Warm temperatures are one of the region’s defining characteristics. Regional temperatures run higher compared to northern climes because the subtropics receive more direct sunlight, and there is little water available to temper its power. In the Southwest, the cool season begins roughly in October and lasts through March, while the warm season smothers the other half of the year. According to the Western Regional Climate Center, in Arizona "high temperatures are common throughout the summer at the lower elevations and temperatures over 125 degrees F have been observed in the desert area. The daily range between maximum and minimum temperatures sometimes runs as much as 50 to 60 degrees F during the drier periods of the year. During winter months, daytime temperatures may average 70 degrees F, with night temperatures often falling to freezing of slightly below in the lower desert valleys."
Temperatures are not independent of precipitation and a related factor, humidity. When humidity levels are low, most commonly during winter and spring, temperature has greater daily swings (Figure 1). When humidity levels are relatively high, such as during the summer monsoon, temperatures often fluctuate less dramatically from day to night. When there’s not much water available for evaporation, more of the sun’s energy can go toward boosting temperatures. That’s partly why Phoenix temperatures can regularly climb above 110 degrees F while the mercury rarely surpasses 100 degrees in more humid climates even closer to the equator. Because water vapor is a greenhouse gas, though, its presence in the atmosphere tends to hold in heat that might otherwise escape. In desert climates, this influence is most noticeable on summer nights, when cloudy skies can keep nighttime temperatures higher than they are on clear nights.