Wind refers to any flow of air relative to the earth's surface in a roughly horizontal direction. Breezes that blow back and forth from a body of water to adjacent land areas—on-shore and off-shore breezes, or land and sea breezes—are examples of local wind. Winds, driven by large pressure systems also exist in great wind belts that comprise the earth's atmospheric circulation.

The ultimate cause of Earth's winds is solar energy. When sunlight strikes Earth's surface, it heats that surface differently. Newly turned soil, for example, absorbs more heat than does snow.

Uneven heating of Earth's surface, in turn, causes differences in air pressure at various locations. On a weather map, these pressure differences can be found by locating isobars, lines that connect points of equal pressure. The pressure at two points on two different isobars will be different. A pressure gradient is said to exist...