Twists and Turns

Think of wind as a river. Water in a river flows from high elevations to low elevations. Likewise, wind wants to flow from a region with high atmospheric pressure to a region with low atmospheric pressure.

High pressure indicates fair weather: It inhibits the formation of clouds and storms, leaving mostly sunny skies, possibly a few popcorn-shaped cumulus clouds in the afternoon, and clear nights. At the center of a high pressure system, air is sinking. Surface-level winds spiral away from the high in a clockwise direction (in the Northern Hemisphere; the direction is reversed in the Southern Hemisphere).

Low pressure means cloudy or stormy weather. Wind spirals inward toward the center of the low in a counterclockwise direction. There's a convergence of air at ground level. Air rises, cooling to its dew point aloft and condensing to form clouds.

• 29.92 inches: the average atmospheric pressure at sea level
• 32.01 inches: the highest pressure ever recorded, during a cold snap in Siberia in 1968
• 25.63 inches: the lowest air pressure measured at sea level, during a typhoon that struck the Philippines in 1979

Figure 2-1: High pressure indicates fair weather: It inhibits the formation of clouds and storms, leaving mostly sunny skies, possibly a few popcorn-shaped cumulus clouds in the afternoon, and clear nights. (Photo courtesy of NOAA/Department of Commerce; http://www.photolib.noaa.gov; photo taken by Michael Van Woert, NOAA NESDIS, ORA