Arizona’s climate is a dynamic system defined by extremes, regional differences, and a powerful seasonal event, making it difficult to categorize as simply dry or humid. The state is fundamentally characterized by an arid environment—a long-term lack of moisture where water demand exceeds precipitation supply. This baseline condition is modified by massive elevation changes and a yearly influx of atmospheric moisture that temporarily changes the air’s nature. Understanding the overall climate requires looking at these geographical factors alongside predictable seasonal shifts.
Defining Arizona’s Dominant Aridity
Arizona’s default state is aridity, characterized by low average annual precipitation and high rates of evapotranspiration. This imbalance is primarily influenced by the state’s geographic location near the 30th parallel. Here, descending air associated with a persistent subtropical high-pressure system suppresses cloud formation, creating conditions that favor clear skies and dry air for much of the year.
Arizona is also placed within a significant rain shadow cast by mountain ranges to the west. Pacific storms moving eastward must rise over the Sierra Nevada mountains in California, causing the air to cool and shed moisture as precipitation on the western slopes. By the time this air descends into Arizona, it is depleted of water vapor, contributing to the extremely low relative humidity experienced in the state.
This geographic and atmospheric setup results in characteristic dry conditions, particularly during the late spring months of May and June. The lack of moisture allows for a large diurnal temperature range; the difference between day and night can often exceed 40 degrees Fahrenheit. Since dry air cannot hold heat efficiently, rapid cooling occurs once the sun sets.
Regional Climate Variation Across the State
Arizona’s vast size and dramatic elevation changes create distinct microclimates that vary significantly from the arid norm. The state is broadly divided into two major climate zones: the Low Desert in the south and the High Desert and Mountain zones in the north and central regions. This topographical diversity means not all parts of the state are classified as true desert.
The Low Desert, including areas like Yuma and Phoenix, sits at elevations near or below 1,500 feet and has a subtropical arid climate. This region receives the least annual precipitation, sometimes averaging only three to four inches per year, with annual average relative humidity values as low as 33 percent. Summer temperatures regularly exceed 105 degrees Fahrenheit, defining the hot and dry experience associated with Arizona.
In contrast, the mountainous regions and the Colorado Plateau in the central and northern parts of the state experience cooler, semi-arid, or even humid continental conditions. Elevations exceeding 5,000 feet, such as near Flagstaff and the White Mountains, can receive between 25 and 30 inches of annual precipitation. These areas sustain extensive pine forests, have lower summer temperatures, and see average annual humidity values rise to around 55 percent, departing significantly from the dry desert air of the lowlands.
The Seasonal Shift to Humidity: The Monsoon
The most profound, temporary shift in Arizona’s climate occurs during the North American Monsoon (NAM), which typically runs from mid-June to mid-September. This seasonal weather pattern dramatically alters the state’s moisture profile, temporarily introducing humidity to the predominantly dry air. The shift involves intense solar heating over the Southwest, which creates a thermal low-pressure area.
This low-pressure system reverses typical wind patterns, drawing in warm, moist air from the Gulf of California and, to a lesser extent, the Gulf of Mexico. This influx of tropical moisture, referred to as a “gulf surge,” causes a significant rise in the air’s dew point and relative humidity, particularly in the Low Desert. The increased atmospheric moisture fuels powerful, sudden, and intense convective thunderstorms.
The monsoonal rains are responsible for a substantial portion of the state’s annual precipitation, often accounting for more than 50 percent of the yearly total. This seasonal moisture results in heavy downpours and flash flooding, and also affects nighttime temperatures. Since water vapor is an effective greenhouse gas, the increased humidity traps heat, leading to warmer overnight lows and reducing the large diurnal temperature swing characteristic of the arid season.