Weather
is the state of the atmosphere, describing for example the degree to which it
is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather
phenomena occur in the lowest level of the atmosphere, the troposphere, just
below the stratosphere. Weather refers to day-to-day temperature and
precipitation activity, whereas climate is the term for the averaging of
atmospheric conditions over longer periods of time. When used without
qualification, "weather" is generally understood to mean the weather
of Earth.
Weather
is driven by air pressure, temperature and moisture differences between one
place and another. These differences can occur due to the sun's angle at any
particular spot, which varies with latitude. The strong temperature contrast
between polar and tropical air gives rise to the largest scale atmospheric
circulations: the Hadley Cell, the Ferrel Cell, the Polar Cell, and 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 (−40 °F to 100 °F) annually. Over thousands of years, changes in Earth's
orbit can affect the amount and distribution of solar energy received by the
Earth, thus influencing long-term climate and global climate change.
Surface
temperature differences in turn cause pressure differences. Higher altitudes
are cooler than lower altitudes, as most atmospheric heating is due to contact
with the Earth's surface while radiative losses to space are mostly constant.
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 Earth's
weather system is a chaotic system; as a result, small changes to one part of
the system can grow to have large effects on the system. Human attempts to
control the weather have occurred throughout history, and there is evidence
that human activities such as agriculture and industry have modified weather
patterns.
Studying
how the weather works on other planets has clarified 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.
