Cold winters are expensive. Not least, they are expensive for the local authorities whose job it is to keep our roads passable and safe during periods of ice and snow. The dangers and inconvenience of such conditions can be alleviated by spreading grit or salt on the more important routes, but the cost in labour and materials is high.
Economy demands that traffic engineers know when and where to grit to maximum effect, and also that unnecessary gritting be avoided.
Traditionally road engineers would rely on a local weather forecast, and trust their experience, local knowledge and native cunning in deciding when, and to what extent, to undertake anti-icing or de-icing measures. And it is a complex issue: night-time road temperatures depend on several meteorological parameters like air temperature, humidity, wind speed and cloud cover, but also on local factors such as altitude, slope, exposure, and road construction materials.
Here in Ireland, as elsewhere, modern techniques have allowed the exercise to be streamlined.
The National Roads Authority has weather sensors at more than 50 locations along the main roads around the State. These provide data on air temperature and humidity, wind speed, road surface temperature, and if the surface is dry, wet or icy, all of which is fed by computer to Met Eireann in Glasnevin. There it is combined with output from Met Eireann's weather forecast models to provide the road authorities with spot predictions, for specified times during the coming night, for these 50 or so key locations.
The next step is to translate these spot forecasts into a detailed picture for the entire road network, using a technique called "thermal mapping". This is based on a series of surveys of a road network to discover which parts of it are most susceptible to very low temperatures, and which parts are affected only in severe conditions.
Each route is analysed several times in different weather conditions, and the result is a comprehensive picture of the extent to which each point on a road network is likely to be warmer or colder than the key locations for which the forecast conditions have already been prepared.
Combining the forecast data and the appropriate thermal map, the result is a display of the road network on a computer screen showing in different colours those roads which will need gritting later on, those roads which can be ignored safely, and roads that will need to be monitored to see how things develop.