The summer visitor to Owens Valley at midday may well think the valley and the apparently barren, flat-lighted slopes of the White Mountains to the east to be under the full possession of the sun, with a forbidding aspect of heat. The same visitor to elevations above 10,000 ft (3,049 m) would likely comment — perhaps complain — about coolness, or even the cold. In July, the warmest month, average daily temperatures are 70°F (21°C) or higher on all the adjacent valley floors; the maximum daily temperature reached 109°F (43°C) at Bishop in June 1969 and July 1982, 106°F (41°C) in June 1961 at Dyer, and 104°F (40°C) at Deep Springs in June 1964. The July average is 51.7°F (10.9°C) at White Mountain I and 46.7°F (8.2°C) at White Mountain II, with maximum summer temperatures reaching 79°F (26°C) at I in July 1967, and 73°F (23°C) at II in August 1978. The decline in average July temperatures with an increase in altitude is close to the 3.6°F drop per 1,000 ft (6.5°C per km) rise in elevation regarded as the normal lapse rate throughout the world. It is a rarity for any of the summer months at either mountain station not to have one or more readings of 32°F (0°C) or below, and even the adjacent valleys have such readings in June and August. For any location on earth, trees are generally absent if the average of the warmest month is below 50°F (10°C). This critical value occurs between White Mountain I and White Mountain II; the former has trees, and the latter has none.
In January, the coldest month, the average temperature is 37.2°F (2.9°C) at Bishop, with a winter low of -7°F (-21.5°C) in January 1982. Temperatures are somewhat colder in the valleys to the east, with Dyer at 31.4°F (-0.3°C) with a winter low of -21°F (-29.5°C) in January 1962 and 1974, and Deep Springs at 30.6°F (-0.8°C), with a low of -10°F (-23.5°C) in January 1973. Fish Lake and Deep Springs Valley are colder than Owens Valley because they are at higher elevations, they are farther from the maritime influence of the Pacific Ocean, and they are more open to invasions of cold air from the north and east. At White Mountain I January is the coldest month, with an average of 20.6°F (-6.3°C) and a winter low of -25°F (-31.5°C) in March 1968. February is the coldest month at White Mountain II, with an average of 14.8°F (-9.6°C) and a winter low of -35°F (-37°C) in March 1964. In winter there is less of a decrease in monthly average temperatures with a rise in elevation than in summer. Cold air settling in the lower valleys at night from radiational cooling and downslope drainage seems to be the major reason: the temperature difference between the valleys and the slopes or highlands is less pronounced for minimum readings than for daily maxima, especially on clear nights. This phenomenon is characteristic of high mountains everywhere. At White Mountain I, located in a valley, average minima for the three coldest months
(January, February, and March) are 2°F (1°C) warmer than at White Mountain II, which is 2,320 ft (707 m) higher on a slope; average maxima for those same months are 9.5°F (5.3°C) higher. Thus, a typical early winter morning at the lower station would be just about as cold as at the higher station, but midafternoon would be noticeably warmer.
There is also a much greater variation in average monthly temperatures in winter than in other seasons at all stations and elevations. At some time during most winters, there is an invasion of continental polar or arctic air from northern Canada and Alaska, which brings below-normal temperatures. The frequency and duration of these incursions of cold air vary greatly from year to year; on rare occasions they dominate the weather for weeks. The last such major occurrence was in January and February 1949, and in January 1937 before that. In 1937, a minimum of -42°F (-41°C) was recorded in January at Fish Lake Valley; readings close to that may have occurred within the mountains. In general, the White Mountains are protected from common and prolonged cold air invasions by the many mountain ranges to the north and east.
February and March averages are lower than those for January at White Mountain II, and they are only slightly higher than January averages at White Mountain I. This is not the case at the lowland stations in the area or at most inland stations in cold-climate regions elsewhere in the United States, where January is nearly always the coldest month, with definite warming occurring in February and, particularly, March. At the two high-elevation stations March is colder than December, an anomaly for the latitude. A continous snow cover during the winter and spring could partially account for this delay in warming at high elevation, but other regions in the United States with persistent snow cover do not show this effect. It is probably the result of the frequent passage of closed low-pressure systems in late winter and spring over the White Mountain area. An analysis of 500 mb (near 18,000 ft or 5,500 m) weather maps shows that these closed lows commonly contain very cold air, which could affect the highest elevations of the White Mountains. At lower elevations the increased solar radiation from longer days and a higher angle of the sun would offset the influence of the cold air aloft. Thus, there is a pronounced lag in temperature increase in late winter and early spring at high elevation in the White Mountains. Significant warming there usually does not occur until mid-May, when the frequency and intensity of the upper-level closed lows diminish. Major cooling throughout the area generally comes in late September and October, at a slower rate at high elevation than the warming in late spring, with a marked decrease in November at all stations.
Midwinter temperatures at White Mountain I are comparable to those in central Iowa, and at White Mountain II to southern Minnesota or Anchorage, Alaska. Summer temperatures at White Mountain I are similar to those at the northern limit of trees in Alaska and Canada, and at White Mountain II to the treeless Arctic Slope of Alaska. At comparable elevations and latitudes, temperatures in the White Mountains are generally warmer in winter and cooler in summer than in Utah or Colorado, reflecting less maritime influence farther inland. From meager data and personal ex-
perience, there seem to be no significant temperature differences between the White Mountains and the Sierra Nevada at similar elevations, although the Sierra might be expected to have warmer winters and cooler summers because of closer proximity to the Pacific Ocean. It is difficult to compare climates in different mountain ranges because of topographical variations at individual recording sites — north slope, south slope, valley, orientation to prevailing winds, and other factors.