What is valley inversion?

In high mountains or deep valleys, sometimes, the temperature of the lower layers of air increases instead of decreasing with elevation along a sloping surface. Cold air at the hills and mountains, produced during the night, flows under the influence of gravity. Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply in pockets and valley bottoms with warm air above. This type of inversion of temperature is called valley inversion.

Further Reading:

• Factors Controlling Temperature Distribution
• Geography Notes