Winter Gritting - Ice, Frost and Snow
When temperatures fall to sub-zero, frost, icy roads or snow will be forecasted.
Frost formation1 is a complex process, and conditions have to be "right" for it to occur. Frost forms on surfaces directly from the vapour state, without condensing as dew. If dew forms, it is unlikely that frost formation will occur, even if the temperature drops below freezing.
However, a dawn frost can occur on dry surfaces, when early morning dew forms and freezes on impact with the cold surface. It can be difficult to predict when or where this condition will occur.
Frost is more likely to form on surfaces above the ground first, such as house roofs, or vehicles, because the air immediately above the ground is usually a few degrees warmer than the air a few feet higher. This is because there is some heat transfer from the ground to the air a few centimetres above it.
On a calm night the ground temperature can be as much as 5-7 degrees cooler than the standard temperature reading.
Conditions for black ice to form are wet and frosty. The ice is clear, but looks black against the road surface. It’s exceedingly difficult to see from a vehicle and very dangerous. Black ice may be formed when rain or drizzle fall on a road surface which is at a temperature below zero.
Freezing rain is that which converts to ice as soon as it lands on the freezing road surface, it rarely occurs, but can make the roads, pathways and car parks treacherous.
Hoar Frost is white ice crystals that appear on grass and trees and occasionally on the road surface. Hoar frost can make the surface very slippery.
On nights with heavy frost, it is not uncommon to have an ice fog, in which super cooled water vapour forms ice crystals in the air and hoar frost on exposed objects. Heat transfer between the ground and air, plays an important role, as the ground supplies energy to heat the air near the ground, unless of course it has been very cold for a long time and the ground is frozen, or there is snow on it.
As beautiful as snow can look, compacted snow may turn to ice, especially if it gets compressed. Compacted snow, which turns to ice, is difficult to treat effectively with grit, it is therefore important to ensure that areas have been gritted ahead of the snow being driven over, being compacted or in the early stages of snow falling.
Remember: Road surfaces can differ from the air temperature!
Reference:
1. D. R. Cook, "Predicting Frost At Your House", Journal of Meteorology, Vol. 15, no. 153, November 1990.