December 26th 2000 Hoarfrost

Hoarfrost blankets the area...December 26th, 2000

For those of you who were out early this morning...you may have noticed many surfaces coated with a thick silvery-gray layer of ice crystals. This phenomenon is known as hoarfrost (hoar, Old English meaning gray). Hoarfrost is similar to the formation of frost...but at colder temperatures with the dew point, or frost point in this case, well below 32 degrees. Hoarfrost forms and grows when the air is cooled to the frost point and nearby water vapor bypasses the liquid stage as it condenses and is immediately deposited as solid ice crystals. The amount of hoarfrost can be especially great on tree limbs and branches...sources of extra water vapor via evapotranspiration and freely exposed to the air...but it can also be found on metal and wood poles, utility wires and other structures. Some sunshine Christmas afternoon helped to evaporate a little of the snow cover and increase the amount of water vapor in the lowest layer of the atmosphere. The air temperature determines the structure and growth of the ice crystals. The morning low temperature at the National Weather Service Office was 0? Fahrenheit or -17.8? Celsius which is very favorable for large dendritic ice crystal formation. The great amount of hoarfrost that formed may have mistakenly given the impression that it snowed overnight.

This frost formation is slightly different from the more typical frost found on the blades of grass during the spring and fall and artfully painted on window panes when the temperature is at or just below freezing. In this scenario the water vapor passes, at least temporarily, through the liquid phase before changing to its frozen state as frost. A hard "killing" frost occurs with temperatures in the middle to upper 20s...when water inside the living cells of a plant freeze subsequently killing the plant.

Take a moment to check out the pictures below!

This story and pictures were contributed by Patrick Murphy

hoarfrost