UNITED STATES WEATHER 

The weather in the United States, with minor exceptions, is typical of all weather types within the temperate regions of the North American, European, and Asiatic continents. The general air circulation in the United States, as in the entire temperate zone of the Northern Hemisphere, is from west to east. All closed surface weather systems (highs and lows) tend to move with this west-to-east circulation. However, since this is only the average circula-tion and weather systems move with the general flow, the fronts associated with the migratory lows also tend to move southward if they are cold fronts and northward if they are warm fronts. Surface low-pressure centers, with their associated weather and frontal systems, are referred to as cyclones. Knowledge of the mean circulation in the temperate region makes it possible to observe and plot average storm tracks and to forecast future movement with a reasonable degree of accuracy.

Certain geographical and climatic conditions tend to make specific areas in the United States favorable for the development of low-pressure systems such as west Texas, Cape Hatteras, cen-tral Idaho, and the northern portions of the Gulf of Mexico. Once a low has formed, it generally follows the same mean track as the last low that formed in that area. The average, or mean paths, are referred to as storm tracks.

These storms (lows) are outbreaks on the polar front or the generation or regeneration of a storm along the trailing edge of an old front. The low pressure along these fronts intensifies in certain areas as the front surges southward ahead of a moving mass of cold polar air. Much of the weather, especially the winter weather, in the temperate zone is a direct result of these storms.

Air-mass weather also affects temperate climates. Air-mass weather is the name given to all weather other than the frontal weather in the temperate region. 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. For the purpose of this discussion, we have divided the con-tinental United States into seven regions as indicated in figure 6-7-3.

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.

Cold fronts rarely penetrate the southwest Pacific coast region. The weather over this region is due to the circulation of moist Pacific air from the west being forced up the slope of the coastal range. In the summer, air is stable, and stratus and fog result. In the winter, unstable air which is forced over the mountain ranges causes showers or snow showers in the mountains.

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