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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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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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Storm of the century satellite.gif
Extratropical cyclones, sometimes called mid-latitude cyclones, are a group of cyclones that occur in the middle latitudes of the Earth, and which have neither tropical nor polar characteristics. They are connected with fronts and feature changes in temperature and dew point horizontally, otherwise known as "baroclinic zones". Extratropical cyclones are the everyday phenomena which, along with anticyclones, drive much of the weather on Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms. The image on right is a picture taken by a weather satellite in infrared of the 1993 North American Storm Complex, an extremely strong extratropical cyclone known to many in North America as the "Storm of the Century".

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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...

August 9

1878: The deadliest tornado in Connecticut history, and one of the worst ever in the Northeastern United States, destroyed the town of Wallingford, killing 34 people.

August 10

2017: Hurricane Franklin made landfall in Veracruz, Mexico, causing relatively minor damage.

August 11

1999: A rare tornado struck Salt Lake City, Utah, killing one person and injuring more than 100 people.

August 12

1993: Hurricane Fernanda reached peak intensity over the northeastern Pacific Ocean, with maximum sustained winds of 145 miles per hour (233 km/h). It would bring high waves to the Hawaiian Islands, but only caused minimal damage.

August 13

2004: Hurricane Charley, after changing direction and rapidly intensifying in the Gulf of Mexico, surprised residents of Punta Gorda, Florida as a Category 4 hurricane.

August 14

1974: Tropical Storm Alma made landfall in northern Venezuela, one of only 4 tropical cyclones in history to do so.

August 15

1787: The Four-State Tornado Swarm, the most extensive tornado outbreak in early American history, killed two people in New England.

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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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