Weather
Weather is an all-encompassing term used to describe all of the many and varied
phenomena that can occur 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 in length. Average atmospheric conditions over significantly longer periods are known as
climate. Usage of the two terms often overlaps and the concepts are obviously very closely related.
Weather phenomena result from temperature differences around the globe, which arise mainly because areas closer to the
tropics, around the equator, receive more energy from the
Sun than more northern and southern regions, nearer to the Earth's
poles.
A secondary cause of temperature differences on the Earth is that different surface areas (such as
ocean waters,
forested lands, and
ice sheets) have differing reflectivity (
albedo), and therefore absorb and radiate different amounts of the solar energy they receive.
Surface
temperature differences cause vertical wind currents. A hot surface heats the air above it, and the air expands and rises, lowering the
air pressure and drawing colder air into its place. Rising and expanding air gives up its heat and so cools, which causes it to shrink and sink, increasing air pressure and displacing the air already below it.
Horizontal wind currents are formed at the boundaries between differentially heated areas and can be intensified by the presence of sloped surfaces. The simple systems thus formed can then display
emergent behaviour to produce more
complex systems and thus all other weather phenomena. A large scale example of this process can be seen in the
Hadley cell and other forms of
atmospheric circulation. A smaller scale example would be
coastal breezes.
The
Coriolis effect is important in causing large scale rotations known as
Cyclones.
The fundamental causes of weather are thus surface temperature, and to a lesser extent,
elevation.
Because the Earth's
axis is tilted (not perpendicular 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. Any
precession in a planet's orbit will affect the amount of energy received at a particular spot throughout the
year and may influence long-term weather patterns. See
Milankovitch cycles.
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 recognised weather phenomena on Earth 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
[http://www.gsfc.nasa.gov/topstory/20011018windsurface.html].
The
Earth's atmosphere is one large
chaotic system so small changes to one part can have large effects elsewhere. This makes it very difficult to accurately predict short term weather changes more than a few days in advance, though
weather forecasters are continually working to extend this limit through the science of the study of weather,
Meteorology.
Chaos theory explains that there are an infinite number of variables which affect the weather. The slightest variation in the motion of any molecule in the air influences the atmospheric situation. The slightest difference could affect the troposphere sufficiently to lead to utterly different conditions. Thus it will never be possible to forecast perfectly.
Shaping the planet
Along with
plate tectonics and
ocean circulation, weather is one of the fundamental processes that have shaped the Earth since its creation and will continue to do so as long as it exists. 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, if indirect, role in
erosion of the surface, moving surface constituents around.
Human history
|
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 that is celebrated is the saving of
Japan from invasion by the
Mongol fleet of
Kublai Khan by the
Kamikaze winds in
1281. The
Little Ice Age of the 14th to 18th centuries had wide ranging effects in the northern hemisphere, including decimating the fledgling
Viking colonies of
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. A series of great storms throughout the
13th century caused the powerful English
Cinque Ports to be silted up and hence lose their influence. More recently,
Hurricane Katrina forced the temporary abandonment of the entire city of
New Orleans in
2005.
Because of the large effect that weather has on day-to-day life and due to the impossibility of any type of forecasting before the advent of modern technology, a large body of
folklore aimed at trying to explain the weather has grown up, some of which is fairly accurate, most less so. A well known example is the
Groundhog Day celebrated 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 adulterated 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).
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 -88.8ºC (-127.8ºF), and that was at
Vostok,
Antarctica on
July 21, 1983. The hottest air temperature ever recorded on earth was 57.7ºC (135.9º F), which accured in
Al 'Aziziyah,
Libya, on
September 13, 1922. The highest recorded average annual temperature was 34.4ºC (94ºF) at
Dallol,
Ethiopia. The coldest recorded average annual temperature is -50.6ºC (-59ºF) at
Vostok,
Antarctica. And the coldest average annual temperature in an permanently inhabited location is at
Resolute, Nunavut, in
Canada.
 |
Jupiter's Great Red Spot |
Weather phenomena and systems on other planets are thought to be similar to those on Earth, but often occur on a much bigger scale or involve different substances to those familiar to Earth dwellers. 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 (900 mph) have been measured on the planet
Neptune. This has created a puzzle for
planetary scientists: The weather is created by the differential action of the Sun's energy on different places and the amount of energy received by Neptune is very, very small, relative to the Earth, yet the strength and magnitude of weather phenomena on Neptune is far, far greater than on Earth. This mystery is still to be solved
[http://hubblesite.org/newscenter/newsdesk/archive/releases/1998/34/text/].
Earth's weather appears to behave based on about a half-dozen latitudinal weather zones. 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.
[http://www.space.com/scienceastronomy/solarsystem/solar_system_weather_010306-1.html]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.
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Climate*
Meteorology*
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List of songs about weatherzh-yue:天氣
