Air takes on different characteristics according to where it has been: air from a tropical ocean, for instance, is warm and wet, while air from an arctic land mass is cold and dry. When two distinct air masses meet, the boundary between them is called a front.
If the air masses are moving, with warm air following cool air, it is called a warm front. If cool air follows warm air, it is a cold front. On a weather map, a warm front is usually marked with semicircles or coloured red, and a cold front is marked with triangles or coloured blue.
Cold fronts generally move faster than warm fronts. Where a cold front catches up with a warm front, they combine to form an occluded front or occlusion, which may be described as warm or cold depending on the change of temperature as it passes. Wind direction is likely to change as a front passes.
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Rain, hail and snow
At all three types of front, the warm air rises, forced upwards by the cooler air. As it rises, it expands and cools, so any water vapour it contains condenses to form clouds and then drops as rain, hail or snow. The slope of a cold front is much steeper than that of a warm front, so the process is more turbulent, producing more cumulus clouds and shorter but heavier bursts of rain.