Relations between minimum temperature and topography in great britain

Abstract The effects of topography on two measures of temperature, the daily minimum and the diurnal range, were investigated using the conventional network of climatological stations in the U.K. The effects of large bodies of water and buildings were minimized by restricting the analysis to data obtained from 145 inland rural sites. Topographic influences were considered to be mainly due to the effects of ‘local’ and ‘large‐scale’ shelter. Local shelter is associated with ‘height above the valley’ and is represented by the drop in altitude within 3 km, whereas large‐scale shelter is associated with nocturnal drainage of cold air, and is represented by the average height of the terrain above the valley over a radius of 10 km. The height above the valley is the most important variable and its maximum effect on diurnal range occurs in the early autumn, when the difference in soil moisture deficit between valleys and summits reaches its maximum. Large‐scale shelter is more important in winter than in summer, and for diurnal range, for rare rather than common events. This represents the increasing importance of nocturnal air drainage with the length of the night and suitable radiating conditions. For minimum temperatures in winter, however, the maximum impact of large‐scale shelter occurs for events with a return‐period around 2 years, and which are loosely associated with cold spells lasting a day or so. More extreme temperatures are usually attained at the end of cold spells lasting several days, when continued radiational cooling over level ground can produce temperatures as low as those attained through cold air drainage.