Road Weather Information System

The NRA operates a Road Weather Information System. This provides local authorities with real time weather data for their area and road weather forecasts to assist them in making informed decisions about salting. Fifty four Road Weather Stations collect local meteorological data. The sensors give continuous data on humidity, air, road surface and sub-surface temperatures and salt concentration on the road surface.

This data and road weather forecasts produced by Met Eireann are passed electronically to operational staff at each local authority.

Watching for ice or snow

Met Éireann transmits the road ice forecasts to the local authority engineers' computers every day. The computers display thermal maps showing the variation in predicted road surface temperatures and condition (frost, ice, snow etc.) across the network of roads. The engineers can also view the road weather station data, providing them with actual real time information on weather, road surface temperature and salt concentration, 24 hours a day.

These facilities enable engineers to decide on the appropriate action for the salt spreader vehicles and for snow ploughing.

The scale of the operation

In severe weather, up to 200 spreaders can be mobilised. The local authorities have over 150 snowblades and blowers and make arrangements with private contractors where appropriate.

Maximum salt storage is about 20,000 tonnes of which about 8000 tonnes is stored in buildings (Salt Barns). This is regularly topped up during the winter. Annual consumption is about 50, 000 tonnes. The service costs about ¤ 6 million to run.What arrangements are made?

Precautionary salting may be carried out during the evening or early the next morning. Salting crews are normally held on alert at home and if ice or snow occur, are brought in early to do the precautionary salting and then to deal with problems as they arise.

In emergency conditions, considerable additional resources are mobilised.

The list is extended when snow and ice persist.

Efficiency constraints

Salt is ideally spread before the road becomes icy or snow starts to fall. However, salt is NOT normally spread during rain, as the salt will simply wash away, or on roads expected remain dry when the temperature falls below zero.

It can often take 2 hours for spreaders to salt a route. It will then be 2 hours before some roads are treated.

No matter how accurate the forecast is, there are situations where a local authority may not be able to prevent ice forming:

• On a wet night followed by rapidly clearing skies, salting will normally start after the rain has stopped. The temperature may fall rapidly and roads freeze before arrival of a salt spreader.
• "Dawn frost" - On a dry road, this involves the development of early morning dew which falls on a cold road and freezes on impact. This situation is impossible to forecast accurately at present.
• Rush hour - When ice or snow formation coincides with rush hour traffic and early salting has not been possible e.g. due to rain, the salt spreaders can not achieve adequate coverage of the network due to traffic congestion.

About salt

• Rock salt is used responsibly as it comes from a non-renewable source.
• Salt spread rates vary from 10 grams/m² for precautionary treatment to 40 grams/m² on snow.
• Salt does not act immediately when it is spread on existing ice or snow. Hence the need for precautionary salting. The ice or snow surrounding each salt granule has to be turned into a saline solution. The action of traffic is essential by moving the salt granules around and eventually melting all the ice or snow. On more lightly trafficked roads the surface will remain icy for some time after salting.
• It must therefore be stressed that the spreading of salt does not mean that the road surface will instantly become ice-free. In freezing or near-freezing conditions, always drive with great care even if the road has been salted.