pressure, temperature, humidity, and wind direction and speed to predict changing weather.

The development of radar has enabled meteorologists to track the movement of cyclones and anticyclones (depressions and highs) and their associated fronts and storms.

The use of advanced radar and computers in the second half of the 20th century has enabled weather patterns to be tracked worldwide.

These developments have improved the accuracy of local forecasts and have led to extended and long-range forecasts, although the high variability of weather in the mid-latitudes makes these longer-range forecasts less accurate.

In tropical regions, by contrast, daily weather variations are minor, with regularly occurring phenomena and perceptible change associated more with seasonal cycles (dry weather and monsoons); tropical cyclones are the main variable.