Frost pockets are spots where frost is more likely to form. Since cold air sinks, it tends to pool in valleys, thus making it easier for frost to form there than in nearby higher terrain.
So if your farm isn't generally flat, you're more likely to see frost in the pockets where the ground dips. On a larger scale, places at the bottom of valleys would be more likely to see large frost pockets than nearby hills.
Areas with frost pockets are also more likely to see earlier frosts in autumn and later frosts in spring, and heavier frost on days with widespread frost.
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