What causes Arizona monsoons?

Dear Tom,

I just moved to Arizona after 72 years in Chicago, and the monsoons are a new experience. What causes them?

— Daniel Murphy, Chandler Ariz.

Dear Daniel,

The term monsoon refers to a seasonal change in wind direction driven by huge temperature differences that develop between land masses and a nearby ocean. In winter, when the air over land is relatively cool and oceanic air is warm, air flows from land to water. In summer, it’s the opposite; air flows from water to land. Most people think of southern Asia when monsoons are mentioned, but the Arizona (and southwestern U.S.) monsoon is real, extending from about mid-July through mid-September. While producing only a fraction of south Asia’s huge monsoon rain totals, the Arizona monsoon rainfall isn’t always light. In 1964, Tucson recorded 13.49 inches of rain, well above its two-month monsoon-season average of 5.46 inches.