Arizona’s low desert regions, including Phoenix and Tucson, experience some of the most intense summer heat in North America. Daytime temperatures routinely surpass 100°F for months, creating a sustained period of extreme heat. The transition to comfortable weather is not abrupt but follows a distinct seasonal pattern influenced by changes in atmospheric circulation. Understanding these local climate mechanisms helps define when a true, lasting cool-down can be expected in the lower elevations.
Defining the Seasonal Transition in the Low Desert
The true, sustained cooling period in the low desert typically begins in late September and reliably settles in throughout October. The first significant temperature drop happens as the sun angle lowers and atmospheric conditions shift. September functions as a transitional month, with average high temperatures in Phoenix still around 99°F, meaning triple-digit heat remains frequent early in the month.
The defining metric for the end of summer is when the average daytime high consistently dips below 90°F. Historically, this reliable transition occurs around the end of October or the beginning of November. The average last date for a 100°F day in Phoenix is October 5th, and the average last date for a 90°F day is October 30th.
This period is often called “second summer” because the intense heat can linger past the astronomical start of autumn. Once November arrives, the average high temperature in Phoenix drops to about 75°F. This shift marks the beginning of the mild winter season, which is a direct result of the desert land retaining heat and high solar radiation continuing into the fall.
The Role of the Arizona Monsoon
The Arizona Monsoon is a seasonal shift in wind patterns occurring from approximately June 15th through September 30th. This change draws moisture from the Gulf of California and the Eastern Pacific, altering the weather from the intense, dry heat of early summer. The monsoon introduces atmospheric moisture but does not signal the end of the heat.
This influx of humidity creates conditions for afternoon and evening thunderstorms, producing heavy rainfall, lightning, and dramatic dust storms (haboobs). While storms temporarily cool the air, the added moisture increases the dew point, resulting in muggier heat. The heat index can increase, making the days feel heavier despite a potential drop in the thermometer reading.
The increased moisture and cloud cover limit the most extreme daytime high temperatures compared to June. However, the humidity prevents efficient nighttime cooling, meaning low temperatures often remain elevated, sometimes above 80°F. The monsoon provides a temporary break in the dry heat, but it is distinct from the final seasonal cooling that arrives in the fall.
Altitude and Regional Variations in Cooling
The cooling timeline varies significantly across Arizona due to its vast size and diverse topography. The primary difference is dictated by altitude, separating the climate of the low desert from the high country. For every 1,000 feet of elevation gain, the temperature can decrease by as much as 5.5°F.
High-altitude regions, such as Flagstaff, Prescott, and the White Mountains, sit on the Colorado Plateau. These areas experience a temperate climate that dramatically predates the low desert’s transition, with elevations often between 5,000 and 7,000 feet. Flagstaff, for instance, sees average summer highs in the 70s or low 80s, not the triple digits common in Phoenix.
By September, the high country is already experiencing comfortable fall temperatures, and nighttime frost becomes a possibility by October. This rapid cooling contrasts sharply with the low desert’s prolonged summer. This highlights that Arizona’s climate is a collection of distinct regional environments defined by elevation.