Comparing Temperature and Humidity on a Mountain Slope and in the Free Air Nearby

Abstract

Surface temperature and humidity data measured by eight remote weather stations on a south-facing slope in the San Bernardino Mountains of Southern California were compared with temperature and humidity data measured by a rawinsonde at the same times and elevations in the free air. The eight stations, located on slopes, in canyons and on ridges, ranged in elevation from 590 to 1621 m. The data were for 0400, 1000 and 1600 PST on 19 days during June?October 1975, the primary forest-fire and brush-fire season in Southern California. The temperature on the mountain slope at 0400 PST was 2.0?4.8°C cooler than in the free air nearby, whereas at 1000 PST the mountain slope was 1.9?5.1°C warmer than the free air, and at 1600 PST the slope was only 0.5?2.9°C warmer than the free air. The average lapse rate of temperature in the free air (590?1621 m) was 0.01°C (100 m)?1 at 0400 PST, ?0.4°C (100 m)?1 at 1000 PST, and ?0.8°C (100 m)?1 at 1600 PST, while on the mountain the lapse rate, as expected, was strongly affected by the complex terrain. The mixing ratio at 0400 PST was 0.1?2.3 g kg?1 lower than in the free air, except at 875 m where the mixing ratio was 0.1 g kg?1 higher. At 1000 PST the mountain slope had mixing ratios that were 0.9?2.1 g kg?1 higher than the free air, and at 1600 PST the mixing ratios on the slope were still 0.5?1.6 g kg?1 higher than the free air. Regression models were developed to diagnose and predict surface temperature and mixing ratio at eight sites on the mountain slope from observed or forecast data in the free air nearby. The standard errors of estimates were ≤ 1.90°C for temperature and ≥2.2 g kg?1 for mixing ratio.