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Salt - the myths and the facts

Myth: Salting a road prevents the formation of ice

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

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 -10 degrees centigrade.

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 to ice or snow will melt and remove it quickly without any other actions

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 -40 degrees centigrade 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 centigrade

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. Here the precipitation once again starts off 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 on to the network as they too will have little, if any, traction.

Myth: You knew it was going to snow but have not gritted

The council have two weather stations in the borough, one on Blackstone Edge and one between Heywood and Middleton. Readings of the current weather conditions are sent from both stations to the Met Office every 10 or 20 minutes. From these readings, the Met Office produce two forecasts daily at 12 noon and 5.30pm which show the forecast air temperature, road surface temperature, wind speed and precipitation.

The forecasts are received in both text and graphical form, and from the information they contain, gritting treatments are planned. If there is any uncertainty in the information received, a telephone call is made to the Met Office forecaster.

Should any major changes to the forecast occur at any time of the day, then the forecaster alerts the council's Winter Service Duty Officer with a telephone message.

Additionally a 2 to 5 day forecast is received daily, along with a 21 day outlook throughout the core winter months of December, January and February. This allows gritting resources to be planned well in advance.

Myth: I’ve not seen a gritter on the network

We have 7 gritting vehicles to cover 350km of the road network across the borough. It takes approximately 3 hours for one vehicle to treat its entire route. The vehicles normally commence gritting 4 hours before the forecast says that the air or road surface temperature is to reach freezing point.

In the case of snowfall, the gritting fleet may be on the road continuously. However it will still be 3 to 4 hours before they pass the same point twice as they will have to return to the depot to be refilled with salt.

When heavy snowfall is occurring then our priority is to keep the strategic network open. At these times, the gritting vehicles will be concentrated on the main roads and fitted with snowploughs, meaning estate roads may not be treated as frequently.

Myth: A gritter passed me but no salt was being spread

The current fleet of gritting vehicles are far more sophisticated than those of years gone by. No longer does salt spray in all directions covering the windscreens of cars and the legs of pedestrians. The computer controlled mechanics now dispense the required amount of salt directly down on to the road. They can also 'throw' the salt to one side or the other to ensure the whole carriageway is covered even if the vehicle is driving down one side only.

However this is not always a myth as the vehicle may not have reached the starting point of its treatment route, or may be returning to the depot at the end of its route or to refill. Where a vehicle is not gritting, the operatives have been asked to turn off their yellow beacons if it safe to do so, to prevent confusion.

Each gritting vehicle has a GPS system which tracks its route, speed, whether it is spreading salt, and if so, the amount being spread. Checks can be made as to the date and time a vehicle was on a certain road and what it was doing. The system also alerts the contractor's Duty Manager should the vehicle deviate from its usual route.

Myth: You have never refilled the salt bin on my street

Hopefully this is a myth, but with over 520 salt bins spread across the borough, there is always the chance one may be missed. There are a further 90 bins on the database that do not belong to us and are therefore not refilled as part of the winter service.

The salt in the bins is for use on public roads, pedestrian areas and footways only. It is not there to be used on private properties.

The bins are refilled at the start of the winter season and then usage is monitored. We refill them once per week, or as often as staff resources allow, during periods of high demand. The refilling takes place on a rota basis per Township across the borough.

If there is a need to preserve salt stocks in order to keep the strategic highway network open, then the frequency of refilling salt bins may be reduced.

Where salt from the bins is seen to be being misused then this can be reported in confidence to the Council. If it is seen being stolen, then this should be reported directly to the Police along with the make, model and registration number of any vehicle involved.

Contact

0300 303 8879

Highways and Engineering Service
Number One Riverside
Smith Street
Rochdale OL16 1XU ​