Air Pressure and Humidity Air pressure is the weight of the atmosphere pressing down on the earth. It is measured by a barometer in units called millibars. One thousand millibars equals one bar. Warm air is light and reduces pressure, so the barometer reading will be low. Cold air is heavy and increases the pressure, so the barometer reading will be high.
Warm air holds more moisture than cold air, so low pressure often means stormy weather.
Relative humidity is the amount of moisture the air can hold before it rains. Humidity is measured by a psychrometer, which indicates the amount of water in the air at any one temperature. Clouds are little drops of water or ice hanging in the atmosphere. A ceilometer measures the height of clouds. All Rights Reserved worldwide.