When the temperature of items on the ground are below freezing, condensation of water vapor can occur. Similar to the formation of dew, moist air will be cooled when it comes in contact with the cold surfaces. The air will quickly reach the dew point temperature, which, in this case, is below freezing. The water vapor will then convert directly into ice and leave needle-like coatings of hoar frost on various objects including grass, trees, telephone wires, and more.
Areas prone to heavy fogs are likely to see hoar frosts often in the spring and fall. Hoar frost can best be described as a white and 'furry' form of ice.
Hoar frost is NOT frozen dew. As temperatures decrease throughout the night, dew will often freeze into a clear coating of ice. Hoar frost is a result of the direct sublimation of gaseous water to ice crystals.
Hoar frost can cause extensive damages to crops, plants, and delicate flowers.