Atmospheric pressure at the Earth's surface is one of the keys to weather High and low are pressure are important because they affect the weather.

A "high" is an area where the air's pressure is higher than the pressure of the surrounding air.

A "low' is where the air pressure is lower. Meteorologists don't have any particular number that divides high from low pressure; it's the relative differences that count.

The pressure is high at the surface where air is slowly descending - much to slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally - but not quite always - associated with good weather.

The air that descends in high pressure areas, has to get to high altitudes in some way, and that way is by rising in areas where the pressure at the surface is low.

High Atmospheric Pressure:	Low Atmospheric Pressure

As air rises it cools.
As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there is enough water, or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather.

As shown in the graphic above, the air descending in high pressure flows out in a clockwise spiral in the Northern Hemisphere. Air flowing into an area of low pressure to rise, makes a counterclockwise spiral on the way in.

Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.

Back to Earth's Atmosphere Layers -Page 1:

Back to Air Pressure Measurement / Barometers - Page 2

To Wind: What Causes it? Notes page 4

To Fog: What causes fog and types of fog - Page 5

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