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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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A picture of a tree branch following a period of freezing rain. Freezing rain occurs when precipitation falls into an area where the temperature is above freezing (0º C, 32º F), melting any frozen precipitation. Closer to the ground, if the temperature drops back below freezing, the melted precipitation (now rain) becomes supercooled, and freezes instantly upon hitting an object. If freezing rain occurs for a long period of time, it can deposit a layer of ice on the ground and any objects which are not above the freezing point. This makes freezing rain an especially dangerous form of precipitation, as it can cause traffic accidents by making roads slippery, and can also cause trees and branches to fall, resulting in power outages and injuries to people who may be hit by falling trees or ice. The North American ice storm of 1998 was a long period of freezing rain which resulted in more than 30 deaths and billions of dollars in damage.

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Hurricane Vince was an unusual hurricane which developed in the northeastern Atlantic basin. Forming in October during the 2005 Atlantic hurricane season, the waters over which it developed were considered too cold for tropical development. Vince was the 20th named tropical cyclone and 12th hurricane of the extremely active season. Vince developed from a non-tropical system on October 8, becoming a subtropical storm southeast of the Azores. The National Hurricane Center (NHC) did not officially name the storm until the next day, shortly before Vince became a hurricane. The storm weakened at sea and, on October 11, made landfall on the Iberian Peninsula as a tropical depression. Vince was the first tropical system to do so since the 1842 Spain hurricane. It dissipated over Spain, bringing much needed rain to the region, and its remnants passed into the Mediterranean Sea...

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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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November 16

2007: Cyclone Guba reached peak intensity over the Coral Sea. Flooding from the storm's rainbands eventually killed more than 200 people in Papua New Guinea.

November 17

1999: Hurricane Lenny made landfall on Saint Croix. It was an unusual storm in many ways, striking the island as the strongest Atlantic hurricane on record for the month of November, while moving in an unprecedented west-to-east direction through the Caribbean Sea.

November 18

2014: A major cold snap through much of the US reached its peak, with some temperatures reaching 45 degrees below normal.

November 19

1421: St. Elizabeth's flood killed 2,000 or more people in the present-day Netherlands.

1912: Robert Simpson, former director of the National Hurricane Center and co-inventor of the Saffir–Simpson Hurricane Scale, was born in Corpus Christi, Texas.

November 20

1996: Hurricane Marco, the longest-lived tropical cyclone ever recorded in the Atlantic Ocean in November, reached its peak intensity while meandering over the Caribbean Sea.

2007: Cyclone Guba reached peak intensity as it drifted near Papua New Guinea, causing flooding which would kill over 100 people.

November 21

1992: A three-day tornado outbreak began in the Southern United States, eventually producing six F4 tornadoes, two tornadoes with paths of more than 100 miles (161 km), and 26 deaths.

2006: The Late November 2006 nor'easter gave parts of Georgia and South Carolina their earliest snowfalls on record.

November 22

1970: Typhoon Patsy struck the coast of Vietnam, causing major damage and killing more than 200 people.

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John Park Finley (April 11, 1854 – November 24, 1943) was an American meteorologist and Army Signal Service officer who was the first person to study tornadoes intensively. He wrote the first known scientific book on the subject of tornadoes, as well as many other manuals and booklets. He was responsible for the first national network for reporting tornado touchdowns and outbreaks, and kept an archive of tornado reports from across the United States. He also collected vast climatological data, set up a nationwide weather observer network, started one of the first private weather enterprises, and opened an early aviation weather school.

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Hurricane Katrina's eye

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Hurricane Isabel

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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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