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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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D Hoarfrost1.jpg

Hoar frost is a loose covering of ice crystals which forms on objects due to radiational cooling. This scene is in Lower Saxony, Germany.

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Satellite image of Tip at its peak intensity

Typhoon Tip was the largest and most intense tropical cyclone on record. The nineteenth tropical storm and twelfth typhoon of the 1979 Pacific typhoon season, Tip developed out of a disturbance in the monsoon trough on October 4 near Pohnpei. Initially, a tropical storm to its northwest hindered the development and motion of Tip, though after it tracked further north Tip was able to intensify. After passing Guam, it rapidly intensified and reached peak winds of 305 km/h (190 mph) and a worldwide record low sea-level pressure of 870 mbar (hPa, 25.69 inHg) on October 12. At its peak strength, it was also the largest tropical cyclone on record with a diameter of 2,220 km (1,380 mi). It slowly weakened as it continued west-northwestward, and later turned to the northeast under the influence of an approaching trough. Tip made landfall on southern Japan on October 19, and became an extratropical cyclone shortly thereafter.

U.S. Air Force Reconnaissance flew into the typhoon for 60 missions, making Tip one of the most closely observed tropical cyclones. Rainfall from the typhoon breached a flood-retaining wall at a United States Marine Corps training camp in the Kanagawa Prefecture of Japan, leading to a fire which injured 68 and killed 13 Marines. Elsewhere in the country, the typhoon led to widespread flooding and 42 deaths, and offshore shipwrecks left 44 killed or missing.

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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 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 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.

Selected biography

Anders Celsius. Portrait by Olof Arenius (1701-1766)

Anders Celsius (November 27, 1701 – April 25, 1744) was a Swedish astronomer who is best known for his pursuit to develop a standardized temperature scale. He determined that the melting point and boiling point of water are constant regardless of latitude, and the boiling point of water is dependant on elevation.

In 1742 he proposed a temperature scale which now bears his name, the Celsius scale, which is used worldwide for meteorological observations around the world. In his scale, the boiling point of water was 0 degrees and the freezing point was 100, while the modern Celsius scale is the reverse of this. In addition to his temperature work, he was an avid observer of the aurora borealis and participated in an expedition to measure an arc of the meridian in northern Sweden.

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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


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