Air Temperatures

Air temperatures have been collected from automated weather stations at Red Butte #2, Red Butte #4, and Red Butte #6 since 1982. Mean monthly air temperatures at Red Butte #2 were below freezing in December and January and above 20 C in June, July, and August (Figure 8). In contrast, mean monthly temperatures at Red Butte #6 were below freezing only slightly longer, from November through February, and above 20 C in July and August. During the main growing period (May through September), daytime maximum temperatures ranged between 18.7 and 31.8 C (66 - 89 F) at Red Butte #2, while nighttime minimum temperatures ranged between 5.2 and 16.4 C (41 - 62 F) (Figure 9). At the higher-elevation stations, daytime maximum air temperatures were lower. The difference in maximum temperatures was negatively related to elevation (maximum temperature [°C] = 34.3 - 0.00494 • elevation [m], r = .91) at approximately half the dry adiabatic lapse rate. On the other hand, nighttime minimum temperatures were not related to elevation, because of cool-air drainage effects (Figure 9). Red Butte #4 is located streamside within the canyon, whereas the other two stations are above the channel of cold air that develops at higher elevations and pours down the canyon at night. As seen in Figure 9, this cold-air drainage effect at Red Butte #4 (1890 m [6180 ft] elevation) depressed nighttime minimum air temperatures by 4 - 8 C (7 - 14 F) below that observed at Red Butte #6 (2230 m [7292 ft] elevation).

Fig. 8. Mean monthly air temperature, vapor pressure, and photosynthetically active solar radiation (400-700 nm) measured at Red Butte #2 between 1982 and 1990.

Fig. 9. Mean monthly maximum and minimum air temperature at Red Butte #2 (1653 m elevation), Red Butte #4 (1890 m elevation), and Red Butte #6 (2195 m elevation) during the growing season between 1982 and 1990.