Air Pressure and Humidity
Updated February 21, 2017 | Factmonster Staff 
- Air pressure is the weight of the atmosphere pressing down on the earth. It is measured by a barometer in units called millibars. Most barometers use mercury in a glass column, like a thermometer, to measure the change in air pressure.
- When the weather is calm the mercury in the barometer seldom moves more than half-an-inch below the 30-inch mark.
- If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.
- The weight pressing down on a one square-inch sample of air at sea level is 14.7 pounds, which is equivalent to a column of mercury 29.92 inches in height (1,000 millibars).
- Air pressure changes with altitude. When you move to a higher place, say a tall mountain, air pressure decreases because there are fewer air molecules as you move higher in the sky.
- Relative humidity is the amount of moisture the air can hold before it rains. The most it can hold is 100 percent. Humidity is measured by a psychrometer, which indicates the amount of water in the air at any one temperature.
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