Radiation fog (or ground fog) generally occurs at night, when radiational cooling of Earth's surface cools the shallow moist air layer near the ground to its dew point or below. This causes moisture in the nearby layers of air to condense into fog droplets. Radiation fog usually occurs under calm weather conditions, when no more than light winds mix the air layers near the surface. Strong winds normally mix the lower-level cold air with the higher-level dry air, thus preventing the air at the bottom from becoming saturated enough to create observable fog. The presence of clouds at night can also prevent fog formation of this type, because they reduce radiational cooling. Radiation fog often forms in late fall and winter nights, especially in lower areas, because cold and heavy air moves downhill to gather in valleys or other relatively low-lying areas. Accordingly, radiation fog is also called valley fog. In the morning, radiation fog usually dissipates or "burns off" when the Sun's heat warms the ground and air above the dew point temperature (i.e., the temperature at which moisture in the air condenses).