Overrunning occurs when warm air flows over cold air at the earth's surface.
Warm air rises because it is lighter and less dense than the cold air.
As a result, the warm air can not push the cold air out of the way. As air rises it cools and moisture condenses into clouds and precipitation. Depending on the temperature of the warm air and the temperature and thickness of the cold air beneath it, rain, mixed precipitation , or snow can fall during overrunning. The most common form of overrunning is that associated with a storm 's warm front . However, overrunning can occur without a warm front or a storm, which can lead to significant rain or snow without any storms nearby.