Orographic Clouds
forced by the earth's topography

Orographic clouds are clouds that develop in response to the forced lifting of air by topographical features on the earth's surface, like mountains for example.

Air passing over a mountain will oscillate up and down as it moves downstream (see diagram below). Initially, stable air encounters a mountain and is lifted upwards. The lifted air parcels undergo cooling through expansion, and eventually grow heavier than the surrounding air. If a parcel cools to its saturation point during this process, the water vapor within will condense and become visible as a cloud.

Upon reaching the mountain top, the air is heavier than the environment and will sink down the other side, warming as it descends. Once the air returns to its original height, it has the same temperature and same buoyancy as the surrounding air, but the air has momentum carrying it downward and does not stop immediately. With continued descent, the air becomes warmer than the surroundings, and begins to accelerate back upwards towards its original height (beginning the cycle again). During the upper-most ascent phase of the cycle is when clouds develop.

The lifting of moist air can result in the generation of clouds, while in contrast, the lifting of drier air may not produce any clouds at all. In contrast, where the air is moving downwards, skies are clear. These oscillations continue as the air moves further downstream from the mountains and are eventually dampened out by mixing and friction.

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Credits and Acknowledgments for WW2010.
Department of Atmospheric Sciences (DAS) at
the University of Illinois at Urbana-Champaign.