In spite of the shade afforded the ground by the oak brush, temperatures reached the same maximum values as at the drainage site, owing to the orientation of the slope. South-facing slopes receive more direct insolation throughout the day and throughout the year than north-facing slopes and mesa tops (Geiger, 1965:374). In Mesa Verde, south-facing slopes tend to be more arid; snow melts rapidly, and most of this moisture evaporates. As a consequence, south-facing slopes have less soil moisture and more widely-distributed vegetation than north-facing slopes where snows often persist all winter and melt in spring. (For a detailed discussion of climates on northeast-versus-southwest-facing slopes in Mesa Verde, see Erdman, Douglas, and Marr, in press.)
Pinyon-Juniper-Muttongrass Site, 7,600 feet elevation
The station was in the trapping grid at D5b ([Fig. 13]). The pinyon-juniper woodland surrounding this site resembles much of the woodland on the middle part of the mesa. The forest floor is well shaded by the coniferous canopy, and muttongrass is the dominant plant in the ground cover. P. truei lives in this habitat.
The climate at this site is moderate. Shade from the canopy greatly moderates the maximum air temperatures during the day; minimum air temperatures, however, are about the same as at the other stations ([Table 4]). Mean temperatures are somewhat lower at this site than at the others because of the lower maximum temperatures. Relative humidities do not differ markedly from those at other stations.
[Figure 14] shows hygrothermograph traces at all stations for a typical week. An interesting phenomenon is illustrated by several of these traces. By about midnight, air temperatures have cooled to within a few degrees of their nightly low. At this time, heat is given up by the surface of the ground in sufficient quantities to elevate the air temperature at ground level. This release of reradiated energy lasts from one to several hours, then air temperatures drop to the nightly low just before sunrise. A depression in the percentage of relative humidity accompanies this surge of warmer air. On some nights winds apparently disturb, or mix, the layers of air at ground level. On such nights the reradiation of energy is not apparent in the traces of the thermographs. Reradiation of energy is restricted to ground level, and traces of hygrothermographs in standard Weather Bureau shelters, approximately four feet above the ground surface, at other sites on the mesa top did not record it.
Fig. 12: (left) Photograph of microclimatic station at the Oak Brush Site, at trapping station L4a of the grid south of Far View Ruins. (right) General view of the stand of Gambel oak in unit L of the trapping grid.