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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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Snow crystals.png

Flakes of snow highly magnified by a low-temperature scanning electron microscope (SEM). The colors are called "pseudo colors"; they are computer generated and are a standard technique used with SEM images.

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In meteorology, precipitation is a term for any product of the condensation of atmospheric water vapour that is deposited on the Earth's surface. It occurs when the atmosphere, becomes saturated with water vapour and the water condenses, falling out of solution. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapour to the air. Precipitation forms via collision with other rain drops or ice crystals within a cloud.

Precipitation that reaches the surface of the earth can occur in many different forms, including rain, freezing rain, drizzle, ice needles, snow, ice pellets or sleet, graupel and hail. While snow and ice pellets require temperatures to be near or below freezing at the surface, hail can occur during much warmer temperature regimes due to the process of its formation. Precipitation also occurs on other celestial bodies—including snow on Mars and a sulfuric acid rain on Venus—though both of these evaporate before reaching the surface.

Moisture overriding associated with weather fronts is a major method of precipitation production. If enough moisture and upward motion is present, precipitation falls from convective clouds such as cumulonimbus and can organize into narrow rainbands. Precipitation can also form due to forced ascent up the windward side of a mountain or mountain range. Precipitation is a major component of the water cycle, and is responsible for depositing essentially all of the fresh water on the planet. Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year; 398,000 km3 (95,000 cu mi) of it over the oceans.

Long-term mean precipitation by month

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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 16

1998: An F2 tornado struck downtown Nashville, Tennessee, killing one. In the same outbreak, a rare F5 tornado traveled more than 60 miles and killed 3 people.

April 17

1979: Flooding along the Pearl River in Jackson, Mississippi crested at 43.28 feet (13.19 m), exceeding the previous record by more than 5 feet (1.5 m).

April 18

2014: An avalanche killed 16 Sherpa guides working on Mount Everest.

April 19

2008: Typhoon Neoguri made landfall in Guangdong province of southern China, the earliest tropical cyclone to strike the nation in recorded history.

April 20

2004: A strong tornado, part of a surprise outbreak of 29 tornadoes, killed 9 people in Utica, Illinois.

April 21

1965: A devastating tornado outbreak killed 58 people near Chicago.

April 22: Earth Day

1992: A subtropical cyclone, the first ever recorded in the Atlantic Ocean in the month of April, reached peak intensity far south of Bermuda.

Selected biography

Portrait of John Dalton FRS

John Dalton FRS (6 September 1766 – 27 July 1844) was an English chemist, meteorologist and physicist. He is best known for his research into colour blindness and his pioneering work in the development of modern atomic theory, which came out of his initial interest in meteorology and atmospheric chemistry. He aided in the rediscovery of George Hadley's theory of atmospheric circulation: the Hadley cell, and published a book, Meteorological Observations and Essays, where he speculated on the nature of the atmosphere, its motion, and its chemistry.

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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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