Cold air drainage often flows over the top of even colder air that has accumulated in low lying areas, sometimes referred to as cold air pools.  The airflow and turbulence within cold pools are quite weak and downward transport of warmer air is minimal.  Cold pools normally have a diffuse top in contrast to some schematics found in the literature.  A summary of implications for  frost potential is presented from a relatively simple point of view.

Cold air drainage down a gentle slope (left to right) rises over deeper colder air on the valley floor.  The rising drainage flow marks a microfront between two local air masses.
Cold air drainage down a gentle slope flows horizontally over colder air on the valley floor (on the right).
Early morning fog (not snow) defines the valley floors in complex terrain in Central Pennsylvania (photo by Geoffrey Mahrt).
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