The heaviest coatings of frost usually do not occur at the coldest temperatures because very cold air can not hold very much water vapor. Instead, thick deposits of frost usually occur when the air temperature is close to 32 deg. F (0 deg. C). HOAR FROST: Huge ice crystals of frost (see the above photo) can develop if a cold surface below 32 degree F (0 deg, C) is exposed for a long period of time to unusually humid air. This can happen on the underside of ice covering a river if the water level of the river falls after the ice was formed.