A mountain breeze is characterized by a downhill flow of air from the slopes toward the valley floor. It is part of a larger, daily cycle of air movement driven by the differential heating and cooling of the landscape. The breeze generally occurs after sunset and continues through the night, transporting cold air from higher elevations to the lower-lying valleys. This wind is one half of a pair of thermal circulations that define daily wind patterns in mountain regions.
The Physics of Slope Winds
The mountain breeze forms due to nocturnal radiational cooling. After sunset, mountain slopes rapidly lose the heat absorbed during the day through longwave radiation. This rapid cooling causes the air immediately in contact with the slopes to cool down.
The chilled air becomes denser compared to the air mass at the same elevation further out over the valley. Gravity pulls this cold, dense air downslope in a process known as katabatic flow. This downward-moving air drains into the valley, often following existing channels like canyons and riverbeds. The mountain breeze is typically a gentle wind, flowing at speeds in the range of 5 to 10 miles per hour.
The Full Diurnal Cycle: Mountain vs. Valley Breeze
The mountain breeze is the nighttime phase of the diurnal cycle of local wind patterns. Its counterpart is the valley breeze, or anabatic flow, which dominates during the day. The entire cycle is driven by the regular shift between solar heating and terrestrial cooling.
During the day, solar radiation heats the mountain slopes, which warm the adjacent air. This warmer, less dense air rises up the slope, creating the valley breeze. The valley breeze can reach speeds of 10 to 15 miles per hour and often contributes to cloud formation over the peaks. As the sun begins to set, the upslope flow weakens and reverses as the slopes cool faster than the valley air, initiating the transition to the mountain breeze.
The transition periods, occurring shortly after sunrise and just before sunset, are characterized by a brief time of calm or variable winds as the circulation shifts direction. The mountain breeze establishes itself in the evening, moving cold air down the slopes and along the valley floor until sunrise. Once the sun reappears, the lower valley air heats up and the downslope flow is replaced by the rising valley breeze.
Influence on Local Weather and Ecology
The mountain breeze influences local microclimates and ecosystems. As the cold, dense air flows down the slopes, it pools in the lowest parts of the valley floor, displacing the warmer air upward. This process leads to a temperature inversion, where the air temperature increases with height for a few hundred feet above the valley bottom.
These cold air pools create “frost pockets,” making the valley floor susceptible to early and late frosts. Conversely, the mid-slopes, positioned above the cold air pool, experience warmer temperatures, creating a “thermal belt.” This thermal belt influences agricultural and vegetation patterns. The mountain breeze also influences air quality and moisture, often contributing to the formation of radiation fog in the valley or concentrating pollutants.