Air-mass weather is the net effect of local surface circulation, terrain, and the modifying effect of significant water bodies. There are many subdivisions of weather regions in the United States. The northwest Pacific coast area has more precipitation than any other region in North America. Its weather is primarily the result of frontal phenomena, consisting mainly of occlusions which move in over the coast from the area of the Aleutian low and orographic lifting of moist, stable maritime air. Predom-inant cloud forms are stratus and fog, which are common in all seasons.
Rainfall is most fre-quent in the winter and least frequent in the summer.
The southwest Pacific coast area experiences a Mediterranean-type climate and is distinctively different from any other North American climate. This climate occurs exclusively in the Mediterra-nean and southern California in the Northern Hemisphere. In the Southern Hemisphere, it occurs over small areas of Chile, South Africa, and southern Australia. This climate is characterized by warm to hot summers, tempered by sea breezes, and by mild winters during which the temperatures seldom go below freezing. Little or no rainfall occurs in the summer and only light to moderate rain in the winter.