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What causes low pressure?
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Low pressure areas form when an airmass warms, either from being over a warm land
or ocean surface, or from being warmed by condensation of water vapor in large rain or snow systems.
The warming causes the air layer to expand upward, becoming slightly thicker. This expansion then causes
air in the upper troposphere to flow away, leaving less mass, and so less weight (pressure) at the surface.
The lower pressure air at the surface then causes higher pressure air around it to flow toward lower pressure,
but as it does, the rotation
of the Earth turns the wind to the right, resulting in the counter-clockwise wind flow around
low pressure (in the Northern Hemisphere...it flows in the opposite direction in the Southern Hemisphere).
Outside of the tropics, low pressure centers are usually associated with extratropical cyclone systems,
along with their fronts and precipitation systems.
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| Interesting facts:
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| The lowest air pressure in the world occurs in intense
tropical cyclones where condensation of water vapor to form
clouds and rain releases heat that gets concentrated in the eye of the storm. The
lowest pressure ever recorded was in the eye of Typhoon Tip, in the
tropical western Pacific Ocean, on October 12, 1979: 25.69 inches of mercury (870 millibars).
Since average sea level pressure is 29.92 inches (1013.23 millibars),
this record pressure was over 14% lower than normal.
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