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Weather

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Part of the Nature series on
   Weather
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             Seasons

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              Storms

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              Topics

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   Weather Portal ·

   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

   Fair weather Cumulus clouds
   Enlarge
   Fair weather Cumulus clouds

   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

   Badly Flooded New Orleans after Hurricane Katrina.
   Enlarge
   Badly Flooded New Orleans after Hurricane Katrina.

   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

   Jupiter's Great Red Spot
   Enlarge
   Jupiter's Great Red Spot

   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

   Aurora Borealis
   Enlarge
   Aurora Borealis

   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.
   Aurora australis
   Enlarge
   Aurora australis
   Retrieved from " http://en.wikipedia.org/wiki/Weather"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
