<P> 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 . </P> <P> 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 . </P> <P> Wind is caused by differences in the atmospheric pressure . When a difference in atmospheric pressure exists, air moves from the higher to the lower pressure area, resulting in winds of various speeds . On a rotating planet, air will also be deflected by the Coriolis effect, except exactly on the equator . Globally, the two major driving factors of large - scale wind patterns (the atmospheric circulation) are the differential heating between the equator and the poles (difference in absorption of solar energy leading to buoyancy forces) and the rotation of the planet . Outside the tropics and aloft from frictional effects of the surface, the large - scale winds tend to approach geostrophic balance . Near the Earth's surface, friction causes the wind to be slower than it would be otherwise . Surface friction also causes winds to blow more inward into low - pressure areas . </P> <P> Winds defined by an equilibrium of physical forces are used in the decomposition and analysis of wind profiles . They are useful for simplifying the atmospheric equations of motion and for making qualitative arguments about the horizontal and vertical distribution of winds . The geostrophic wind component is the result of the balance between Coriolis force and pressure gradient force . It flows parallel to isobars and approximates the flow above the atmospheric boundary layer in the midlatitudes . The thermal wind is the difference in the geostrophic wind between two levels in the atmosphere . It exists only in an atmosphere with horizontal temperature gradients . The ageostrophic wind component is the difference between actual and geostrophic wind, which is responsible for air "filling up" cyclones over time . The gradient wind is similar to the geostrophic wind but also includes centrifugal force (or centripetal acceleration). </P>

What is the main source of energy for wind currents on earth