Mountain Valley Breezes

Mountain-valley breezes are common in areas with significant topographic relief. During the day, as the Sun heats up the land and air at the valley bottom and sides, a valley breeze develops. When the air heats, it becomes less dense and more buoyant, allowing it to start flowing up the valley sides. The vertical rise of the air along the sides of the mountain is restricted by a temperature inversion layer, confining the airflow to the valley (not letting the rising air escape into the atmosphere). Once the rising air pushes against the inversion, it moves horizontally toward the center of the valley, and drops toward the valley floor. Like the other local systems illustrated in this chapter, it creates a self-contained circulation system. Oftentimes, this cycle of air will develop cumuliform clouds at the mountain peaks.

During the night, the opposite happens. The air along the mountain slopes cools quickly. As it cools, the air becomes denser and begins to

The leeward side of a slope in an orographic wind flow pattern receives little moisture from an air mass being pushed up and over a mountain barrier. Vegetation on this side is drier and scarcer. (Nature's Images)

flow downhill, causing a mountain breeze. It converges on the valley floor and forces the air to move vertically upward. This upward movement is usually stopped by the temperature inversion. Similar to daytime flow, it forces the air to move horizontally, which then allows the airflow to complete the cycle in a cell. Where canyons and valleys are narrow, they can funnel the winds; some winds have reached speeds as high as 93 miles per hour (150 km/hr). Mountain-valley breezes are important for firefighters to understand when they battle wildfires, which are one of the consequences of global warming (global warming leads to drought and excessive evaporation, which dries out the exist-

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