Weather
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Weather is a term that encompasses phenomena in the atmosphere of a planet. The term is normally taken to mean the activity of these phenomena over short periods of time, usually no more than a few days (see also Weather report). Average atmospheric conditions over significantly longer periods are known as climate. Usage of the two terms often overlaps and the physical concepts underlying them are closely related.
Basic mechanism
When used without qualification, "weather" is understood to be the weather of Earth. At large scales, weather results from temperature differences around the globe, which arise mainly because areas closer to the tropics receive more energy per unit area from the Sun (see also Sunrise and Sunset) than regions nearer to the Earth's poles. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, or ice sheets) have differing physical characteristics such as reflectivity, roughness, or moisture content.
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 horizontal pressure gradient accelerates the air from high to low pressure, creating wind. The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell and other forms of atmospheric circulation. An smaller scale example would be coastal breezes.
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, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate). This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence long-term climate (see Milankovitch cycles).
Terrestrial weather
On Earth, regularly occurring weather phenomena include such things as wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere). Weather does occur in the stratosphere and does affect weather lower down in the troposphere, but the exact mechanisms are poorly understood .
The Earth's atmosphere is a chaotic system, so small changes to one part can have large effects elsewhere. This makes it very difficult to accurately predict weather changes more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, Meteorology.
Chaos theory says that the slightest variation in the motion of the air will grow with time. This idea is sometimes called the butterfly effect, from the idea that the motions caused by the flapping wings of a butterfly eventually could produce marked changes in the state of the atmosphere. Because of this sensitivity to small changes it will never be possible to forecast perfectly, although there still is potential for vast improvement.
Shaping the planet
Along with plate tectonics and ocean circulation, weather is one of the fundamental processes that shape the Earth. The process of weathering breaks down rocks and soils into smaller fragments and then into their constituent substances. These are then free to take part in chemical reactions that can affect the surface further (e.g. acid rain) or are reformed into other rocks and soils. Weather also plays a major role in erosion of the surface.
Human history
Weather has played a large, and sometimes direct, part in human history. Aside from climatic changes that have caused the gradual drift of populations (for example the desertification of the Middle East, and Ice ages in Northern Europe), extreme weather events have caused smaller scale population movements and intruded directly on the course of human history. One such event is the saving of Japan from invasion by the Mongol fleet of Kublai Khan by the Kamikaze winds in 1281. A series of great storms throughout the 13th century caused the powerful English Cinque Ports to be silted up and hence lose their influence. The Little Ice Age of the 14th to 18th centuries had wide ranging effects in the North Atlantic region, including the demise of the Viking colonies in Greenland, catalysing the formation of leagues among the Native American groups in North America, and forcing the change of patterns of agriculture across Europe to accommodate the shortened growing season. More recently, Hurricane Katrina forced the temporary abandonment of the entire city of New Orleans in 2005.
Because of the effect that weather has on day-to-day life, prior to the advent of scientific methods of weather forecasting a large body of weather folklore developed to explain the weather has grown up. An example is the Groundhog Day celebration near the end of winter in parts of the United States.
The effect of seasons on the life of primitive peoples also caused them to observe and celebrate certain events during the calendar, some of which, in altered form, are still observed today. Christmas, for example, is the Yule of the pagans, celebrated around the winter solstice, the shortest day of the year (in the Northern Hemisphere, the summer solstice in the Southern Hemisphere).
Forecasting
Weather forecasting is the application of current technology and science to predict the state of the atmosphere for a future time and a given location. The history of weather forecasting goes back millennia, however the techniques used have changed significantly since then. Today, weather forecasts are made by collecting as much data as possible about the current state of the atmosphere (particularly the temperature, humidity and wind) and using understanding of atmospheric processes (through meteorology) to determine how the atmosphere evolves in the future. However, the chaotic nature of the atmosphere and incomplete understanding of the processes mean that forecasts become less accurate as the range of the forecast increases.
In the future
It is the goal of some scientists to control the weather. Experiments have been carried out for many years, but the results are usually ambiguous. On a grander scale, science fiction authors have long posited the idea of terraforming other planets in order to make them habitable by human beings. While this may be possible in the distant future, this is far beyond current technology.
Extremes
The coldest air temperature ever recorded on Earth is -89.2°C (-127.8°F), and that was at Vostok, Antarctica on July 21, 1983. The hottest air temperature ever recorded on earth was 57.7°C (135.9°F), which occurred in Al 'Aziziyah, Libya, on September 13, 1922. The highest recorded average annual temperature was 34.4°C (94°F) at Dallol, Ethiopia. The coldest recorded average annual temperature is -50.6°C (-59°F) at Vostok, Antarctica. And the coldest average annual temperature in a permanently inhabited location is at Resolute, Nunavut, in Canada.
Extra-terrestrial weather
Weather on other planets follows many of the same physical principles as weather on Earth, but occurs on different scales and in atmospheres having different chemical composition from Earth. The Cassini-Huygens mission to Titan, for example, discovered clouds formed from methane or ethane which deposit rain composed of liquid methane and other organic compounds.
Extra-terrestrial weather systems can be extremely stable; one of the most famous landmarks in the solar system, Jupiter's Great Red Spot is an anticyclonic storm known to have existed for at least 300 years. On other gas giants, the lack of a surface allows the wind to reach enormous speeds: gusts of up to 400 metres per second (ca. 1440kmph / 900 mph) have been measured on the planet Neptune. This has created a puzzle for planetary scientists: The weather is ultimately created by solar energy and the amount of energy received by Neptune is only about 1/900th of that received by Earth, yet the intensity of weather phenomena on Neptune is far greater than on Earth.
Earth's weather includes about six latitudinal circulation zones, three in each hemisphere (see Hadley cell). Jupiter's banded appearance shows over a dozen such zones, while Venus appears to have no zones at all. Studying how the weather works on other planets has been seen as helpful in understanding how it works on Earth.
Extra-planetary weather
Weather is not limited to just planetary bodies, however. A star's corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the solar system, known as the solar wind.
Inconsistencies in this wind and larger events on the surface of the star, such as Coronal Mass Ejections, form a system that has features analogous to conventional weather systems (i.e. pressure and wind), and though not true weather, is generally known as space weather. The activity of this system can affect planetary atmospheres and occasionally surfaces. The interaction of the solar wind with the terrestrial atmosphere can produce spectacular aurorae, but can play havoc with electrically sensitive systems such as electricity grids and radio signals.
