Because air is fluid, it can move easily from place to place. Differences in air pressure cause the air to move. A wind is the horizontal movement of air from an area of high pressure to an area of lower pressure. Winds are caused by differences in air pressure.

 

Most differences in air pressure are caused by the unequal heating of the atmosphere. Convection currents form when an area of Earth's surface is heated by the sun's rays. Air over the heated surface expands and becomes less dense. As the air beomes less dense, its air pressure decreases. If a nearby area is not heated as much, the air above the less-heated area will be cooler and denser. The cool, dense air with a higher pressure flows underneath the warm, less dense air. This forces the warm air to rise.

Local winds are winds that blow over short distances. Local winds are caused by the unequal heating of Earth's surface within a small area. Local winds form only when large-scale winds are weak.

 

Unequal heating often occurs along the shore of a large body of water. It takes more energy to warm up a body of water than it does to warm up an equal area of land. As the sun heats Earth's surface during the day, the land warms up faster than the water. As a result, the air over the land becomes warmer than the air over the water. The warm air expands and rises, creating a low-pressure area. Cool air blows inland from over the water and moves underneath the warm air, causing a sea breeze. A SEA BREEZE or a lake breeze is a local wind that blows from an ocean or lake. Below you will see a diagram of a sea breeze.

 

At night, the process is reversed. Land cools more quickly than water, so the air over the land becomes cooler than the air over the water. As the warmer air over the water expands and rises, cooler air from the land moves beneath it. The flow of air from land to a body of water is called a LAND BREEZE. Below you will see a diagram of a land breeze.

 

 

Global winds are winds that blow steadily from specific directions over long distances. Like local winds, global winds are created by the unequal heating of Earth's surface. But unlike local winds, global winds occur over a large area. 

 

Temperature differences between the equator and the poles produce giant convection currents in the atmosphere. Warm air rises at the equator, and cold air sinks at the poles. Therefore air pressure tends to be lower near the equator and greater near the poles. This difference in pressure causes winds at Earth's surface to blow from the poles toward the equator. Higher in the atmosphere, however, air flows away from the equator toward the poles. Those air movements produce global winds.

 

If Earth did not rotate, global winds would blow in a straight line from the poles toward the equator. Because Earth is rotating, however, global winds do not follow a straight path. As the winds blow, Earth rotates from west to east underneath them, making it seem as if the winds have curved. The way Earth's rotation makes winds curve is called the Coriolis effect. Because of the Coriolis effect, global winds in the Northern Hemisphere gradually turn toward the right. A wind blowing toward the south gradually turns toward the southwest. In the Southern Hemisphere, winds curve toward the left.