IN response to Mr Johnson's letter about gritting (February 8), the Met Office provides a dedicated daily winter forecast for York, based on data collected from weather stations in the north east and south west of the city.

It is this forecast which City of York Council uses as the basis for its gritting response.

The weather stations record air, road surface and dew temperatures throughout the winter. The road surface temperature (RST) is the critical factor for gritting purposes and is often several degrees below the air temperature.

National weather forecasts only quote general regional air temperatures and frost warnings and are unable to deal with detailed local information. The situation last Tuesday, to which Mr Johnson referred, was a typical example. The possibility of ice and frost formation on parts of the city's defined gritting network cannot be ignored. As the highway authority, City of York Council has a duty to prevent the formation of ice, frost and snow, if reasonable and practicable, on the defined gritting network.

York's road network is gritted in its entirety, because the geographical area is too small to have distinct weather domains. This means that at certain times, the RST may remain above zero when other areas are below zero, particularly in urban areas.

Forecasting the RST is not an exact science and the decision to grit is taken by one of the council's winter maintenance engineers using all the available information and in consultation with the Met Office. Check out the council's website www.york.gov.uk/roads/gritting or at local libraries for details.

Brian Hebditch,

Winter maintenance engineer,

City of York Council,

Development and transportation,

Foss Islands Road, York.

Updated: 09:35 Friday, February 11, 2005