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The air moving away from the High pressure system leaves a “hole” to be filled, so air from above sinks into that “hole”.
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High Pressure Systems

You may have seen a weather map with a blue H on it. This blue H denotes a high pressure system in that area of the map. Just what does that mean?

The H for high pressure systems and alternatively the L for low pressure systems are relative measures of the pressure in that particular system compared to the pressure around the system. There are no exact measurements that would make a particular system a High and another a Low. It's all relative!

The differences in pressure from one place to another on Earth are what cause wind. You see, air wants to move from areas of high pressure to areas of low pressure. Away from areas of high pressure and into areas of low pressure.

The Coriolis Force also adds rotation to the movement of air around pressure systems. In the Northern Hemisphere, air moves clockwise around a High and counter-clockwise around a Low. In summary, air moves away and clockwise from a High and into and counter-clockwise from a Low.

Now the air moving away from the High pressure system leaves a "hole" to be filled, so air from above sinks into that "hole". As the air sinks, it experiences an increase in pressure and an increase in temperature. Any water in the air tends to evaporate because of the increase in temperature. This means the water is not in the condensed form needed to make clouds and precipitation. That's why many people equate seeing a blue H on a weather chart with good weather and clear skies!

Last modified April 19, 2009 by Jennifer Bergman.

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