Humidity
- What is humidity?
- How do we measure relative humidity?

The World's Water

The World' s water About 70% of the Earth is covered with water. Most of this lies in the oceans. The Pacific Ocean alone covers almost half the world. Much of the rest of the water is in the ice sheets, glaciers and underground.

Water's disguises There are 3 forms of water in the air:
1. The gas water vapour.
2. Liquid water droplets.
3. Solid ice crystals. It changes from one form to another by evaporation, freezing, melting and condensation.

Did You know?
If all the water in the air fell at the same time, it would cover the whole Earth with 25 mm of rain. This amount of rain would fill enough buckets to reach from the Earth to the Sun 57 million times.

The Water Cycle

No new water is ever made. The rain you see has fallen millions of time before. In the water cycle the water on the Earth is used again and again. The Sun heats the oceans and lakes and millions of gallons of water rise into the air as invisible water vapour. This is called evaporation. As the vapour rises, it cools and turns back into liquid water. This is called condensation. It falls as rain and snow and is carried back to the ocean by rivers and streams. Then the cycle begins all over again.

What is Humidity?

The 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.

How do we measure relative humidity?

We use 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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