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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

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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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Satellite image of the storm

The Christmas 1994 nor'easter was an intense cyclone along the East Coast of the United States and Atlantic Canada. It developed from an area of low pressure in the southeast Gulf of Mexico near the Florida Keys, and moved across the state of Florida. As it entered the warm waters of the Gulf Stream in the Atlantic Ocean, it began to rapidly intensify, exhibiting traits of a tropical system, including the formation of an eye. It attained a pressure of 970 millibars on December 23 and 24, and after moving northward, it came ashore near New York City on Christmas Eve. Due to the uncertain nature of the storm, the National Hurricane Center (NHC) did not classify it as a tropical cyclone.

Heavy rain from the developing storm contributed to significant flooding in South Carolina. Much of the rest of the East Coast was affected by high winds, coastal flooding, and beach erosion. New York State and New England bore the brunt of the storm; damage was extensive on Long Island, and in Connecticut, 130,000 households lost electric power during the storm. Widespread damage and power outages also occurred throughout Rhode Island and Massachusetts, where the storm generated 30-foot (9.1 m) waves along the coast. Due to the warm weather pattern that contributed to the storm's development, precipitation was limited to rain. Two people were killed, and damage amounted to at least $21 million dollars.

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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...

January 13

1979: A major snowstorm affected the Chicago area, dropping record snows of more than 16 inches (410 mm) in one day.

1982: Air Florida Flight 90 crashed into the Potomac River as a result of severe icing in a snowstorm, killing 78 people.

January 14

1888: The severe cold wave associated with the blizzard two days earlier reached its peak. The temperature reached −65 °F at Fort Keogh in Montana, the second lowest temperature ever recorded in the Contiguous United States.

2007: European windstorm Per struck parts of Scandinavia, killing six people.

2016: Hurricane Alex, an unusual January hurricane, reached peak intensity near the Azores.

January 15

1919: The Boston Molasses Disaster, caused in part by unusually warm temperatures, killed 21 people in Boston, Massachusetts.

January 16

1362: The Grote Mandrenke, Dutch for "Great Drowning of Men", produced an unprecedented storm tide in the Netherlands, killing 25,000 or more people.

January 17

1706: Benjamin Franklin, who would invent the lightning rod and introduce revolutionary concepts to meteorology, was born in Boston, Massachusetts.

1977: Snow was reported for the first time in West Palm Beach, Miami, and as far south as Homestead, Florida.

1982: An unprecedented outbreak of cold weather known as Cold Sunday brought record low temperatures to much of the Eastern United States.

January 18

1978: A subtropical storm formed in the central North Atlantic Ocean. It one of only four known tropical or subtropical cyclones to form in the North Atlantic in the month of January.

1993: An avalanche killed 59 people in Bayburt Province in northeastern Turkey.

January 19

1995: A helicopter crashed in the North Sea after being struck by lightning. Despite the high seas and stormy weather, all 18 passengers and crew were rescued.

2007: European windstorm Kyrill moved over the Baltic Sea towards northern Russia, after leaving a path of destruction from winds greater than 200 kilometers per hour (120 mph) across northern Europe. At least 44 people were killed.

2008: The Pacific Star, a cruise ship based out of New Zealand, ran into large waves caused by Cyclone Funa, suffering significant damage and several injuries to passengers.

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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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.

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