In this video, upslope flow quickly yields to downslope drainage of about 1 m/s down a gentle slope.
With sufficiently weak regional flow, most of the Earth’s surface may at times experience cold air drainage and formation of cold air pools in low lying areas. With weak regional flow, even very weak slopes generate downslope flows so that there is some air flow, even if not readily observable by a human without adequate instrumentation. Gentle slopes often occur simultaneously on different scales. Drainage flows from different slopes may interact. Drainage flows are often non-stationary, evidently related to transient submeso motions. The literature concentrates on flows over isolated well-defined slopes and drainage flows over more typical less organized terrain is poorly understood.
A larger-scale slope may generate nocturnal downslope flow that appears as a regional flow with respect to local drainage flow down a smaller-scale slope. On a night with strong surface cooling and weak larger-scale flow, an idealized parcel originating from upland areas may drain into small gullies, which in turn drains into little valleys, which in turn drain into larger valleys, and so on, to the sea.
On north facing slopes, cold air drainage will persist well past sunrise and begin well before sunset. With tall dense canopies, daytime heated upslope flow at the canopy top may coexist with downslope cold air drainage in the subcanopy. With low winter sun angles, cold air drainage may persist the entire daytime period on north facing slopes.
Implications with respect to frost damage can be found on the frost potential page.