General and Local Winds

Wind is created by a variety of  differing conditions.  In order to help understand them better, we will divide them into two categories. 

Gradient winds or general winds are named because of the gradient that exists between a high and low pressure system.  A high pressure system is a large invisible mountain of air in the atmosphere.  A low pressure system is like a sinkhole.  Around a low pressure system the air flows in a counterclockwise manner into the area of low pressure.  Around a high pressure system the air flows in a clockwise manner.  These highs and lows are caused by the differential heating of the earth’s surface.

Local winds are so named because of the effect that local conditions and terrain have in its creation.  Land surfaces warm rapidly when exposed to the direct rays of the sun (radiation). Earth is a poor conductor of heat so it stays at or very near the surface. Warming is less rapid over water because the heat penetrates deeper.  As a result, the heat is spread out through a large volume of water.  As daytime temperatures increase, the warmer air over the land begins to slowly rise.  The cooler, more dense air located over the water moves inland to fill the area of low pressure created by the rising hot air.  In turn this air becomes heated also and rises.  An air circulation pattern forms, causing a convective cell over a very small, localized area.  The result is a “sea-breeze.”

Vertical movement of air in the atmosphere.

The reverse of the air circulation takes place at night.  The land mass cools more rapidly  and becomes more dense.  As the air over the water becomes less dense and warmer than the air over the land mass, the air over the water rises.  Cooler air from the land moves in to replace the air that has risen.  The cool air becomes heated and continues to rise, causing a convective circulation.