This is a photo of some frost crystals that were on our car windows yesterday morning. This morning there was no frost on the windows, even though it was just as cold. Why the difference?
Answer:
Just as with dew, for frost to form the air has to reach its dew point. That is the point where the air is cooled enough so that more water vapor is condensing than is evaporating. As the humidity decreases, the air temperature has to get colder to reach the dew point. If the air is humid and cold, you will get frost. If the air is dry and cold, you do not.
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