The weather portal

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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Ground fog is a type of fog formed by the ground cooling a warm layer of air near the surface to its dew point. This scene is in East Frisia, Germany just after sunrise.

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Major damage from the tornado

The Evansville Tornado of November 2005 was a powerful tornado that formed early in the morning of November 6, 2005, outside of Evansville, a city in Southwestern Indiana on the Ohio River. It was the first of several significant tornado events in the month of November 2005. The tornado resulted in 25 confirmed fatalities across the region, making it by far the deadliest and most destructive tornado in the United States in 2005, and it was also the deadliest single tornado in the US since 36 died in Oklahoma on May 3, 1999. Significant tornadoes were also reported in western Kentucky on the same day, but none were as damaging or deadly as the Evansville 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...

September 15

1995: Hurricane Ismael struck the Mexican state of Sinaloa, killing 116 people.

2004: The outer rainbands of Hurricane Ivan began spawning tornadoes in the Southeastern United States. This tornado outbreak, the second-largest ever caused by a tropical cyclone, killed 7 people.

September 16

1903: President of the United States Theodore Roosevelt was caught aboard his yacht in a hurricane, but made it to shore safely. Dozens of other boats in the area were sunk.

1928: Just two years after a Category 4 hurricane hit Miami, Florida, killing 373, another hurricane came ashore just north of the city, killing 2500 in Florida.

1961: Silver iodide was dropped into the eyewall of Hurricane Esther, marking the beginning of the project which would eventually become Project Stormfury, an attempt to weaken Atlantic hurricanes through cloud seeding.

1971: Hurricane Edith made its third and final landfall in western Louisiana.

September 17

1947: A category 4 hurricane struck Fort Lauderdale, Florida, killing more than 50 people and causing more than $100 million in damage.

September 18

1974: Hurricane Fifi began skimming the northern coast of Honduras, killing 10,000 people and causing $4 billion (2007 USD) in damage.

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).

Selected biography

Portrait of Sir George Gabriel Stokes

Sir George Gabriel Stokes (August 13, 1819 - February 1, 1903) was an Irish mathematician and physicist. He contributed greatly to fluid dynamics and optics, as well as other fields such as biochemistry to a lesser extent. One of his best known contributions is the Navier-Stokes equations, which has many applications in computational meteorology and atmospheric science today.

Stokes was a long-time professor at Cambridge, and was secretary, then president, of London's Royal Society.

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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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