Low-pressure systems can become storms. A low-pressure system is a large weather generating system that surrounds a center of low pressure. In the Northern Hemisphere, the air in a low-pressure system circles in a counterclockwise direction. It moves downward toward the lowest pressure then is forced upward to higher altitudes. Upward motion of air creates more falling pressure which, in turn, causes high winds and stormy weather. On a weather map, low-pressure centers are represented by a red, capital L.

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A high-pressure system is formed when air moves around a high-pressure center. High pressure centers are large and slow-moving. The air in these systems moves in a clockwise direction in the Northern Hemisphere (due to the Coriolis Effect). Air slowly sinks downward and as it gets closer to the ground it spreads out towards areas of lower pressure. Generally, high-pressure systems bring clear skies and calm air. On a weather map, high-pressure centers are represented by a blue, capital H.
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Large weather systems are created by the interaction of high and low-pressure systems. The air moves down, out and around a high-pressure center then it swirls around and into a low-pressure center and moves upward. Highs and Lows will affect each other as they move across the surface of the Earth.
Weather Changes as Air Masses Move

Low Pressure Systems Can Become Storms

Vertical Air Motion Can Cause Severe Storms

Weather Forecasters Use Advanced Technologies