When you are standing on the ground the atmosphere above you is exerting pressure on you. This pressure is exerted in all directions ( remember demo with pop bottle and collapsing pop can). The pressure map (above right) shows the isobars, lines that connect points of equal pressure (std pressure 1013mb), This difference in pressure generates winds as the air moves horizontally from an area of high pressure to an area of low pressure. The closer these isobars are to each other, steep pressure gradient, the stronger the wind. remember these differing areas of pressure are caused by the unequal heating of the Earth's surface by the Sun. In the northern hemisphere low pressure systems blow counterclockwise toward the center- rising air. High pressure systems blow out from the center and rotate clockwise.
Winds are influenced by friction with the Earth's surface and the Coriolis effect, above left. TheCoriolis effect influences the direction of the wind. In the northern hemisphere all free-flowing objects are deflected to the right and in the southern hemisphere to the left.
If you recall from our daily weather reports low pressure systems, mid-latitude cyclones, move from west to east across the United States. Low pressures, also known as cyclones, typically produce precipitation. High pressure systems, anticyclones, typically produce fair-weather. Also, remember from our daily weather reports that these systems are also steered by the jet stream- a high altitude, high velocity wind.