From AMS Glossary
Nocturnal cold-air jet flowing out of the mouth of a valley or canyon as it opens onto a plain.
When the flow is fully developed, its depth near the exit is about the same as the height of the valley or canyon sidewalls there. It represents a continuation or extension of the mountain (downvalley or downcanyon) breeze generated in the valley by surface cooling. The jet often achieves peak wind speeds outside the valley. This acceleration is probably due to the conversion of potential to kinetic energy, as the column of cold air flowing out of the valley, freed of the confining sidewalls, fans out in the horizontal and compresses vertically. Another factor is the release of the flow from surface friction along the sidewalls. Maximum speeds in the jet begin near the surface early in the evening, but for well-developed jets at exits from deep valleys the highest speeds may be at 300–500 m or more above the ground. In deep, extensive valley drainage systems, peak speeds may exceed 10 m s-1 on clear nights.