Severe winter weather is defined as when a severe weather warning has been issued by the Met Office.
These are issued when the probability of a specified event occurring exceeds a pre-defined threshold, for example, if there is greater than 60% probability of snow falling at a rate of more than 2cm per hour for more than 2 hours.
It is the road surface temperature and whether the road is wet or dry that determines the course of action taken, rather than the air temperature.
Delivery of the winter service therefore depends on the receipt of accurate forecasts of road surface temperature and state, together with humidity which is critical when conditions are marginal, such as when temperatures are close to 0°C.
Derbyshire has a significant proportion of marginal nights in a winter season therefore our local knowledge and expertise is vital in deciding what to do.
Our engineers have electronic access to real time weather data. They can view the forecasts for each geographical domain and the associated thermal mapping information, which gives the variation in predicted road surface temperature and condition (frost, ice and snow) across the county.
Road weather station data can also be viewed giving actual real time information on weather, road surface temperature, road wetness and salt concentration 24 hours per day.
Access is also available to Met Office weather which shows the type and intensity of precipitation. These facilities allow our engineers to make appropriate and timely decisions.
Despite using detailed and regular weather reports sometimes circumstances mean we cannot continue as planned.
Road weather forecasts on average are up to 90% accurate. This can mean that there can be days in winter when frost is not forecast but actually occurs.
On a wet night followed by rapidly clearing skies, salting will usually start after the rain has stopped to avoid salt being washed away. If the road surface temperature falls rapidly the roads may freeze before the gritter arrives.
A 'dawn frost' is the development of early morning dew which falls on a cold dry road and freezes on impact. It is currently impossible to forecast this accurately.
When snow or ice formation coincides with the rush hour and early salting has not been possible due to rain, then traffic congestion can significantly delay the gritters.
Snow is more difficult to forecast and to counteract. Salt has a limited effect on snow and where accumulations occur, ploughing will be necessary before salting can take place and the service will take much longer to deliver.