Radiation fog is the most common type of fog in continental areas and forms during clear nights with calm or very light wind and high relative humidity. The mechanism is straightforward: in the absence of clouds, the land surface loses heat efficiently through infrared radiation to space. The ground cools rapidly and, through conduction, cools the air layer in contact with it. When this layer reaches the dew point, water vapour condenses to form fog. Radiation fog is especially dense and persistent in valleys and river basins, where cold, dense air accumulates by gravity (katabatic drainage). Rivers and reservoirs provide additional moisture that promotes condensation. In continental regions, river valleys, plains, and inland basins are the most prone areas, with fogs that can persist for several days in winter under anticyclonic conditions with temperature inversions. This type of fog typically dissipates after sunrise, when solar radiation heats the ground and breaks the temperature inversion, although in winter it can persist all day if the inversion is strong and sunshine insufficient. Vertical thickness rarely exceeds 200–300 metres, allowing hilltops and elevated points to remain clear while valleys are covered—the spectacular "sea of clouds" phenomenon often photographed from mountain summits.