Wikipedia
Weather
From Wikipedia, the free encyclopedia

Part of the Nature series on
Weather
 
Seasons
Temperate
Spring · Summer
Autumn · Winter
Tropical
Dry season
Wet season
Storms
Thunderstorm · Tornado
Tropical cyclone (Hurricane)
Extratropical cyclone
Winter storm · Blizzard
Ice storm
Precipitation
Fog · Drizzle · Rain
Freezing rain · Ice pellets
Hail · Snow · Graupel
Topics
Meteorology
Weather forecasting
Climate · Air pollution
Weather Portal
 v • d • e 
This article is missing citations or needs footnotes.
Using inline citations helps guard against copyright violations and factual inaccuracies. (November 2007)
For other uses, see Weather (disambiguation).
For the geological process, see Weathering and Erosion.
The weather is a set of all extant phenomena in a given atmosphere at a given time. It also includes interactions with the hydrosphere. The term usually refers to the activity of these phenomena over short periods (hours or days), as opposed to the term climate, which refers to the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.
Weather most often results from temperature differences from one place to another. On large scales, temperature differences occur because areas closer to the equator receive more energy per unit area from the Sun than do regions closer to the poles. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or man-made objects) have differing physical characteristics such as reflectivity, roughness, or moisture content.
Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands, lowering the air pressure. The resulting horizontal pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation then causes curvature of the flow via the Coriolis effect. The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell while a smaller scale example would be coastal breezes.
The strong temperature contrast between polar and tropical air gives rise to the jet stream. Most weather systems in the mid-latitudes are caused by instabilities of the jet stream flow (see baroclinity). Weather systems in the tropics are caused by different processes, such as monsoons or organized thunderstorm systems.
Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate). This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence long-term climate (see Milankovitch cycles).

Find more weather info on Wikipedia

News Results

Get more news results on google.com

Blog Posts

Find more blog posts on google.com