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Dry Clouds

It is that time of the year when all of us look to the sky, waiting for the first drops of rain to fall on our faces. We wish the monsoon would come soon. But for people who live in polluted cities like Delhi or Tokyo, there's some bad news. Israeli scientists have discovered that air pollution may actually stop rain from falling.

To know how pollution may prevent rain from falling, it is necessary to know how and when rain falls. The sun beats down on the earth and heats up the land and water.

When the land and water have absorbed as much heat as they can, they throw the rest back, and that heats up the air. The air warms up and rises. It contains water vapour, or water in the form of gas.

As the air rises, it becomes cooler. The water vapour hits particles of dust that are always present in the air. It condenses, or gets changed into a dense or thick form. It forms droplets. When these droplets become too big and heavy, they fall back to the earth as rain.

Now, the smoke released by cars, trucks, buses, scooters and factories adds to the particles in the air around us. Because there are so many of these particles, many small droplets form, but they are not big enough to fall down as rain. These clouds are called dry clouds. Clouds that contain no rain.

The Israeli scientists took many pictures of these dry clouds caused by air pollution. They showed water droplets half the size of those present in clean clouds.

The only way to get back good rains is by cleaning up the environment. Factories and traffic must not release too much of smoke and trees must be planted wherever possible. Or, a time may come when clouds will form but no rain will fall.

Dry Clouds [Eureka: Science for Kids]

By Mohuya Choudhary

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