The strongest influence on the temperature of Baja California is the dissimilarity of its surrounding bodies of water. The Pacific Ocean dominates the temperature regime of western Baja California with its south-flowing, cold California Current, which is responsible for the coastal advection fog (Shreve 1951) and cloud cover that, with steady onshore breezes, keep the coastal temperatures relatively low. In consequence, the west coast of Baja California, from approximately El Rosario south to Todos Santos, is a cool coastal desert that Humphrey (1974) referred to as the Pacific Coastal Desert. In fact, the northern portion of this area, south to Punta Eugenia and inland 6 to 10 km, is one of only three fog deserts in the world (Meigs 1966). Its fogginess results from warm, moist belts of air cooling as they descend and contact the cold surface waters of the California Current. As this air cools, its moisture forms fog near the surface of the water and is carried inland by continuous onshore breezes. The California Current is submerged beneath the north-flowing, slightly warmer Davidson Current in the vicinity of Bahía Magdalena during the fall. Here, along the Magdalena Plain, the Pacific Ocean also influences the climate of the peninsula, although cloud cover is less and annual mean temperatures are slightly higher (Hastings and Turner 1969).
The temperature of the southern and eastern portions of Baja California is controlled primarily by the Sea of Cortés. A much warmer body of water than the Pacific (Robinson 1973), the Sea of Cortés offers little amelioration to the peninsula in the form of precipitation or cooling. The areas east of the Peninsular Ranges receive no cooling from onshore Pacific breezes, and consequently the Gulf coast becomes extremely hot during the summer (Markham 1972).
