We had a beautiful morning skiing amoung the frost-rimed trees just outside our front door on the day before New Year’s.
On the morning of the 30 December, a dense fog greeted us, however, it dissipated the same day. The next day was fog-free, but the frost was intense, covering every blade of grass, twig, branch and wire. While I first thought of it as a hoar frost (from the Old English har “hoary, grey, venerable, old,“) I began to wonder. What was the difference between a rime frost and a hoar frost?
To answer this, I resorted to the good ol’ Encyclopedia Britannica. Hoar frost (often hoarfrost), is a deposit of ice crystals on objects exposed to the free air, such as grass blades, tree branches, or leaves. It is formed by direct condensation of water vapour to ice at temperatures below freezing and occurs when air is brought to its frost point by cooling. Besides needing freezing temperatures, hoar frost needs a sufficient amount of dampness in the air to form. Rime frost happens when supercooled water droplets (at a temperature lower than 0° C [32° F]) in fog come in contact with a surface that is also at a temperature below freezing; the droplets are so small that they freeze almost immediately upon contact with the object.
We had woken up to mostly clear skies. The weather report showed that it had been a partially cloudy night before, but I could find no mention of any fog, so this frost was most likely a true hoarfrost, as the crystalline structure in the above photograph suggests.
In the modern meteorological sense, knowing why the frosts are different is important. While hoarfrost is light and insubstantial, rime frost has the potential to create hazardous conditions on roads as well as build up on trees and powerlines, causing breakage and power failures.
In the quotation below, Charles Dickens uses both forms of frost in a single sentence, describing David Copperfield’s memories upon his return to his school at Salem House. Note that the hoar frost is seen as “ghostly” or insubstantial through the fog, while the rime is a result of the moisture particles freezing on his hair:
How well I recollect the kind of day it was! I smell the fog that hung about the place; I see the hoar frost, ghostly, through it; I feel my rimy hair fall clammy on my cheek; I look along the dim perspective of the schoolroom, with a sputtering candle here and there to light up the foggy morning, and the breath of the boys wreathing and smoking in the raw cold as they blow upon their fingers, and tap their feet upon the floor.Charles Dicken: David Copperfield, chapter nine.