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