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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.

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Cyclone Catarina from the ISS on March 26 2004.JPG

Cyclone Catarina, the only major tropical cyclone ever observed in the Southern Atlantic Ocean, as it appeared from the International Space Station on March 26, 2004.

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Template:/box-header The 1999 Sydney hailstorm was the costliest natural disaster in Australian history, causing extensive damage along the east coast of New South Wales. The storm developed south of Sydney on the afternoon of April 14, 1999 and struck the city's eastern suburbs, including the central business district, later that evening.

The storm dropped an estimated 500,000 tonnes of hailstones in its path. Insured damages caused by the storm were over A$1.7 billion, with the total damage bill (including uninsured damages) estimated to be around A$2.3 billion, equivalent to US$1.5 billion. It was the costliest in Australian history in terms of insured damages, overtaking the 1989 Newcastle earthquake that had resulted in A$1.1 billion in insured damages. Lightning also claimed one life during the storm, and the event caused approximately 50 injuries.

The storm was classified as a supercell following further analysis of its erratic nature and extreme attributes. During the event, the Bureau of Meteorology was consistently surprised at the frequent changes in direction, as well as the severity of the hail and the duration of the storm. The event was very unusual, as the time of year and weather conditions in the region were not conducive for a severe thunderstorm to form.

1999 Sydney hailstorm stones.jpg

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...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 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?

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September 19

1914: A tropical storm, the only tropical cyclone of the 1914 Atlantic hurricane season, dissipated over coastal Louisiana. This was the least active hurricane season on record.

September 20

1978: Hurricane Greta, after having weakened to a tropical depression due to its passage over Central America, restrengthened to a tropical storm in the far eastern Pacific Ocean. Because naming conventions for tropical cyclones in the Pacific are different from the Atlantic Ocean, the storm was renamed "Olivia", becoming a rare two-name storm.

September 21

1938: The Long Island Express crossed over Long Island and passed into New England as a Category 3 hurricane, the only major hurricane known to affect the area.

1961: Hurricane Esther passed over Nantucket Island as a weakening Category 3 hurricane, but caused no deaths.

2006: Typhoon Yagi, the strongest storm of the 2006 Pacific typhoon season, reached a peak intensity of 910 hPa, with winds of 195 km/h (120 mph 10-minute winds).

September 22

1989: Hurricane Hugo, the most damaging tropical cyclone in US history at the time, made landfall on Isle of Palms, South Carolina.

September 23

1551: (possibly 1556): The deadliest tornado in European history destroyed a shipping armada in the Grand Harbour of Malta.

1815: One of the most powerful tropical cyclones ever to strike New England came ashore along the coast of Long Island.

September 24

2001: Just two weeks after the September 11, 2001 attacks, several tornadoes hit the Washington, D.C. metro area, including one which passed a few hundred feet from The Pentagon and skipped over Capitol Hill.

September 25

1846: Wladimir Köppen, developer of the Köppen climate classification system, was born in Russia.

1939: A tropical storm struck central far-southern California, one of only two tropical cyclones to do so. As many as 93 people were killed.

1997: Hurricane Nora struck central Baja California.

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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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Cumulonimbus cloud

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Hurricane Katrina flooding

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Template:/box-header 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 Template:/box-footer

Template:/box-header 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! Template:/box-footer


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