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Weather is an all-encompassing term used to describe all of the many and varied phenomena that occur in the atmosphere of a planet at a given time. The term usually refers to the activity of these phenomena over short periods of 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, caused by the Sun heating areas near the equator more than the poles, or by different areas of the Earth absorbing varying amounts of heat, due to differences in albedo, moisture, and cloud cover. 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 pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation causes curvature of the flow via the Coriolis effect. These simple systems can interact, producing more complex systems, and thus other weather phenomena.

The strong temperature contrast between polar and tropical air gives rise to the jet stream. Most weather phenomena in the mid-latitudes are caused by instabilities of the jet stream flow (see baroclinity) or by weather fronts. 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, while in December it is tilted away, causing yearly changes in the weather known as seasons. In the mid-latitudes, winter weather often includes snow and sleet, while in both the mid-latitudes and most of the tropics, tropical cyclones form in the summer and autumn. Almost all weather phenomena can occur year-round on different parts of the planet, including snow, rain, lightning, and, more rarely, hail and tornadoes.

Related portals: Earth sciences (Atmosphere  · Atmospheric Sciences)  · Tropical cyclones Featured article  · Disasters  · Water

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Lightning over Oradea Romania 2.jpg

Lightning strikes in the outskirts of Oradea, Romania, during a thunderstorm on August 17, 2005. This storm system went on to cause major flash floods over Southern Romania.

Recently selected pictures: Snow-swept trees, Low pressure

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Storm of the century satellite.gif
Extratropical cyclones, sometimes called mid-latitude cyclones, are a group of cyclones that occur in the middle latitudes of the Earth, and which have neither tropical nor polar characteristics. They are connected with fronts and feature changes in temperature and dew point horizontally, otherwise known as "baroclinic zones". Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive much of the weather on Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms. The image on right is a picture taken by a weather satellite in infrared of the 1993 North American Storm Complex, an extremely strong extratropical cyclone known to many in North America as the "Storm of the Century".

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Did you know...

...that Hurricane Debbie is the only known tropical cyclone ever to strike Ireland?

...that the Tempest Prognosticator, one of the earliest attempts at a weather prediction device, employed live leeches in its operation?

...that eyewall replacement cycles are among the biggest challenges in forecasting tropical cyclone intensity?

...that the Braer Storm of January 1993 is likely the strongest extratropical cyclone ever recorded in the north Atlantic Ocean?

...that in medieval lore, Tempestarii are magicians with the power to control the weather?

...that the omega equation is essential to numerical weather prediction?

Recent and ongoing weather

This week in weather history...

April 23

1792: John Thomas Romney Robinson, inventor of the cup-anemometer, was born in Dublin, Ireland.

April 24

2007: Two tornadoes struck the towns of Piedras Negras, Coahuila, Mexico and Eagle Pass, Texas, United States, killing 10 people.

April 25

1994: An F4 tornado killed three people on the first day of a 3-day tornado outbreak.

April 26

1989: The deadliest tornado in world history destroyed areas of the Manikganj District, Bangladesh. More than 1300 people were killed, and over 80,000 people were left homeless.

April 27

2011: The deadliest American tornado outbreak in 76 years killed more than 300 people in the Southern United States.

April 28

2003: Santa Fe, Argentina, was struck by the worst flooding in the city's 400-year history.

April 29

1991: The second-deadliest tropical cyclone in world history struck the Chittagong area of Bangladesh, killing at least 138,000 people.

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Selected biography

Vilhelm Friman Koren Bjerknes (March 14, 1862 – April 9, 1951) was a Norwegian physicist and meteorologist who did much to found the modern practice of weather forecasting. In 1895, he became professor of applied mechanics and mathematical physics at the University of Stockholm where he elucidated the fundamental interaction between fluid dynamics and thermodynamics. His major contribution was the primitive equations which are used in climate models. His son, Jacob Bjerknes, would introduce the ideas of weather fronts and the life cycle of extratropical cyclones, some of the most important advances in the history of meteorology.

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Weather: Meteorology | Atmosphere | Basic meteorological concepts and phenomena | Climate | Clouds | Cyclones | Floods | Precipitation| Seasons | Severe weather and convection | Snow | Storms | Tornadoes | Tropical cyclones | Weather events | Weather lore | Weather hazards | Weather modification | Weather prediction | Weather warnings and advisories| Winds


WikiProject Meteorology is a collaborative effort by dozens of Wikipedians to improve the quality of meteorology- and weather-related articles. If you would like to help, visit the project talk page, and see what needs doing.

WikiProject Severe weather is a similar project specific to articles about severe weather. Their talk page is located here.

WikiProject Tropical cyclones is a daughter project of WikiProject meteorology. The dozens of semi-active members and several full-time members focus on improving Wikipdia's coverage of tropical cyclones.

WikiProject Non-tropical storms is a collaborative project to improve articles related to winter storms, wind storms, and extratropical weather.

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