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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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This dust storm occurred around Spearman, Texas on April 14, 1935. This was in the heart of the Dust Bowl, a period of severe dust storms and drought, which contributed to the Great Depression in the United States.

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The initial frontal wave (or low pressure area) forms at the location of the red dot on the image. It is usually perpendicular (at a right angle to) the leaf-like cloud formation (baroclinic leaf) seen on satellite during the early stage of cyclogenesis. The location of the axis of the upper level jet stream is in light blue.
Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area). Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. It can occur at various scales, from the microscale to the synoptic scale. Extratropical cyclones form as waves along weather fronts before occluding later in their life cycle as cold core cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high-pressure areasAnticyclogenesis.

Recently selected articles: 1933 Atlantic hurricane season, Jet stream, More...

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...

November 15: Start of the South-West Indian Ocean cyclone season

1989: Dozens were killed in a severe weather outbreak across the eastern half of North America, mainly in the Southeastern and Northeastern United States.

2006: Hurricane Sergio, the strongest Pacific hurricane on record in the month of November, reached its peak intensity, with maximum sustained winds of 110 miles per hour (180 km/h).

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.

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

1912: Robert Simpson, former director of the National Hurricane Center and co-inventor of the Saffir–Simpson scale of hurricane intensity, 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

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

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Vilhelm Friman Koren Bjerknes (March 14, 1862 – April 9, 1951) was a Norwegian physicist and meteorologist who did much to found the modern practice of weather forecasting. In 1895, he became professor of applied mechanics and mathematical physics at the University of Stockholm where he elucidated the fundamental interaction between fluid dynamics and thermodynamics. His major contribution was the primitive equations which are used in climate models. His son, Jacob Bjerknes, would introduce the ideas of weather fronts and the life cycle of extratropical cyclones, some of the most important advances in the history of meteorology.

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