However, if clouds are present, some of the heat emitted from the earth's surface is trapped by the clouds and remitted back towards the earth. As a result, temperatures decrease more slowly than if the skies were clear. Factors to consider when forecasting precipitation. Clouds and precipitation are formed by the upward motion of air. Therefore, there must be a mechanism present to lift the air.
Fronts often serve as such a mechanism.
Air on one side of the front typically blows in a different direction from the wind on the other side, causing the air to converge, or pile up right along the frontal surface. Since this air has to go somewhere, it rises. As air rises, the moisture in the rising air cools, condenses and forms clouds and precipitation. For example, a cold front lifts warm moist air ahead of it as it advances. The rising air cools and the water vapor condenses out to form clouds, most commonly ahead of and along the cold front.