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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

Selected picture

Lightning strike jan 2007.jpg

A bolt of lightning struck just behind this hill, near Swifts Creek, Victoria, Australia. Lightning kills more people than any other thunderstorm phenomenon (including tornadoes), and strikes approximately 100 times per second across the world.

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The 2008 Super Tuesday tornado outbreak was a deadly tornado outbreak which affected the Southern United States and the lower Ohio Valley on February 5-6, 2008. In total, 87 tornadoes were confirmed in the outbreak's 15 hour span. Several destructive tornadoes struck heavily populated areas, most notably in the Memphis metropolitan area, in Jackson, Tennessee, and the northeastern end of the Nashville metropolitan area. Fifty-seven people were killed in the outbreak by tornadoes across four states and 18 counties, with hundreds injured and property damage totaling more than $500 million (USD).

The outbreak was the deadliest in the U.S. since the May 31, 1985 outbreak that killed 76 across Ohio and Pennsylvania (and also killed 12 in Ontario, Canada). It also was the deadliest tornado outbreak in both Tennessee and Kentucky since the 1974 Super Outbreak. In addition to the tornadoes, the same system produced significant straight-line wind damage, hail as large as softballs, or 4.50 inches (11 cm) in diameter, major flooding, significant freezing rain, and heavy snow across many areas of eastern North America.

Map of deadly tornadoes (in red) and other severe weather reports.

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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...

November 6

2005: A violent tornado killed 25 in Evansville, Indiana.

November 7

2006: Japan's deadliest tornado since World War II destroyed several buildings in Saroma, Hokkaido.

2013: Typhoon Haiyan, the strongest typhoon to ever make landfall, struck the island of Samar in the Philippines. More than 6,000 people were killed by the storm, mostly in Eastern Samar.

November 8

2008: Hurricane Paloma, the second-strongest tropical cyclone ever recorded in the Atlantic Ocean in November, reached peak intensity near the Cayman Islands.

2014: The strongest storm ever recorded in the Bering Sea reached peak intensity.

November 9

1913: The deadliest storm ever to hit the Great Lakes destroyed 19 ships and killed 250 people.

2005: An unusually late-season tornado struck Hamilton, Ontario.

November 10

1979: The SS Edmund Fitzgerald sank during a severe gale on Lake Superior, killing all 29 crew members.

1997: Hurricane Rick made landfall in southern Mexico, the second-latest hurricane ever to do so.

November 11

1940: The Armistice Day Blizzard dumped two or more feet (0.6 meters) of snow on the United States' Midwest, killing 154 people.

November 12

1961: Hurricane Tara struck the Mexican state of Guerrero, killing as many as 500 people.

1970: The Bhola cyclone drove a 20 foot (7 meter) storm surge into areas of present-day Bangladesh, killing more than 500,000 people. It was the deadliest tropical cyclone in recorded history.

Selected biography

John Park Finley (April 11, 1854 - November 24, 1943) was an American meteorologist and Army Signal Service officer who was the first person to study tornadoes intensively. He wrote the first known scientific book on the subject of tornadoes, as well as many other manuals and booklets. He was responsible for the first national network for reporting tornado touchdowns and outbreaks, and kept an archive of tornado reports from across the United States. He also collected vast climatological data, set up a nationwide weather observer network, started one of the first private weather enterprises, and opened an early aviation weather school.

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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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