This horizontal movement of air is the phenomena we call wind . What Causes the Air to Move Horizontally? Wind is the result of horizontal differences in air pressure . Air flows from areas of high pressure to areas of low pressure . Wind is nature’s attempt to balance larger-scale inequalities in air pressure .
What causes air pressure to differ from place to place? Air pressure differences originate from unequal heating of Earth’s surface (both land and water surfaces). Solar radiation is the ultimate driving force of wind. If Earth did not rotate, and there were no friction, air would flow directly from areas of higher pressure to areas of lower pressure. But Earth does rotate , and the atmosphere, especially below 2300 feet (700 meters) , does have friction , so pressure differences from place to place are not the only factors affecting wind direction and speed . There are three factors affecting wind direction and speed: 1) Pressure Gradient Force 2) Coriolis Effect 3) Friction
Pressure Gradient Force Horizontal pressure differences : a) have magnitude (indicated by the spacing of the isobars). b) have direction (always from higher to lower pressure). Horizontal pressure differences cause the wind to blow. The greater those differences , the greater the wind speed . Pressure differences are recorded from hundreds of weather stations across Earth’s surface. Pressure differences are shown on maps by isobars , which are lines connecting places of equal air pressure.
The spacing of the isobars indicates the amount of pressure change occurring over a given distance and is expressed as the pressure gradient . Closely spaced isobars indicate a steep pressure gradient and strong winds . Widely spaced isobars indicate a weak pressure gradient and light winds .
Other Characteristics of the Pressure Gradient Force Pressure gradient force is always at right angles to the isobars. Temperature variations create pressure differences and hence wind; the greater these temperature differences , the stronger the pressure gradient , and the stronger the resultant wind . Daily temperature differences, and the pressure gradients so generated, are confined to a zone that is only a few kilometers thick . Movement of air horizontally is initiated by the pressure gradient. Other factors affecting wind speed and direction (Coriolis Effect & Friction) only modify this movement of wind , they do not produce it .
Earth’s Rotation 24 hours in duration Every point on the Earth takes the same 24 hours to rotate. Rotation 1) determines day length 2) creates the apparent deflection of winds and ocean currents 3) produces the twice-daily rise and fall of the ocean tides (in relation to the gravitational pull of the Sun and the Moon) Rotation is counterclockwise . Rotation is eastward when viewed from the equator.
Coriolis Effect Named after the French scientist who discovered it.
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