Humidity in Phoenix, Arizona Today

Phoenix sits in the Sonoran Desert, and for most of the year that fact defines its humidity story: single-digit relative-humidity readings on a June afternoon are routine, dew points sit in the 30s and 40s°F, and the "dry heat" cliché is genuinely accurate. With annual rainfall averaging roughly eight inches and clear skies the norm, the air in Phoenix is among the driest of any major American city for nine or ten months of the year. Evaporative coolers — swamp coolers — work here precisely because the baseline air is so dry.

Then the North American Monsoon arrives, usually in late June or early July, and the entire equation changes. The monsoon is a seasonal wind shift that draws tropical moisture north from the Gulf of California and the eastern Pacific. Dew points leap from the 30s into the 60s and even low 70s°F within days. The relative humidity can climb from 10% at 3 PM to 60% by midnight, and the combination of 105°F heat with a 65°F dew point produces a heat index that feels radically different from the dry 110°F of June. Haboobs — massive dust storms pushed by thunderstorm outflow — are a direct product of this moisture surge collapsing onto the desert floor.

The monsoon typically lasts through mid-September, then retreats almost as abruptly as it arrived. By October, the desert air is dry again. This sharp seasonal binary is unique among major US cities: Phoenix is either very dry or surprisingly humid, with almost no in-between. For the millions of residents and visitors who experience 110°F in June and think they understand desert heat, a 105°F monsoon afternoon with a 68°F dew point is a completely different — and far more taxing — physical experience.