PHOENIX — Here in the Valley, the milestone temperature is 110 degrees. It inevitably hits every summer, but residents really pay attention when it happens on consecutive days.
The most consecutive days hitting 110 degrees occurred in June of 1974 with eighteen days. Another long heat wave, 17 days, happened 21 years later. The ongoing heat wave ranks fourth between 2007 and 2020 with 11 days over 110 degrees. With more days in the forecast over that mark, there is plenty of time for it to move up in the rankings.
When not considering consecutive day heat waves, 2020 takes the crown of summer months with the highest number of 110 degrees days and it’s not close, with 53 days in total. The next highest years range between 28 days and 33 and all happened after 2000. The other four years are spread farther apart; 1936, 1974, 1979, and 1985.
Not only is hitting 110 degrees or higher a more common occurrence, but daily lows between June 1 and July 9 since temperature recording began in 1896 are trending up. The minimum and maximum annual lows were consistent in the Valley from 1896 to 1960 between 59 degrees and 83 degrees with an average of 71.5 degrees. Today the minimum and maximum lows are between 70 degrees and 90 degrees with an average of 78 degrees. These are the morning temperatures the Valley experiences just as the sun begins to rise.
The increase in lows is primarily due to the urban heat island effect which has a bigger impact on overnight temperatures.
A similar analysis of daytime highs since 1896 does not have the same dramatic rise in the last half of the twentieth century, although average daily highs have increased about five degrees from 100 degrees to 105 degrees.
The rise also makes an impact on the lengthy heat waves. The longest one in 1974 had an average daily temperature of 96.5 degrees. Last year’s extended wave of over 110 degrees lasted 10 days but had a daily average of 100.8 degrees. This year’s average so far is 99.4 degrees.
