What Causes Winds
Wind is caused by differences in atmospheric pressure created, in large part, by the unequal heating of the earth's surface by the sun. Air moves from a region of higher pressure to one of lower pressure and this movement is wind. Any difference in pressure will cause wind, but the greater the difference the stronger the wind.
The direction that wind takes is influenced by the rotation of the earth. On a nonrotating earth wind would move in a straight path from a high- to a low-pressure area. It is deflected from this path—to the right in the Northern Hemisphere and to the left in the Southern—by the turning of the earth on its axis.
Local winds—those that affect a comparatively small area—are often caused by heat transferred by convection. Direct radiation from the sun does little to heat the air. It is warmed chiefly by heat radiated from the earth. Intense local heating of the land causes air directly above to become greatly heated and to expand. As a result, some of the air aloft flows away, lowering the pressure over the heated area and increasing the pressure around it. The cooler, heavier air near the earth then flows to the heated area.
In mountainous areas, winds tend to blow uphill during the day because the mountainside is heated more than the valley below it. At night, when the mountainside cools, the wind blows downhill. In summer, breezes tend to blow from oceans or large lakes to the warmer land during the day. They blow from the land at night, when the land cools.