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Weather is an all-encompassing term used to describe all of the many and varied phenomena that occur in the atmosphere of a planet at a given time. The term usually refers to the activity of these phenomena over short periods of hours or days, as opposed to the term climate, which refers to the average atmospheric conditions over longer periods of time. When used without qualification, "weather" is understood to be the weather of Earth.
Weather most often results from temperature differences from one place to another, caused by the Sun heating areas near the equator more than the poles, or by different areas of the Earth absorbing varying amounts of heat, due to differences in albedo, moisture, and cloud cover. Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands, lowering the air pressure. The resulting pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation causes curvature of the flow via the Coriolis effect. These simple systems can interact, producing more complex systems, and thus other weather phenomena.
The strong temperature contrast between polar and tropical air gives rise to the jet stream. Most weather phenomena in the mid-latitudes are caused by instabilities of the jet stream flow (see baroclinity) or by weather fronts. Weather systems in the tropics are caused by different processes, such as monsoons or organized thunderstorm systems.
Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, while in December it is tilted away, causing yearly changes in the weather known as seasons. In the mid-latitudes, winter weather often includes snow and sleet, while in both the mid-latitudes and most of the tropics, tropical cyclones form in the summer and autumn. Almost all weather phenomena can occur year-round on different parts of the planet, including snow, rain, lightning, and, more rarely, hail and tornadoes.
Recent and ongoing weather
This week in weather history...
October 4
1869: The 1869 Saxby Gale, an Atlantic hurricane, killed more than 100 people around the Bay of Fundy.
1963: Hurricane Flora struck southwestern Haiti, killing 5,000 people in that country alone.
October 6
1981: NLM Cityhopper Flight 431, a short-range passenger flight, flew into a tornado near Moerdijk, Netherlands. All 17 on board were killed.
October 7
1737: A severe cyclone killed 3,000 people in Calcutta, India.
October 10
1780: The Great Hurricane of 1780, the deadliest tropical cyclone ever in the Atlantic Ocean, began its trek through the Carribean Sea, which eventually led to the deaths of 27,500 people.
Wladimir Köppen (September 25, 1846 — June 22, 1940) was a Russian born geographer, meteorologist, climatologist and botanist of German descent. After studies in St. Petersburg, he spent the bulk of his life and professional career in Germany and Austria. His most notable contribution to science was the development of the Köppen climate classification system, which, with some modifications, is still commonly used. Köppen was one of the last scholars with a broad enough background and intellectual ability to make significant contributions to several branches of science.
Recently selected biographies: John Park Finley, Edward Norton Lorenz, More...
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