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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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Mammatus cloud panorama.jpg

Mammatus (also known as mamma or mammatocumulus, meaning "breast-cloud") is a meteorological term applied to a cellular pattern of pouches hanging underneath the base of a cloud. The name "mammatus" is derived from the Latin mamma (breast), due to the resemblance between the shape of these clouds and human female breasts. Mammatus are most often found on the anvil cloud that extends from a cumulonimbus, and therefore are commonly associated with severe weather.

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Ice Storm 98 trees line Noaa6198.jpg

The Ice Storm of 1998 (also known as Ice Storm '98) was a massive ice storm that struck areas of Eastern Ontario, southern Québec, and Nova Scotia in Canada, and bordering areas from Northern New York to Northern Maine in the United States. From January 4-10, 1998, up to 5 inches (120 mm) of ice accumulated on surfaces in these areas due to an unusually long period of freezing rain. The tremendous weight of ice accretion caused massive damage to trees and electrical infrastructure all over the area, leading to widespread power outages. Millions were left without power for periods varying from days to weeks, leading to more than 30 fatalities, a shut down of activities in large cities like Montreal and Ottawa, and an unprecedented reconstruction of the power grid. More than $5 million in damages were attributed to this storm.

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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...

August 3

2008: A rare F4 (T8) tornado struck the town of Hautmont, France, killing three people.

August 4

1980: Hurricane Allen reached category 5 strength for the first time in the late evening. Allen was at this strength for a total of almost three days, the longest of any Atlantic hurricane on record, and the third-longest in the world.

August 5

1997: Hurricane Guillermo reached a peak intensity of 919 millibars (27.1 inHg) over the eastern Pacific Ocean, making it the second-strongest Pacific hurricane on record at the time (since surpassed by two other storms).

August 6

1879: The easternmost major tornado ever recorded in North America struck Bouctouche, New Brunswick.

August 7

1979: Several tornadoes killed 2 people in the Woodstock, Ontario area.

August 8

1899: Hurricane San Ciriaco began crossing the island of Puerto Rico, eventually killing more than 3,000 people. The storm had an accumulated cyclone energy of 73.57, the highest on record in the Atlantic basin.

August 9

1878: The deadliest tornado in Connecticut history, and one of the worst ever in the Northeastern United States, destroyed the town of Wallingford, killing 34 people.

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Robert FitzRoy

Vice Admiral Robert FitzRoy (July 5, 1805 – April 30, 1865) achieved lasting fame as the captain of HMS Beagle and as a pioneering meteorologist who made accurate weather forecasting a reality, also proving an able surveyor and hydrographer as well as Governor of New Zealand.

FitzRoy developed charts to allow predictions to be made using observation stations connected by telegraph to transmit to him daily reports of weather at set times. The first daily weather forecasts were published in The Times in 1860, and in the following year a system was introduced of hoisting storm warning cones at the principal ports when a gale was expected. The Weather Book which he published in 1863 was far in advance of the scientific opinion of the time.

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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.

Wikipedia is a fully collaborative effort by volunteers. So if you see something you think you can improve, be bold and get to editing! We appreciate any help you can provide!

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