Average monthly atmospheric vapor pressure at site #2 showed little annual variation, ranging only about 3 mbar throughout the year (Figure 8). Other sites exhibited a similar pattern. This parameter is largely affected by large air mass movements; and since subtropical air masses do not move into this region during the summer, the monthly changes in atmospheric vapor pressure change little during the course of the year. However, because of the large annual change in air temperature and the nonlinear dependence of the evaporative gradient on temperature, relative humidity levels are substantially lower and evaporative gradients are substantially higher during the summer months.

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.