Hoar Frost Hoar frost forms when water vapor hits a freezing surface. It freezes immediately, leaving spikes called hoar frost. The temperature of the air is usually around freezing when there is hoar frost, but the ground must be a lot colder than freezing. The reason the air is warmer than the ground is because the air must be warm enough to carry water vapor.
Fern Frost Some frost can make very pretty patterns on windows. If the weather is severe you might be able to see fern frost. Fern frost is made when dew is on cold windows. Then the dew freezes until you get fern frost.