Air Pressure Systems — Highs and Lows
AIR PRESSURE is the weight of the air pressing down on a place. It changes constantly with temperature and altitude. Weather maps mark areas of HIGH pressure (H) and LOW pressure (L). The DIFFERENCE between highs and lows drives wind and weather across the planet.
High vs. low. HIGH PRESSURE: cool, dense air SINKS. As it sinks, it warms slightly and dries out. Skies tend to be CLEAR with light winds. LOW PRESSURE: warm air RISES. As it rises, it cools, water vapor condenses into clouds, and PRECIPITATION often forms. Storms (including hurricanes) are intense low-pressure systems. Wind blows FROM high pressure TOWARD low pressure — that's where the energy flow is.
A barometer (which measures air pressure) is FALLING rapidly. What weather change is most likely?
Pressure measurement. STANDARD ATMOSPHERIC PRESSURE at sea level is about 1013 millibars (or 29.92 inches of mercury). Hurricanes can drop to under 900 millibars at the center. Mountains have lower pressure than valleys. Pressure also changes with weather — that's why your ears pop on planes (cabin pressure changes) or in elevators.
Pressure Hunt
Look at a current weather map online (search "US weather map current"). Find the highs (H) and lows (L). What weather is each region having? Notice clear skies under highs and storms near lows.
Air pressure is the engine of weather. Once you understand highs and lows, weather forecasting starts to make sense — and you can read maps like a meteorologist.
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