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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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An atmospheric gravity wave manifests itself as altocumulus undulatus clouds in an arid environment, in the Tadrart Acacus region of southeast Algeria.

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The Ice Storm of 1998 (also known as Ice Storm '98) was a massive ice storm that struck areas of Eastern Ontario, southern Québec, and Nova Scotia in Canada, and bordering areas from Northern New York to Northern Maine in the United States. From January 4-10, 1998, up to 5 inches (120 mm) of ice accumulated on surfaces in these areas due to an unusually long period of freezing rain. The tremendous weight of ice accretion caused massive damage to trees and electrical infrastructure all over the area, leading to widespread power outages. Millions were left without power for periods varying from days to weeks, leading to more than 30 fatalities, a shut down of activities in large cities like Montreal and Ottawa, and an unprecedented reconstruction of the power grid. More than $5 million in damages were attributed to this storm.

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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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July 16

1904: A destructive tornado struck Chappaqua, New York, killing two people.

1997: Hurricane Danny formed as Tropical Depression Four, eventually dropping 36.71 inches (932 mm) of rain on Dauphin Island, Alabama. This was the most rain ever recorded from a tropical cyclone in the state.

July 17

2006: An active period of derechos begins across the Midwestern United States and eastern Canada, with four large derechos spanning five days. Portal:Weather/On this day list/July 18

July 19

1977: The Laurel Run Dam in West Taylor Township, Pennsylvania was overtopped and subsequently failed due to heavy rains, killing 40 people.

1983: A derecho, which moved along a path parallel to Interstate 94 from Minnesota to Illinois, knocked out power to 250,000 people and injured at least 34.

1996: Unprecedented rainfall started the Saguenay Flood, killing at least seven people and causing as much as $1.5 billion (CAD) in damage.

July 20

2007: Severe flooding in the United Kingdom peaked as a storm system dropped as much as 120 millimeters (4.7 in) of rain on southern England.

2008: Hurricane Bertha, the longest-lived July Atlantic hurricane on record, dissipated over Atlantic Canada.

July 21

1961: Hurricane Anna reached peak intensity over the Caribbean Sea, with peak winds of 115 miles per hour (185 km/h).

1987: A rare high-altitude violent tornado downed around 1,000,000 trees in Yellowstone National Park and surrounding wilderness.

July 22

1342: St. Mary Magdalene's flood, the worst flooding in central European history, reached its peak inundating most rivers and valleys across central Europe.

2006: Hurricane Daniel reached peak intensity over the open Pacific Ocean, with maximum sustained winds of 130 knots (150 mph; 240 km/h).

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Robert Simpson

Dr. Robert Homer Simpson (born November 19, 1912) is an American meteorologist and hurricane specialist. He was the first director of the National Hurricane Research Project (1955-1959), and a former director (1967-1974) of the National Hurricane Center. He is also the co-developer of the Saffir-Simpson Hurricane Scale, used for measuring tropical cyclone intensity in the Western Hemisphere.

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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.

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