Wind is the flow of
gases on a large scale. On the surface of the Earth, wind consists of the bulk movement of air. In
solar wind is the movement of gases or charged particles from the Sun through space, while
planetary wind is the
outgassing of light
chemical elements from a planet's atmosphere into space. Winds are commonly classified by their
spatial scale, their
speed, the types of forces that cause them, the regions in which they occur, and their effect. The strongest observed winds on a planet in the
Solar System occur on
Saturn. Winds have various aspects, an important one being its
wind speed); another the density of the gas involved; another its energy content or
wind energy. Wind is also a great source of transportation for seeds and small birds; with time things can travel thousands of miles in the wind.
meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high-speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termed
squalls. Long-duration winds have various names associated with their average strength, such as
hurricane. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of
solar energy between the
climate zones on Earth. The two main causes of large-scale
atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (
Coriolis effect). Within the tropics,
thermal low circulations over terrain and high plateaus can drive
monsoon circulations. In coastal areas the
sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds.
In human civilization, wind has inspired
mythology, influenced the events of history, expanded the range of transport and warfare, and provided a
power source for mechanical work, electricity and recreation. Wind powers the voyages of
sailing ships across Earth's oceans.
Hot air balloons use the wind to take short trips, and powered flight uses it to increase lift and reduce fuel consumption. Areas of
wind shear caused by various weather phenomena can lead to dangerous situations for aircraft. When winds become strong, trees and man-made structures are damaged or destroyed.
Winds can shape landforms, via a variety of
aeolian processes such as the formation of fertile soils, such as
loess, and by
erosion. Dust from large deserts can be moved great distances from its source region by the
prevailing winds; winds that are accelerated by rough topography and associated with dust outbreaks have been assigned regional names in various parts of the world because of their significant effects on those regions. Wind also affects the spread of
wildfires. Winds can disperse seeds from various plants, enabling the survival and dispersal of those plant species, as well as flying insect populations. When combined with cold temperatures, wind has a negative impact on livestock. Wind affects animals' food stores, as well as their hunting and defensive strategies.