On clear, cool nights, when the earth’s heat is radiated away from the ground quickly, the layer of air near the soil is cooler than the air mass above. Because this air is cooler, it’s denser, and on slopes steeper than 2 percent, it starts to slide downhill. The coolest air collects in pools in the lowest places or behind barriers, and when temperatures on the slopes are hovering just above freezing, vegetation in the pockets can be blackened by frost. Valley floor temperatures run from two to four degrees colder than on slopes at night, but about two degrees warmer during the day, since air movement is less and heat buildup occurs in the low spots.
