The contrast in weather between Flagstaff and Phoenix is one of the most dramatic temperature shifts between two major U.S. cities. Despite being separated by only a couple of hours, Flagstaff is typically 20 to 30 degrees Fahrenheit cooler than Phoenix year-round. This difference transforms the climate from the desert heat of the Valley of the Sun to the four distinct seasons of a mountain town. The primary cause of this stark temperature gradient is the significant difference in elevation between the two locations.
The Primary Influence of Elevation
The single greatest factor explaining Flagstaff’s colder climate is its altitude above sea level. Phoenix sits in the Sonoran Desert at a relatively low elevation of about 1,100 feet. In contrast, Flagstaff is situated high on the Colorado Plateau, with the city center resting at approximately 7,000 feet. This nearly 6,000-foot vertical difference exposes Flagstaff to the cooler layers of the Earth’s atmosphere.
Temperature decreases as altitude increases, a principle illustrated by these two cities. Air is thinner at higher elevations, meaning fewer molecules are available to absorb and retain heat energy. This reduced atmospheric mass acts as a less effective insulating blanket, allowing temperatures to drop more quickly. The elevation difference alone accounts for the majority of the constant temperature gap.
Understanding Atmospheric Cooling
The precise mechanism behind this cooling with elevation is the adiabatic lapse rate, a foundational concept in meteorology. As air rises from a low point like Phoenix to a high point like Flagstaff, it encounters progressively lower atmospheric pressure. This decrease in external pressure causes the air molecules to expand, which requires energy.
Since the air parcel is not exchanging heat with its surrounding environment, the energy needed for expansion is drawn from the internal kinetic energy of the air molecules, causing the air to cool. In the dry climate of Arizona, this cooling occurs at the dry adiabatic lapse rate. This rate specifies a temperature drop of about 5.5 degrees Fahrenheit for every 1,000 feet of ascent.
Applying this rate to the 5,900-foot difference shows a theoretical cooling of roughly 32 degrees Fahrenheit, closely matching the observed temperature contrast. This confirms that the physical process of air expansion is the dominant force driving Flagstaff’s lower temperature. Conversely, air that sinks is compressed by increasing pressure, causing it to warm at the same rate.
Secondary Local Climate Factors
Beyond the primary effect of elevation, several local geographic features enhance Flagstaff’s cold climate. One significant factor is the presence of snow cover for much of the winter. Flagstaff averages over 80 inches of snow annually, which often remains on the ground for extended periods.
Snow is highly reflective, a property known as high albedo, meaning it bounces a large percentage of incoming solar radiation back into space. This reflection prevents the ground from absorbing sunlight and warming the air above it. This keeps daytime temperatures lower than they would be over bare ground.
Flagstaff’s position at the base of the San Francisco Peaks also contributes to colder nighttime temperatures through cold air drainage. After sunset, air near the mountain slopes cools rapidly, becoming denser than the surrounding air. This heavy, cold air then flows downhill and settles in valleys where the city is located, causing temperatures to plummet overnight. This topographic effect pushes temperatures even lower than the lapse rate alone dictates.