Air is also lifted by the earth itself. When air encounters a mountain range, for example, air is forced to rise up and over the mountains and if enough lifting occurs, water vapor condenses to produce orographic clouds.

In the United States, the prevailing winds are generally from west to east, so most orographic clouds form on the western side of a mountain.

Why do orographic clouds appear to be stationary?
Air rises on a mountain's windward (upwind) side and sinks on the lee (downwind) side. This sinking motion warms the air and causes the cloud to evaporate, destroying the cloud. Therefore, even though the wind blows over the mountain, condensation processes and associated cloud droplets are confined to the windward side. This is why orographic clouds begin on the windward side of the mountain and end near the summit.

The Rocky and the Sierra-Nevada Mountains are examples of mountain ranges that produce orographic clouds. The large dark cloud in the upper right-hand corner of the picture above and the smaller cloud just above the mountain are both examples of orographic clouds.

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Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW2010. Department of Atmospheric Sciences (DAS) at the University of Illinois at Urbana-Champaign.

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