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Rainfall over Denmark Precipitation occurs when the air becomes saturated with water vapour: it can hold no more. Air containing a lot of moisture is described as humid. Fully saturated air has 100% humidity. When saturated, the water vapour in the air starts to condense (turn from gas to liquid) around tiny particles in the air, such as sea salt or dust. Millions of these tiny droplets gather together to form clouds. In the highest clouds, the water freezes into ice. When the ice or water droplets become too heavy to stay up, they fall or "precipitate" to the ground as rain—or snow if the air below is freezing.

{alt}Time lapse video of storm clouds forming{more}Click to play video


Air may become saturated when it cools (warm air can hold more water vapour than cool air) or when more and more water vapour is added to the air. The temperature at which moist, cooling air becomes saturated and starts to condense is called the dew point. We can see this process at work when clouds form. Air cools when it rises and expands (adiabatic cooling), or when it comes into contact with a cooler surface. Water vapour becomes added to the air as a result of evaporation from oceans and lakes or transpiration from plants.

Not all precipitation reaches the ground. If it evaporates before doing so, it is called virga. Winds may sometimes cause virga to fall at an angle, making the clouds appear to have commas attached.


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