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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 (Atmospheric Sciences)  · Tropical cyclones Featured article  · Disasters  · Water

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Moore Oklahoma Tornado Damage.jpg

A pickup truck was wrapped around this utility pole by the extreme winds from the Moore, Oklahoma tornado which occurred on 1999-05-03. The truck was then pummelled by winds which exceeded 200 mph (320 km/h), stripping most of the sheet metal off the truck. The F5 tornado was one of the most severe ever observed, and destroyed more than 1500 homes, killing 36 people.

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Snow events are a rarity in the U.S. state of Florida, as freezing temperatures in the state are generally caused by the cold and dry winds of anticyclones. Most of the state is in a rare portion of the continental United States which receives a mean maximum monthly snowfall amount of zero, the only other such areas being southern Texas and California. However, snow does occur, especially in the northern interior sections of the state, sometimes more than once in a season. Areas near Jacksonville have seen several inches of snow on occasion, and snow flurries have been reported as far south as Homestead. Generally, for snow to occur, the polar jet stream must move southward through Texas and into the Gulf of Mexico, with a stalled cold front across the southern portion of the state curving northeastward to combine freezing air into the frontal clouds.

Snowfall events by month in Florida

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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...

June 20

1957: The Fargo Tornado, an extremely damaging F5 tornado, killed 11 in Fargo, North Dakota.

June 21

2007: Cyclone Yemyin developed as a tropical depression east of India, eventually crossing the Deccan Plateau to the Arabian Sea and making a second landfall in Pakistan. Almost 1000 people were killed.

June 22

1919: A tornado killed 57 people in Fergus Falls, Minnesota.

1972: Hurricane Agnes made landfall as a strong tropical storm near New York City. It would cause devastating flooding inland, killing 113 people and causing more than $2 billion in damage (1972 USD).

2007: The first F5 tornado in Canadian history struck the town of Elie, Manitoba.

June 23

1544: Artemius of Verkola, a saint in the Russian Orthodox Church, was killed by lightning.

1944: An unusual long-lived tornado tracked through mountainous West Virginia, killing 100 people.

June 24

1996: Hurricane Alma drifted just off the coast of the Mexican state of Michoacán, producing heavy rains and winds of up to 100 miles per hour (160 km/h) which would kill 20 people.

June 25

1977: Roy Sullivan was struck by lightning for the seventh time and survived, a world record.

June 26

1930: A boat carrying dynamite was struck by lightning and exploded, killing 30 people.

1959: The first WSR-57 weather radar went online in Miami, Florida.

2012: The Waldo Canyon Fire advanced into Colorado Springs, Colorado, destroying over 300 homes and killing two people. It was part of a series of wildfires in Colorado caused by record heat and dry thunderstorms that summer.

Selected biography

Wladimir Peter Köppen

Wladimir Köppen (September 25, 1846 - June 22, 1940) was a Russian born geographer, meteorologist, climatologist and botanist of German descent. After studies in St. Petersburg, he spent the bulk of his life and professional career in Germany and Austria. His most notable contribution to science was the development of the Köppen climate classification system, which, with some modifications, is still commonly used. Köppen was one of the last scholars with a broad enough background and intellectual ability to make significant contributions to several branches of science.

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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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