The air all around us contains water. We don't normally see it as it exists as a gas, as water vapour, rather than solid water or ice.
Although it has no smell, you can still feel it, recall walking into a bathroom after someone has just taken a bath or shower, the air feels clammy, just like on a hot day. If the light is right, you may even be see millions of tiny droplets floating around.
Notice how in both situations the air was warm, either from the heat of the bath water, or the sun on a hot day. Hot air is able to hold a lot more water vapour than cold air, and as the air cools so the water vapour is squeezed out. It has to go somewhere, so it condenses into larger droplets of water - the same thing that happens when the warm air from the shower hits the cold window pane.
If you ever watch a cloud floating past you'll noticed that the edges are constantly shifting, growing and decreasing. This is because the air in and around the cloud is warming and cooling as it moves along. If the air is cooling, so the water vapour is squeezed into tiny droplets of water that are blown about by the wind.
As the air cools and more and more vapour is squeezed into droplets, so the clouds grow. The droplets start to clump together and become larger, making the clouds appear denser. Eventually they form large enough drops that the air cannot support them and gravity pulls them down towards the ground as rain.
The conditions for the air warming and cooling vary, but can be broken down into three general groups, or types of rain.
Convectional Rain
Convection, or rising hot air, is caused by the sun warming the ground. This in turn warms the air sitting on the ground. As the air warms it expands and absorbs water vapour. When it expands it becomes less dense, so the cooler air above falls down and the warm air with its fresh load of water vapour rises upwards.
As the air moves away from the ground up into the sky, so it cools down again (usually by about 1 degree Celsius for every 100m it rises). Eventually the air cools and contracts to a point where it starts to squeeze out the water vapour and water droplets form.
The clouds formed in this way are normally cumulus or cumulonimbus (storm) clouds. If it's a hot day, this process can be quite vigourous and produce spectacular storm clouds with heavy rain, thunder and lightning. Quite often a cloud can form over a field or car park in a very short space of time.
Frontal Rain
When you listen to the weather forecast, you'll hear the weather forecaster talk of warm or cold fronts. These are the front edges of large masses of cold or warm air trundling across the surface of the Earth.
When these these air masses meet the warm front will rise up over the top of the cold front. As it rises it starts to cool and clouds start to form. In this case the clouds tend to be vast sheets of unbroken grey cloud that fill the sky.
Relief Rain
Just as the warm air rises up over cold air, it can also rise up over physical features such as hills or mountains. Again, as the warm air rises it cools and if the conditions are right, clouds form.
As the air moves along and falls down the far side of mountains or hills, so it warms again and the water droplets can turn back to vapour, resulting in clear skies. This is known as a rain shadow and such areas are often prone to drought.
About this Author
Andy Coughlan is a freelance writer, filmmaker and Science Fiction geek. When he's not hammering out screenplays, he's usually found looking for bargains on UK Hot Deals or working on his online novel, The Elementalist.
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