Orographic ascent

Orographic ascent is the forced vertical motion of air by mountains or raised terrain. The type of cloud formed depends on the stability of the atmosphere and moisture content of the surface layer. Some examples of cloud formation are depicted in the Figure below:

Orographic cloud forms on higher ground when moist air condenses below the ridgetop in stable conditions. Cumulus clouds form when air flowing over high terrain condenses above the hilltops when the atmosphere is unstable. Lenticular clouds form above hilltops when wave motion in the atmosphere lifts moist air to saturation in a generally stable atmosphere. In the lenticular case, cloud forms in the up motion section of the wave and dissipate in the downward flow past the wave. It is the wave formation that creates the lens shape of the lenticular cloud.

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