The first week of August, 1976, brought unusually low minimum temperatures not only to Yosemite National Park, but to much of the west coast. As an example, Fresno recorded a week of continuous, record-setting low minimum temperatures.

Low temperatures resulted from a stable and persistent upper level low which remained over the state for nearly a week. Upper level lows over the Park are not uncommon occurrences in summer, but this particular one was associated with very cold and clear weather. During August, 1975, an intense upper level low brought snow and cloudy days to many parts of the High Sierra; temperatures in most areas did not reach to the lows observed August, 1976. John Muir noted, in The Mountains of California, that the ‘middle region of the Sierra is usually well flecked with rains and light dustings of snow . . .’ in summer.

Yosemite National Park, as a function of its topography, is blessed with many atypical conditions, weather and climate included, which result in a variety of environments. Topography, elevation and exposure work in concert to produce complex and unusual weather phenomena and microclimates. Often one reads of ‘normal’ temperature lapse rates (air cools approximately 3.2� F. for each 1,000’ rise in elevation), but in mountains, environmental lapse rates are so variable that to consider them ‘normal’ is almost meaningless.

August 1-7, 1976, has shown dramatically the complexity of temperature distributions in Yosemite National Park. Air drainage (cold air flowing to lowest elevations because of its greater density than warm air) and the consequent development of a temperature inversion (warm air lying above a shallow layer of cold air), phenomena so well expressed in the Park during winter, appeared in unusual form in early August.

A glance at minimum temperatures recorded at five different locations in the Park for August 1-7, reveals the effects of microclimate locations and local circulation factors associated with mountainous terrain. Some of the minimum temperatures and their place of observation are truly surprising, as the following table illustrates.

MINIMUM TEMPERATURES Yosemite National Park August 1-7, 1976
August	Valley	Wawona R/S	So. Entrance	Tuolumne Meadows	Tioga Pass
	(3,970')	(3,985')	(5,120')	(8,600')	(9,945')
1	43�F	42�F	50�F	32�F	41�F
2	44	35	54	29	40
3	43	40	54	26	39
4	42	32	42	27	34
5	42	32	44	27	38
6	43	37	43	28	34
7	43	34	45	27	33
Note: Temperatures were recorded by max-min thermometers or hygrothermographs.

Although minimum temperatures in the Valley were not unusually low (similar temperatures have been recorded there often during past summers), Wawona Ranger Station (R/S), at a similar elevation, but in a different geographic setting, registered minimum temperatures from as much as 10�F cooler, to freezing. Note the warm minimum temperatures observed at the South Entrance as compared with any other station, especially Wawona or the Valley. South Entrance, located in a site not conducive to air drainage, averaged the warmest minimum temperature of all five locations, 47�F. On 2 August, 19�F separated minimum temperatures between South Entrance and Wawona.

Most dramatic, however, is the comparison of minimum temperatures between Wawona, Tuolumne Meadows and Tioga Pass. On three of the seven nights Wawona recorded temperatures lower than at Tioga Pass, nearly 6,000’ higher! Even on the other four nights, no large differences in minimum temperatures were observed. Allowing for instrument error, suspect calibration and observational errors, the similarity in minimum temperatures is remarkable. Particular attention is called to Tuolumne Meadows which registered significantly lower temperatures than did Tioga Pass (a 13�F difference on 3 August).

For the most part, these minimum temperatures can be explained in terms of exposure and local circulation. Tuolumne Meadows and Wawona are located in sites somewhat conducive to collection of still and calm air. Under these conditions, air drainage occurs and, with clear nights, radiative cooling produced marked temperature inversions. The Valley also experiences temperature inversions, especially in winter, but because of the large heat sink afforded by the Valley walls and because temperatures are recorded on the warmer north side of the Valley, minimum temperatures remained relatively high.

Often with a temperature inversion only a small elevation gain results in a substantial temperature increase. In North Wawona, less than 150’ higher than Wawona R/S, an unofficial minimum temperature at 38�F was observed while Wawona R/S registered a low of 32�F (4 August). On 3 August, Wawona R/S recorded a minimum temperature of only 40�F, yet campers in Wawona Campground, about 150’ lower, observed formation of ice on sleeping bags and tables, a sure indication of cooler temperatures. The campground approximates the lowest elevation in the area because it is situated along the course of the Merced River’s south fork. Consequently, cold air collects here, stagnates and continues to cool radiatively under clear skies.

Temperature inversions most typically are broken by air movement. Such a condition prevailed at Tioga Pass (9,945’) where strong winds continually removed potentially cold and stagnant air. Air drainage occurred at Tuolumne Meadows (8,600’), but at Tioga Pass, air was kept in constant motion. Consequently, minimum temperatures remained above freezing at Tioga Pass. For all seven days, Tioga Pass had an average minimum temperature 7�F warmer than Tuolumne Meadows.

On a walk to Mono Pass and the Golden Crown Mine on 6 August, we saw much ice in protected spots along the trail and near the pass, even at midday. No ice was seen at the pass itself because, like Tioga Pass, strong winds kept air circulating and the pass is exposed to warmth of the early morning sun. In any case, even without freezing temperatures, the wind chill factor at the pass was significant!

Finally, it was interesting to note during the week that chickarees, gray squirrels and ground squirrels were all very active, rapidly eating green cones. Possibly they were anticipating the early arrival of winter, as were many people in campgrounds!

References

John Muir (1894, 1961), The Mountains of California, The Natural History Library, Anchor Books, Doubleday and Company, Inc. 300 pp. footnote page 209.

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