WEATHER WATCH COLUMN: OCTOBER 2004 ISSUE ------------------------------------------------------------------------- Weather Watch: Valley and Mountain Breezes Rapid evening and nighttime cooling of the air near upper mountain slopes can result in cool downslope winds that flow down drainage channels and stream valleys. These may begin in the late afternoon as the valleys become shaded by the setting sun. Such mountain breezes usually produce steady 10-20 mph winds and can be as much as 10-20 degrees Fahrenheit cooler than air outside the drainage. Conditions favoring such winds are clear skies, weak ambient winds, low relative humidity, and steep rock or snow slopes. Sometimes these winds can be quite gusty as air temporarily pools up along plateaus, then spills downhill in "cold-air avalanches." Beware camping in depressions where the cold air may pool overnight---camping a little higher on the slope may be much warmer! During the daytime, the opposite effect occurs when the air over the mountain slopes is heated more than the air over the valleys. Upslope flows called may produce mountain-top thunderstorms usually beginning after noon. These daytime winds, however, are usually weaker, especially when the overnight mountain breezes are particularly strong. <-David Schultz, University of Oklahoma and NOAA/National Severe Storms Laboratory, Norman, Oklahoma>