Personal tools
  •  
You are here: Home Wildland Resources Wildland Fire Management and Planning Unit 6: Local and General Winds

Unit 6: Local and General Winds

Document Actions
  • Content View
  • Bookmarks
  • CourseFeed

::   Back

Slope and Valley Winds

The figure below illustrates the valley winds. During the day, air in mountain valleys and canyons tends to become warmer than air at the same elevation over adjacent plains or larger valleys, thus creating a pressure gradient and resulting in upvalley winds. The main difference between upslope winds and upvalley winds is that the upvalley winds do not start until most of the air mass in the valley becomes warmed. Usually this is middle or late afternoon, depending largely on the size of the valley. These winds reach their maximum speeds in early afternoon and continue into the evening.

The transition from upvalley to downvalley flow takes place in the early night. The transition is gradual: first the downslope winds, then a pooling of cool, heavy air in the valley bottoms. The cool air in the higher valley bottoms will flow to lower elevations and increase in velocity as the pool of cool air deepens. This continues through the night and diminishes after sunrise.

The velocities of the slope and valley winds vary considerably by terrain and current weather conditions. For example, slope and valley winds develop better under clear skies when the heating and cooling processes are more pronounced.

We can give you some broad ranges to indicate typical windspeeds in mountain topography. Upslope winds usually range from 8 to 12 miles per hour, while downslope winds are somewhat less; 2 to 7 miles per hour. Upvalley winds typically are stronger, 12 to 20 miles per hour, while downvalley winds can be 8 to 14 miles per hour.

Diurnal valley winds Solar radiation affects slope winds

The illustrations of slope and valley winds to this point might suggest that upslope and upvalley winds occur on all slopes at the same time. This is not usually the case. For one example, see the graphic above. Let's suppose we have a ridge line and canyon parallel to each other running north and south. In the morning, the east aspects will be heated by the sun, but the west aspects are shaded. Upslope winds can occur on the east slopes, while downslope winds occur on the west slopes. As the sun passes overhead and into the afternoon positions, the west slopes become heated and the east slopes become shaded. The slope winds can reverse from those of the morning.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, November 08). Unit 6: Local and General Winds. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_6__Local_and_General_Winds_6.html. This work is licensed under a Creative Commons License Creative Commons License