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:''For the geological process, see Weathering or Erosion.''
The 'weather' is the set of all extant phenomena in a given atmosphere at a given time. The term usually refers to the activity of these phenomena over short periods (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.

Contents

Basic mechanism

Terrestrial weather

Shaping the planet

Human history

Forecasting

Weather modification and human impact

Extremes

Extra-terrestrial weather

Extra-planetary weather

References

See also

External link

Basic mechanism

Weather most often results from temperature differences from one place to another. On large scales, temperature differences occur because areas closer to the equator receive more energy per unit area from the Sun than do regions closer to the poles. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or man-made objects) 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, and Earth's rotation then causes curvature of the flow via the Coriolis effect. 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 while a smaller scale example would be coastal breezes.
The strong temperature contrast between polar and tropical air gives rise to the jet stream. Most weather systems in the mid-latitudes are caused by instabilities of the jet stream flow (see baroclinic instability). Weather systems in the tropics are caused by different processes, such as monsoons or organized thunderstorm systems.
Because the Earth's axis are 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, common 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 can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.^[1]
The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole. This makes it difficult to accurately predict weather more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, Meteorology. It is theoretically impossible to make useful day-to-day predictions more than about two weeks ahead, imposing an upper limit to potential for improved prediction skill.[1]
Chaos theory says that the slightest variation in the motion of the air can 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 make perfect forecasts, although there still is much potential for improvement.

Shaping the planet

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. Almost all weather is the cause of heat exchange.
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 the formation of land bridges during glacial periods), extreme weather events have caused smaller scale population movements and intruded directly in historical events. 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. More recently, Hurricane Katrina forced the temporary abandonment of the entire city of New Orleans in 2005.
Though weather affects people in drastic ways, it can also affect the human race in simpler ways. It has been noted that the human immune system is affected in extreme heat or cold. Mood can also be affected by weather, hence the common scene of heavy downpour in Soap operas when a person cries. Weather, in its power, however, cannot affect a person's performance, at work, school or play. It is the person's own mindset that leads to poor performance during times of bad weather, heat, cold or rain.

Forecasting

Weather forecasting is the application of science and technology to predict the state of the atmosphere at a future time. Prior to the advent of scientific methods of weather forecasting, a large body of weather folklore developed to explain the weather. An example is the Groundhog Day celebration near the end of winter in parts of the United States and Canada, which forecasts whether spring is near or far depending on if the groundhog sees his shadow or not. Today, weather forecasts are made by collecting data that describe the current state of the atmosphere (particularly the temperature, humidity and wind) and using physically-based mathematical models to determine how the atmosphere is expected to change in the future. The chaotic nature of the atmosphere means that perfect forecasts are impossible, and that forecasts become less accurate as the range of the forecast increases.

Weather modification and human impact

The wish to control the weather is evident throughout human history: from ancient rituals intended to bring rain for crops to the U.S. Military Operation Popeye, an attempt to disrupt supply lines by lengthening the North Vietnamese monsoon. The most successful attempts at influencing weather involve cloud seeding; they include the fog- and low stratus dispersion techniques employed by major airports, techniques used to increase winter precipitation over mountains, and techniques to suppress hail.^[2]
Whereas there is inconclusive evidence for these techniques' efficacy, there is extensive evidence that human activity such as agriculture and industry results in inadvertent weather modification:^[2]

★ Acid rain, caused by industrial emission of sulfur dioxide and nitrogen oxides into the atmosphere, adversely effects freshwater lakes, vegetation, and structures.

★ Anthropogenic pollutants reduce air quality and visibility.

★ Climate change caused by human activities that emit greenhouse gases into the air is expected to affect the frequency of extreme weather events such as drought, extreme temperatures, flooding, high winds, and severe storms.^[4]
The effects of inadvertent weather modification may pose serious threats to many aspects of civilization, including ecosystems, natural resources, food and fiber production, economic development, and human health.^[5].

Extremes

Main articles: List of weather records

On earth, temperatures usually range between ±40°C. However, the wide range of climates and latitudes offer extremes of temperature well outside this range. The coldest air temperature ever recorded on Earth is -89.2°C (-127.8°F), at Vostok, Antarctica on 21 July 1983. The hottest air temperature ever recorded was 57.7°C (135.9°F), at Al 'Aziziyah, Libya, on 13 September 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. The coldest average annual temperature in a permanently inhabited location is at Resolute, Nunavut, in Canada.

Extra-terrestrial weather

Studying how the weather works on other planets has been seen as helpful in understanding how it works on Earth.^[6] 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. The Cassini-Huygens mission to Titan discovered clouds formed from methane or ethane which deposit rain composed of liquid methane and other organic compounds. Earth's atmosphere includes about six latitudinal circulation zones, three in each hemisphere (see Hadley cell). In contrast Jupiter's banded appearance shows over a dozen such zones, Titan has a single cell covering its entire surface, and Venus appears to have no zones at all.
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 (about 1440 km/h / 900 mi/h) 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.^[7] The strongest planetary winds discovered so far are on the extrasolar planet HD 189733b, which is thought to have easterly winds moving at more than 9,600 kilometers per hour.

Extra-planetary weather

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.
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 (such as pressure and wind) and 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, and can play havoc with electrically sensitive systems such as electricity grids and radio signals.

References

See also

★ Climate

★ Clouds

★ Meteorology

★ Weather forecasting

External link

★ Ogimet - online data from meteorological stations of the world

★ Wunderground (Weather Underground) - online forecast and personal weather station network

★ NOAA's National Weather Service - Local Forecast Search