Source: The USA TODAY Weather Book, second edition
Graphic by Kevin A. Kepple
What's happening inside highs and lows
By Jack Williams, USATODAY.com
Atmospheric pressure at the Earth's surface is one of the keys to weather, which is one reason weather maps feature H's and L's, representing areas of high and low air pressure. High and low pressure areas are important because they affect the weather.
The weather maps, such as those on television, show what's happening at the Earth's surface, and that's what we're talking about here.
As the name says, a "high" is an area where the air's pressure is higher than the pressure of the surrounding air. A "low' is where it's 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 too 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 its done by rising in areas where the pressure at the surface is low.
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's 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 rises, making a counterclockwise spiral on the way in.
For more on why the wind flows in the directions it does, go to the USATODAY.com Guide to the science of the atmosphere and look at the links from the section on "The what and why of the wind."