A frost hollow is a local low point in a landscape where cold dense air collects overnight.
Under clear skies, the heat radiates out of the land. This creates a very cold layer of air near the ground.
Cold air is denser than warm air. As soon as it forms, it flows slowly downhill, like treacle.
This cold heavy air flows downhill with gravity until it reaches the lowest points in the landscape. And this is a frost hollow. It is where the cold air sits, freezing anything it touches.
Frost hollows are very local.
In the photo above the air has flowed down off the hillside, but then stopped as it hits the raised bank at the edge of a road.
Some frost hollows are tiny. I went for a walk on a cold morning in Yorkshire recently.
As I crossed a large field, I only saw one fully frost-covered leaf. It was in the lowest spot in the field, a frost hollow that was only a few inches wide.
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