Weather Regimes

Figure 1.1 shows the principal airflow patterns and air-mass types or source regions that determine regional weather in different seasons of the year. The arrows indicate the directions of air movement near and above the crest of the major mountain ranges, at levels between 10,000 and 20,000 ft (3 to 6 km) above sea level. The open circles are locations from which twice-daily (near 4 A.M. and 4 P.M. PST) rawinsonde balloon ascents are made to obtain data on air temperature, humidity, and wind velocity, from which upper-air (e.g., 500 mb) weather maps are plotted and analyzed. The black circle indicates the location of the Bishop Airport, the nearest (3 mi, or 5 km) National Weather Service station to the White-Inyo Range.

The air that flows across California at any time of year is most likely to have passed over some part of the Pacific Ocean. In summer the Pacific Anticyclone (a large, slow-moving clockwise whirl of air) lies just west of California, bringing an onshore flow of cool marine air, stratus clouds, and fog to the coast and mostly clear, dry air to the Sierra and White-Inyo Range. During much of the summer the Great Basin Anticyclone develops over the warm plateau region of Nevada and Utah. When this whirl expands and shifts westward, a flow of moist maritime tropical air from the Gulf of California or the Gulf of Mexico may persist for a few days before the normally dry Pacific flow reasserts itself. Thus, during the summer season the mountainous terrain of eastern California and western Nevada is contested for by two air masses, with that from the northern or central Pacific usually prevailing.

Figure 1.1
Major airflow patterns and air mass types affecting California and the Great
Basin. Rawinsonde stations shown are:

During fall, winter, and spring a series of traveling upper-air troughs (cyclonic bends with counterclockwise flow) and ridges (anticyclonic bends with clockwise flow) cross California and Nevada. Ridges and anticyclones usually bring subsiding air, few clouds, and "fair" weather, whereas the troughs and associated cyclones, low-pressure centers, and fronts bring much cloudiness and widespread precipitation. Fronts are boundaries between converging air masses from different source regions. The primary air masses affecting California are cold maritime polar air from the Gulf of Alaska and warmer, moist maritime subtropical air from lower latitudes. Occasionally there are invasions of cold continental polar air from northern Canada or the Rocky Mountains.