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Rain, snow, hail, dew

The earth has a role to play in making wind. It also has an important role in making rain. But without the sun and air, earth just cannot cause weather. Weather is the changes happening in the atmosphere and this change is brought about by the sun, air and earth. Earth surface is made of both land and water. Rain, fog, snow, mist, clouds, hail, frost and dew, all these words tell something about water. All of them are water in weather in one form or another. But why does water come down sometimes as rain and sometimes as snow or hail? And how did the water get up into the clouds there in the first place?

Water can exist in three forms – solid (ice), liquid (water) and gas (water vapour) depending on what temperature it is. Cool water and it becomes ice, heat water and it becomes water vapour. Once it becomes water vapour, it joins the other gases in air. Hence air, apart from nitrogen, oxygen, carbon-di-oxide, also contains water vapour. But the amount of water vapour it contains varies from 0 to 4 percent. When there is a lot of water vapour in air, its humidity is said to be high and when these is less water vapour, the humidity is low.

Water evaporates from the earth all the time. It evaporates from damp soil, from wet washed clothes, from puddles, from lakes and rivers and oceans. Transpiration of plants also contributes water vapour. Water vapour is constantly being formed. Remember, to produce water vapour, you need not boil water!

The water vapour stays in the air as long as the air is warm. When air gets cooled, the water vapour becomes water or snow (ice) depending on how cool it gets. Now let us see what role earth has in bringing about precipitation, that is rain, snow, hail, dew, fog, etc.

Dew

At night, when the land cools off, it cools the water vapour in air near its surface. Then the water vapour condenses against any cool surface. It forms a thin film of water on rocks and pebbles, on soil and plants. On the leaves of plants it cools into little beads of water that we call dew. All night long the dew clings to the cool surfaces along the ground. In the morning, sunlight makes the dewdrops sparkle with rainbow colours. Soon as the sunlight warms the ground and the things on it dew evaporates into the air, invisible until the cool of the night comes once again. From this it is also clear that dew can form only when the ground gets cooler than air, as in winter days.

Fog

Sometimes the percentage of water vapour in air is very high. When air cools, vapour begins to condense against anything cool. Some of the water vapour condenses on the dust particles and other specks in the air. Around each particle, a film of water forms as a tiny drop. Billions and billions of these drops swirling in the air form a fog in winter. When sun warms the land, the tiny drops evaporate once more into invisible water vapour and the air is clear again.

Clouds

Sometimes water vapour is carried higher up by rising warm air. High above, the temperature of air is very low. When the water vapour cools up there, it condenses on grains of dust in the same way as fog. Such a high-up fog is called a cloud.

On a clear day, when you look up at what was a cloudless sky, you may see a cloud that wasn't there a few minutes earlier. How did it get there? Water evaporates all the time. It gets into air. Warm air rises and as it gets higher and higher, it starts to get cooler and cooler. When the air gets cool enough, water can no longer exist as vapour and it condenses. Water need some surface to condense and this is provided by small dust particles in air. When lots of water vapour condenses, it forms a cloud. To begin with clouds as small and white. They begin to grow larger and larger as more and more water condenses. This is when the clouds begin to become darker and darker and of course bigger. When the drops of water become very big, they drop down as water. When clouds are very high, the rain water is chill, as it happens in winter. When the clouds are not very high, the rain is warmer.

In some places in winter, water in the clouds get so cold, that they condense into ice and this causes a snowfall.

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