6abc Weather School - What is Pressure?

The Delaware and Lehigh Valleys can see a wide variety of weather.

Dramatic swings in weather are caused by the Earth's atmosphere exerting different amounts of pressure on the surface of the earth, causing movement of air, vertically and horizontally.

Winds around high pressure are usually light and blow in a clockwise direction in the Northern Hemisphere.

High pressure means the atmosphere is exerting a high level of pressure on the surface of the Earth, where we live. That forces the air to sink - that's called subsidence. The sinking air dries and spreads out, typically bringing sunny, dry weather. If you're planning a pool party, you want to see high pressure on the weather map.

With low pressure, winds blow in the reverse direction: counterclockwise and with moist air. Since the atmosphere is exerting low pressure on the surface of the Earth, air is able to rise, allowing moisture to condense into clouds and rain, or snow if it's the winter. If you see low pressure heading our way on the weather map, find the umbrella... or the snow shovel!

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