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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 means there is a high pressure system over that area of the map. Just what does that mean?

There are no exact measurements that would make a system a High. It's all relative! If a certain system is a High pressure system, that means it has higher pressure then the areas around it.

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. In the Northern Hemisphere, air moves clockwise around a High and counter-clockwise around 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, water that was in the condensed form (like clouds) tends to evaporate into water vapor. This means the water is not in the condensed form needed to make clouds and precipitation. That's why many people see a blue H on a weather chart and know good weather and clear skies are coming!

Last modified April 19, 2009 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA