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What causes high pressure?
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| High pressure areas form when an airmass cools over a cool land or ocean surface. The cooling causes the air
layer to shrink, becoming slightly thinner. This shrinkage then causes surrounding air in the upper troposphere to fill up the extra space. The
added weight of the extra air causes higher pressure at the surface. The higher pressure air at the surface then
tries to flow outward toward lower pressure, but as it does, the rotation of the Earth turns
the wind to the right, resulting in the clockwise wind flow around
the high pressure (in the Northern Hemisphere...it flows in the opposite direction in the Southern Hemisphere).
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| Interesting facts:
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| The highest surface air pressures occur in the winter over Asia and North America where cold
air masses form and become very deep. The world record for highest surface pressure (adjusted to sea level) was at Agata Lake in
Siberia on December 31, 1968 at 32.01 inches (1083.8 mb). Average sea level pressure is 29.92 inches (1013.23 mb),
making this record pressure 7% higher than normal.
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