Number of Days with Minimum Temperature above 90 Degrees
This graph shows the number of days that the low temperature, recorded at the Phoenix Sky Harbor International Airport climate monitoring station, was higher than ninety degrees for the years 1948 through 2013. Nighttime minimum temperatures above ninety degrees started occuring in the 1970s. This is an indication of the urban heat island phenomenon of higher nighttime temperatures in the urban core compared to the surrounding rural countryside. In each decade since, the number of days with a minimum temperature above ninety degrees has increased: from three in the 1970s, to six in the 1980s, to eight in the 1990s, to fifty in the most recent decade of 2000-2009. Although much of this increase has to do with the urban heat island effect, rising global temperatures may also contribute to warmer summer temperatures, resulting in higher nighttime lows.