frost pocket

(meteorology) A parcel of cold air in a hollow or at a valley floor, occurring when nighttime terrestrial radiation is greatest on valley slopes.

A concentration of cold air in a hollow or valley floor. This occurs when night-time terrestrial radiation is greatest on valley slopes. Air above these slopes becomes colder and hence denser, therefore it flows downslope. Minimum temperatures in the pocket may be tens of degrees below the surroundings. For this reason, fruit growers try to avoid frost pockets.