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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. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature, precipitation, and other atmospheric conditions, 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 middle latitudes, such as extratropical cyclones, are caused by instabilities of the jet streamflow. Because Earth's axis is tilted relative to its orbital plane (called the ecliptic), sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 104 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by 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. 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, the 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. (Full article...)

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Hurricane Vince eye 2005.jpg
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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Water mill Rosenmühle in Lower Saxony, Germany.jpg

Ground fog is a name given to fog that forms a shallow layer near the ground, sometimes just tens of centimeters thick. It can form due to warm air moving over a colder surface (advection fog), or at night due to the escape of thermal radiation into space (radiation fog). This scene is in Nordstemmen, Lower Saxony, Germany just after sunset.

Previously selected pictures: noctilucent cloud, Elie, Manitoba tornado, Dust storm from space, More...

More did you know...

...that the Flying river is the name given to the transport of water vapor from the Amazon rainforest to southern Brazil?

...that hurricane shutters are required for all homes in Florida unless impact-resistant glass is used?

...that the Joint Institute for Marine and Atmospheric Research is a combined weather and ocean research institute with the cooperation of the Office of Oceanic and Atmospheric Research and the University of Hawaiʻi?

...that the SS Central America was sunk by a hurricane while carrying more than 30,000 pounds (13,600 kg) of gold, contributing to the Panic of 1857?

...that a hurricane force wind warning is issued by the United States National Weather Service for storms that are not tropical cyclones but are expected to produce hurricane-force winds (65 knots (75 mph; 120 km/h) or higher)?

...that the Automated Tropical Cyclone Forecasting System is a software package for tropical cyclone forecasting developed in 1988 that is still used today by meteorologists in various branches of the US Government?

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This week in weather history...

August 27

1989: Hurricane Kiko struck the southern tip of Baja California as a category 3 hurricane, but caused no deaths.

August 28

2012: Typhoon Bolaven struck the Korean Peninsula, killing 19 in South Korea and at least 59 in North Korea.

August 29

2005: Hurricane Katrina pushed an incredible storm surge onshore in Louisiana, Mississippi, and Alabama, killing almost 2,000 people and causing more than $80 billion in damage. It was the costliest Atlantic hurricane in history, as well as the deadliest in the United States since the 1928 Okeechobee Hurricane.

August 30

2008: Hurricane Gustav made landfall in Pinar del Río Province, Cuba. A weather station recorded a wind gust of 184 knots (212 mph; 341 km/h), which at the time was thought to be the strongest wind gust from a tropical cyclone on record.

August 31

1952: Hurricane Able moved ashore near Beaufort, South Carolina, causing flooding and heavy crop damage.

September 1

1958: Hurricane Ella made landfall on the Tiburon Peninsula of Haiti, killing 30 people in Les Cayes.

September 2

1937: A severe typhoon struck Hong Kong with wind gusts over 130 knots (150 mph; 240 km/h), killing 11,000 people.

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Portrait of John Park Finley in 1913

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 also wrote the first known book on the subject as well as many other manuals and booklets, 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. (Full article...)

Previously selected biographies: Edward Norton Lorenz, Vilhelm Friman Koren Bjerknes, More...

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The scope of WikiProject Weather is to have a single location for all weather-related articles on Wikipedia.

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 cyclones.

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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