WW2010
University of Illinois

WW2010
 
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Online Guides
 
  introduction
 
> 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

Hydrologic Cycle
 
  introduction
 
  water budget
 
  evaporation
 
> condensation
 
  transport
 
  precipitation
 
  groundwater
 
  transpiration
 
  runoff
 
  summary

condensation
 
  introduction
 
  convection
 
  cyclones
 
  fronts
 
> topography

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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Topography
forced lifting by the surface of the earth

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.

[Image: orographic cloud (78K)]
Photograph by: Holle

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

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