WW2010
University of Illinois

WW2010
 
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Online Guides
 
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> meteorology
 
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Meteorology
 
  introduction
 
  air masses, fronts
 
> clouds, precipitation
 
  el nino
 
  forces, winds
 
  hurricanes
 
  hydrologic cycle
 
  light, optics
 
  midlatitude cyclones
 
  severe storms
 
  weather forecasting

Clouds, Precipitation
 
  introduction
 
  development
 
> cloud types
 
  precipitation

Cloud Types
 
  introduction
 
  high-level clouds
 
  mid-level clouds
 
  low-level clouds
 
  vertically developed
 
> other cloud types

Other Cloud Types
 
  contrails
 
  billow clouds
 
  mammatus
 
> orographic
 
  pileus

User Interface
 
  graphics
> text

NOTE: We've guessed that you're not using a client that supports colored tables and have tried to compensate. Low graphics mode looks much better on clients that do... we recommend switching to Netscape 3.0 or Microsoft Internet Explorer.
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Orographic Clouds
forced by the earth's topography

Orographic clouds are clouds that develop in response to the forced lifting of air by the earth's topography (mountains for example).

[Image: lee mountain wave clouds (78K)]
Photograph by: Holle

Air passing over a mountain oscillates up and down as it moves downstream (as shown in the diagram below). Initially, stable air encounters a mountain, is lifted upward and cools through expansion as it rises. If the air cools to its saturation temperature during this process, the water vapor within condenses and becomes 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 buoyancy as the surrounding air. However, the air does not stop immediately because it still has momentum carrying it downward. With continued descent, the air becomes warmer than the surroundings and begins to accelerate back upward towards its original height (beginning the cycle again). It is during the upper-most ascent phase of this cycle that clouds develop. In regions where air is descending, skies are clear.

[Image: orographic clouds that resemlle waves in the ocean (72K)]
Photograph by: Holle

The lifting of moist air can result in the generation of clouds, while lifting drier air may not produce any clouds at all. The oscillations continue as the air moves further downstream from the mountains but are eventually dampened out by mixing and friction.



mammatus
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.

pileus