Humidity describes the amount of water vapor suspended in the air. Phoenix is situated within a hot desert environment, which naturally limits the available moisture in the atmosphere. Understanding the humidity requires looking beyond a single number, as the daily and seasonal fluctuations are substantial. This arid setting profoundly affects the local weather experience.
The Dry Reality of Phoenix Relative Humidity
Relative humidity (RH) is the standard measurement, representing the ratio of water vapor in the air to the maximum amount the air can hold at a specific temperature. The overall annual average RH in Phoenix is remarkably low, typically settling around 36% to 37%. Because warmer air holds more moisture, the RH percentage changes dramatically throughout the day, even if the actual amount of water vapor remains constant.
This fluctuation creates a significant difference between morning and afternoon conditions. Early morning readings, when temperatures are coolest, often see the RH rise to a range of 47% to 51%. As the intense desert sun heats the air, the afternoon RH plunges, frequently bottoming out around 23%. This daily cycle highlights the extreme dryness that characterizes the desert environment.
Seasonal Fluctuations and the Monsoon Effect
Phoenix experiences its most extreme dryness during the late spring and early summer, typically in May and June. During this period, the average monthly relative humidity can drop to its lowest annual point, sometimes registering between 16% and 19%. Afternoon readings often fall into the single digits.
The atmospheric moisture changes drastically with the arrival of the North American Monsoon season, which officially runs from mid-June through September. This seasonal shift draws moisture from the Gulf of California and the Gulf of Mexico into the region. This influx causes the average RH to climb noticeably, with July and August averages reaching approximately 27% to 32%.
During active monsoon periods, the air’s dew point—an absolute measure of moisture—can temporarily exceed 70°F, creating a noticeably muggy feel. Individual days with active storm cells can push the relative humidity above 50%. This temporary elevation in moisture leads to thunderstorms and disrupts the city’s otherwise consistently arid climate.
How Low Humidity Affects Comfort
The low relative humidity allows for efficient evaporative cooling, which significantly impacts how high temperatures feel. When moisture on the skin, such as sweat, evaporates quickly into the dry air, it draws heat away from the body. This effect makes the extremely hot temperatures, particularly in May and June, far more tolerable than equivalent temperatures in a humid climate.
In the summer, the heat index often remains close to the actual air temperature due to the minimal moisture. However, during the monsoon season, temporary humidity spikes can make the heat index feel significantly worse. A 105°F day with 50% relative humidity feels much more oppressive than a 115°F day with 15% relative humidity.
The persistent dryness also affects health. Extremely low humidity levels pull moisture from the body, often leading to common complaints like dry skin, irritated eyes, and respiratory discomfort. The dry air can dehydrate mucous membranes, potentially increasing susceptibility to sinus irritation.