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.

Heat island

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.

The monsoon is a seasonal shift in wind that brings more moisture into Arizona, triggering severe thunderstorms and forcing temperatures down. Serious monsoons occur in India and in much of Mexico (Acapulco averages 51.8 inches of rain during its summer monsoon and just 3.3 inches the rest of the year). Phoenix is on the northern fringe of the Mexican monsoon, and this year, it's been beyond the fringe much of the summer.

Phoenix gets about 7 inches of rain a year, and about a third of that comes during the monsoon. But this year, amid a 12-year drought, the city is already 2.4 inches below normal. The lack of summer rains have allowed heat to rule.

What is normal?

Another factor: "The term 'normal' does not adequately describe our typical weather," Haffer said. "Rather, in a desert, we experience wide shifts in the weather, from very hot to not so hot."

Reliable climate records in the United States go back just a century, so records of all sorts are bound to be broken frequently for decades to come. Breaking a record like Phoenix's number of hot days "is to be expected in a climate where extremes are common," Haffer said.

Finally, studies have suggested that global warming, while not fueling such dramatic numbers globally as what Phoenix has seen, will lead to more extreme weather events . One prediction: An extreme, 90-year drought for the American Southwest.

So does global warming have anything to do with this year's record heat in the Valley of the Sun?

"It is impossible to determine the impact of the warming of the global atmosphere relative to a given point on Earth," Haffer said. So: maybe, mabye not.