Gritting Frequently Asked Questions
Gritting is an important part of how we keep the roads throughout Redcar and Cleveland safe for travel during the winter period. Below are some general questions and answers about how gritting works.
Does 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 de-bonding layer so if snow falls, it doesn't freeze onto the road surface and can be ploughed off or churned off by vehicular movements.
Does spreading salt on to ice or snow cause it to 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.
Can it be 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.
Does All water freeze 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 super cooled.
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.
Why have I not seen a gritter on the roads?
We have 6 gritting vehicles to cover 302km 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 could 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 snow ploughs, meaning estate roads may not be treated as frequently.
Why did a gritter pass 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.