From Wikipedia, the free encyclopedia
The weather is a set of all the phenomena occurring in a given atmosphere at a given time.[1] Weather phenomena lie in the hydrosphere and troposphere.[2][3] Weather refers to current activity, as opposed to the term climate, which refers to the average atmospheric conditions over longer periods of time.[4] When used without qualification, "weather" is understood to be the weather of Earth.
Weather occurs due to density (temperature and moisture) differences
between one place to another. These differences can occur due to the
sun angle at any particular spot, which varies by latitude from the
tropics. The strong temperature contrast between polar and tropical air
gives rise to the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth's axis is tilted relative to its orbital plane, sunlight
is incident at different angles at different times of the year. On
Earth's surface, temperatures usually range ±40 °C (±72 °F) annually.
Over thousands to hundreds of thousands of years, changes in Earth's
orbit affect the amount and distribution of solar energy received by
the Earth and influence long-term climate.
Surface temperature
differences in turn cause pressure differences. Higher altitudes are
cooler than lower altitudes due to differences in compressional
heating. Weather forecasting is the application of science and
technology to predict the state of the atmosphere for a future time and a given location. 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. Human attempts to control the weather have occurred throughout human history, and there is evidence that human activity such as agriculture and industry has inadvertently modified weather patterns.
Studying how the weather works on other planets has been helpful in
understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, 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.
Cause
On Earth, common weather phenomena include 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).[3] Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.[5]
Weather occurs primarily due to density (temperature and moisture)
differences between one place to another. These differences can occur
due to the sun angle at any particular spot, which varies by latitude
from the tropics. In other words, the farther from the tropics you lie,
the lower the sun angle is, which causes those locations to be cooler
due to the indirect sunlight.[6] The strong temperature contrast between polar and tropical air gives rise to the jet stream.[7] Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow (see baroclinity).[8] Weather systems in the tropics, such as monsoons or organized thunderstorm systems, are caused by different processes.
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).[9] 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).[10]
Uneven solar heating (the formation of zones of temperature and moisture gradients, or frontogenesis) can also be due to the weather itself in the form of cloudiness and precipitation.[11] Higher altitudes are cooler than lower altitudes, which is explained by the lapse rate.[12][13] 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 and its density.[14] 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.[15] 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 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.[16] 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.[17]
Chaos theory says that the slightest variation in the motion of the
ground 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.
The sun and oceans
can also affect the weather of land. If the sun heats up ocean waters
for a period of time, water can evaporate. Once evaporated into the
air, the moisture can spread throughout nearby land, thus making it
cooler.
Shaping the planet Earth
-
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.[18] These are then free to take part in chemical reactions that can affect the surface further (such as acid rain) or are reformed into other rocks and soils. In this way, weather plays a major role in erosion of the surface.[19]
Effect on humans
Badly flooded New Orleans, Louisiana after strong Category 3 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 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.[20]
French claims to Florida came to an end in 1565 when a hurricane
destroyed the French fleet, allowing Spain to conquer Fort Caroline.[21] More recently, Hurricane Katrina redistributed over one million people from the central Gulf coast elsewhere across the United States, becoming the largest diaspora in the history of the United States.[22]
Though weather affects people in drastic ways, it can also affect the human race in simpler ways. The human body is negatively affected by extremes in temperature, humidity, and wind.[23] Mood is also affected by the weather.[24]
Forecasting
-
Classical
from 
