The effect of synoptic scale weather and topography on road surface temperatures in Devon
Microclimates of various road weather stations in Devon were examined. Road surface temperatures were measured during various synoptic conditions. Data from the thermal mapping exercise conducted by Vaisala TMI were analysed and categorised as clear and calm, cloudy and windy, and a condition between the two extremes. Results indicate valleys to be relative cold spots in clear conditions, and high altitude stations are cold spots during cloudy conditions. A separate case study during the cold spell of February 1991 reinforced the conclusion and extended these views county wide. Coastal stations with surface winds were observed to be 1 to 3 deg.C warmer than inland stations. Reaction times, the delay in road temperature reaction due to synoptic change, showed for the most difficult forecasting scenario on the passage of a cold front, small reaction times (less than 30 minutes) at exposed sites when clear skies resulted, sheltered sites having a reaction time of up to 2 hours. Sheltered sites in valleys had the largest temperature drop due to cold air drainage. A surface climate model was used in retrospect to predict road surface temperatures at night, each station having its own characteristic exposure. Cloud change was estimated from satellite images. Results indicate predicted minimum temperature within 0.5 deg.C of observation. Real time forecasts were tried and an accuracy of 65% at this level was achieved. These results were equal or better than the existing "Open Road" forecasts.