Weather Watch: 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 usually will produce steady 10-20 knot winds and can be as much as 10-20 degrees 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. Beware camping in depressions where the cold air may pool overnight---camping a little higher on the slope may be much warmer! -David Schultz, NOAA/National Severe Storms Laboratory