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Thunderstorm near Garajau, Madeira

Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the lowest level of the atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature and precipitation activity, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.

Weather is driven by air pressure, temperature and moisture differences between one place and another. These differences can occur due to the sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley Cell, the Ferrel Cell, the Polar Cell, and the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 100 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by the Earth, thus influencing long-term climate and global climate change.

Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes, as most atmospheric heating is due to contact with the Earth's surface while radiative losses to space are mostly constant. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. The Earth's weather system is a chaotic system; as a result, small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to control the weather have occurred throughout history, and there is evidence that human activities such as agriculture and industry have modified weather patterns.

Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, weather is not limited to planetary bodies. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind.

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Hurricane Isabel 18 sept 2003 1555Z.jpg

The eye of Hurricane Isabel approaches North Carolina's Outer Banks in this true-color Moderate Resolution Imaging Spectroradiometer (MODIS) image captured by the Terra satellite on September 18, 2003 at 11:55 am US Eastern time.

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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.

Recently selected articles: Typhoon Tip, Hurricane Floyd, Hurricane Dean, More...

Did you know...

...that the Oklahoma Mesonet is a network of 121 weather instruments across US state of Oklahoma, providing surface weather observations across the state every five minutes?

...that a SIGMET is an advisory issued for airline crews regarding weather that may impact the safety of the aircraft, including atmospheric convection, icing conditions, or even dust storms?

...that the Antarctic Circumpolar Wave is thought by some scientists to be a very slow-moving wave in the atmosphere and ocean of the far southern hemisphere that circles the globe once every 8 years?

...that the Papagayo Jet is a strong wind that often blows across the Gulf of Papagayo west of Central America, sometimes reaching speeds as high as 30 meters per second (110 km/h; 67 mph)?

...that atmospheric optics is the study of different optical phenomena in the atmosphere, including sunsets, rainbows, and sun dogs?

...that the GME was a numerical weather prediction model run by Deutscher Wetterdienst, the German national meteorological service?

Recent and ongoing weather

This week in weather history...

September 14

1996: An unusual hybrid sub-tropical cyclone formed over Lake Superior.

September 15

2016: Typhoon Meranti, one of the most intense tropical cyclones on record at its peak, made landfall at Xiamen, Fujian, China, killing 47 people in China and Taiwan.

September 16

1984: Tropical Storm Fran passed just south of the Cape Verde islands, causing severe flooding that killed at least 29 people.

September 17

2010: Hurricane Karl, the strongest hurricane on record in the Bay of Campeche, made landfall near Veracruz, Mexico, killing 22 people.

September 18

1974: Hurricane Fifi began skimming the northern coast of Honduras, eventually killing at least 8,000 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 Atlantic hurricane season on record.

September 20

1995: Hurricane Juliette, the strongest hurricane of the 1995 Pacific hurricane season, reached peak intensity over the open ocean, with maximum sustained winds of 150 miles per hour (240 km/h).

Selected biography

Edward Norton Lorenz (May 23, 1917 – April 16, 2008) was an American mathematician and meteorologist, and a pioneer of chaos theory. He discovered the strange attractor notion and coined the term butterfly effect. Lorenz was born in West Hartford, Connecticut. He studied mathematics at both Dartmouth College in New Hampshire and Harvard University in Cambridge, Massachusetts. During World War II, he served as a weather forecaster for the United States Army Air Corps. After his return from the war, he decided to study meteorology. Lorenz earned two degrees in the area from the Massachusetts Institute of Technology where he later was a professor for many years.

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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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