Mountain and valley breezes are predictable, localized wind systems characteristic of mountainous areas, collectively known as slope winds. These atmospheric circulations are driven by the daily cycle of heating and cooling between the mountain slopes and the adjacent air mass in the valley. This local wind pattern reverses direction daily.
The Driving Force: Differential Heating
The fundamental mechanism driving mountain and valley breezes is differential heating: the difference in how quickly land and air absorb and release heat. During the day, mountain slopes receive solar radiation at a more direct angle than the valley floor, causing the slope surface to warm rapidly. The air in contact with this heated surface warms quickly, becoming less dense than the air at the same altitude over the valley. This temperature difference creates a pressure gradient; the warmer, lighter air over the slope begins to rise, while the colder, denser air in the valley sinks. At night, this process reverses as the mountain slopes radiate heat away more efficiently than the air mass in the valley.
Valley Breezes (Anabatic Flow)
The valley breeze (anabatic flow) is the daytime component of this circulation, typically beginning in the mid-to-late morning. As the sun climbs, it heats the mountain slopes, transferring heat efficiently to the overlying air. This heated air becomes buoyant and begins to flow upward along the mountain’s incline. Cooler air from the valley floor is drawn up the slope to replace the rising air, creating the noticeable up-slope breeze. This flow often peaks in the early afternoon, with speeds commonly reaching 10 to 15 miles per hour. The upward movement of air can carry moisture aloft, leading to convection and the formation of clouds near the ridgeline or mountain crests.
Mountain Breezes (Katabatic Flow)
Following sunset, the process flips, initiating the mountain breeze (katabatic flow), the nighttime wind component. The mountain slopes quickly lose heat through radiation, cooling the surface and the air above it much faster than the air in the valley. This air becomes significantly colder and denser. Gravity then pulls this heavy, cold air to flow down-slope and into the valley floor. This drainage wind begins shortly after sunset and continues through the night, often following natural drainage lines like ravines or canyons. This flow often results in the pooling of cold air in the lowest parts of the valley, leading to a temperature inversion. This inversion traps pollutants near the ground and increases the likelihood of frost formation on the valley floor.