Salt myths and facts

Ice and snow myths

Here are some myths about salting ice and snow along with the factual information.

Myth: salting a road prevents the formation of ice

Salt lowers the freezing temperature of water, which prevents ice or frost from forming on the carriageway as it would normally when the temperature of the road or the air falls to zero degrees.

The higher the concentration of salt, the lower the temperature at which freezing will occur. Generally, on the roads, salt loses its effectiveness once the temperature falls below minus 10 degrees.

Pre-salting the road forms a separating layer so if snow falls, it doesn't freeze onto the road surface and can be ploughed off or churned off by vehicular movements.

Myth: spreading salt on ice or snow will melt and remove it quickly

Salt comes in grain sizes of 6mm or 10mm and is spread at rates between 10 and 40 grams per square metre depending upon the forecast road surface temperatures and if snow is forecast or is falling.

When spread on top of ice or snow, each grain will begin to melt the surrounding ice working its way outwards. As it melts the ice, it forms a pool of salty water, which in turn helps to melt the surrounding ice and so on. Without any traffic to move the salt and salty water around and mix it into the thawing ice, the melting process can take some considerable time.

Where snow falls on top of salt then it begins to melt the snow from beneath. Again vehicular movements will speed up this process. However, the first vehicles over the snow will actually compress the snow into ice in much the same way as a snowball is created. If there is little traffic or very slow-moving traffic, then a layer of ice may form on top of the road until the salt works its way up from below.

Myth: it’s too cold for snow

There is a relationship between the temperature and the amount of moisture the air can hold. However, it is only once the temperature gets below minus 40 degrees that the air has so little moisture content that snow can rarely occur.

In this country, most rainfall begins as snow in the upper atmosphere throughout the year. As the snow falls through the lower atmosphere, the air is warmer and it turns to rain.

In the winter, the air in the lower atmosphere is also cold, and, if it is at or below zero then the snow can make it to the ground. However, very slight temperature changes at ground level due to factors like wind and altitude can change the type of precipitation over short distances. This is why weather forecasters are often very cautious and say it could hail, sleet or snow.

Myth: all water freezes at zero degrees

Except in the case of freezing rain! This phenomenon thankfully occurs rarely and is often associated with the approach of warm air after a prolonged cold spell.

Precipitation once again starts as snow in the upper atmosphere, then it passes through a region of warm air which turns it to rain before finally passing through a thin layer of cold air just above the surface. The moisture cools to a temperature below freezing point, but the water droplets do not freeze themselves and become supercooled.

When the droplets strike the ground or any surface, they instantly freeze and coat everything in a film of ice. This coating will cover the grains of salt rendering them almost ineffective until the air temperature rises and the ice begins to melt. Road travel during a period of freezing rain will be severely disrupted and it is unsafe to send heavy gritting vehicles onto the network as they too will have little if any, traction.