Advection fog forms when a warm, moist air mass moves horizontally over a significantly colder surface, cooling from below until it reaches the dew point and producing condensation. Unlike radiation fog, advection fog can form with moderate winds (10–30 km/h) and at any time of day or night, as it depends not on radiative cooling but on the thermal contrast between the air and the surface. The most typical scenarios include: tropical maritime air moving over cold waters (California coast, coasts of Portugal and Galicia, Grand Banks of Newfoundland), warm continental air over snow-covered or frozen surfaces, and moist sea breezes over cold coasts. In Atlantic regions, advection fog is especially frequent when southwesterly winds bring humid subtropical air over cold ocean currents. Advection fog can be extremely dense and persistent—more so than radiation fog—because the mechanism continues as long as the warm air flow persists. On coasts, it can penetrate several kilometres inland. It poses a significant hazard for maritime navigation (it was the cause of the SS Andrea Doria disaster in 1956) and coastal aviation. Satellites are the best tool for delineating its horizontal extent, which can cover thousands of square kilometres.