PHOENIX—The forecasted high here today is 111 degrees. If the mercury hits that mark, the city will set a record for the most days in a year above 110 degrees, at 29.

The record was tied yesterday when the high hit 111.

The new mark is the pinnacle of a stark trend. The average number of 110+ days in Phoenix has climbed from 6.7 per year in the 1950s to 21.6 per year so far this decade.


The urban heat island is partly to blame, said Tony Haffer, a meteorologist at the National Weather Service office in Phoenix. Concrete, pavement and buildings retain more heat than the natural landscape. That heat is released slowly, causing morning temperatures to be about—ballpark estimate, Haffer said—5 degrees warmer in the city.

Afternoon highs are less influenced by the heat island, but are likely warmer than they would be, Haffer said in a telephone interview, adding that the issue needs more study.

The lack of monsoon rains this summer has played a role, too.