Daly City is usually blanketed with heavy fog and swept by brisk sea breezes from mid-May to mid-September. While Daly City shivers, the summer fog and sea breezes provide natural air conditioning for the rest of the Bay Area. The breezes begin when California's Central Valley heats up in summer; as the inland valley air warms, it rises and draws cool coastal air in to replace it. The coast is cooled by summer fog that forms when warm moist sea air blows over cold ocean waters upwelling along the coast in summer. Daly City gets an extra share of fog and wind because the coastal mountains keep most of the fog from moving inland, but the San Bruno Gap, a break in the mountains, funnels fog and wind inland right over the top of Daly City.