measure of the water vapour content in the atmosphere is called
humidity. All air contains some water vapour but the amount varies
greatly. Warm air can hold more water vapour than cold air. As a
result, when the humidity is high, the weather is humid. When the
humidity is low, the weather is dry.
do we measure relative humidity?
a hygrometer to measure the relative humidity of the air. A hygrometer
consists of a dry-bulb thermometer and a wet- bulb thermometer.
One end of the wet- bulb thermometer is covered with wet muslin.
Evaporation of water from the wet bulb lowers the temperature. When
the air is dry, more water will evaporate and the wet-bulb thermometer
will show a lower reading. The difference in the readings of the
two thermometers tell us the relative humidity of the air. Relative
humidity is expressed as a percentage (%). On very wet days, relative
humidity may be 100%. When the relative humidity is at 100%, the
air is saturated. If there is any excess water vapour, condensation
will take place.
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