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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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Helkivad ööpilved Kuresoo kohal.jpg

A noctilucent cloud photographed from Soomaa National Park, Estonia. Noctilucent clouds are the highest clouds that form on Earth, being found in the mesosphere at altitudes of more than 70 kilometres (43 mi) above the ground. They are also among the rarest seen types of clouds: they are very dim (can only be seen after sunset illuminated by the sun below the horizon), and are typically only seen at latitudes between 50 and 70 degrees from the equator during the summertime.

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A surface weather analysis is a type of weather map which provides a view of weather elements at a specified time based on information from ground-based weather stations. Weather maps are created by plotting or tracing the values of relevant quantities such as sea level pressure, temperature, and cloud cover onto a geographical map to help find synoptic scale features such as weather fronts.

The first weather maps in the 19th century were drawn well after the fact to help devise a theory on storm systems. After the advent of the telegraph, simultaneous observations of weather became possible for the first time, and beginning in the late 1840s, the Smithsonian Institution became the first organization to draw real-time surface analyses. Use of surface analyses began first in the United States, spreading worldwide during the 1870s. Use of the Norwegian cyclone model for frontal analysis began in the late 1910s across Europe, with its use finally spreading to the United States during World War II.

Surface weather analyses have special symbols which show frontal systems, cloud cover, precipitation, or other important information. For example, an H may represent high pressure, implying good and fair weather. An L on the other hand may represent low pressure, which frequently accompanies precipitation. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. The surface weather analysis is useful for visualizing general trends in the weather across a relatively large geographic area.

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Did you know...

...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?

...that a cryoseism is a sudden ground or glacier movement that can occur due to water freezing or ice cracking after drastic temperature changes?

...that BUFR is a binary data format standardized by the World Meteorological Organization for storing observation data from weather stations and weather satellites?

...that the Northwest Weather and Avalanche Center issues weather forecasts for conditions that can cause avalanches in the mountains of western Washington and northwestern Oregon?

...that a wind chill warning is issued by the National Weather Service when a combination of wind and cold temperatures is expected to cause life-threatening conditions for anyone caught outside?

Recent and ongoing weather

This week in weather history...

July 19

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.

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.

July 21

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.

July 23

1961: Hurricane Anna made landfall in Honduras, causing minor damage.

July 24

1959: Hurricane Debra made landfall near Galveston, Texas, bringing more than 15 inches (380 mm) of rain in Orange County that resulted in severe flooding.

July 25

2019: A historic heat wave reached its peak across western Europe, with temperatures reaching all-time record highs in Germany (42.6 °C, 108.7 °F), Belgium (41.8 °C, 107.2 °F), Luxembourg (40.8 °C, 105.4 °F), and the United Kingdom (38.7 °C, 101.7 °F).

Selected biography

Portrait of C. H. D. Buys Ballot c. 1857

Christophorus Henricus Diedericus Buys Ballot (Dutch pronunciation: ['boeys 'b?l?t]; October 10, 1817 – February 3, 1890) was a Dutch chemist and meteorologist after whom Buys Ballot's law and the Buys Ballot table are named. He was first chairman of the International Meteorological Organization, the organization that would become the World Meteorological Organization. (Full article...)

Previously selected biographies: Sir William Napier Shaw, Johannes Peter Letzmann, More...

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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.

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