Dr Dennis Wheeler
Reader in Geography, University of Sunderland
The very low -21.1°C was recorded in February 1941 and March 1947
It is a common experience to find that mean temperatures tend to decrease with altitude.
But a study of the very lowest absolute temperatures recorded in the British Isles, yields the remarkable finding that many of them occur at low level.
This curiosity is a result of local topographical features that give rise to what are known as "frost hollows" - low-lying ground, often in a valley floor, into which relatively dense cold air will drain overnight.
This cold air then accumulates towards dawn to provide minima far lower than those found on the adjacent slopes and hilltops.
There are many such sites around the British Isles, but some are notorious for the marked effect that they exercise.
Houghall, not far from Durham City, is one such location. It lies in a deep valley carved by the River Wear.
On clear and calm nights - such cold air drainage cannot occur at other times - heat is lost from the ground by radiation and the cooling air immediately above the surface becomes denser and drifts slowly downslope to accumulate in the valley floor.
Houghall has recorded the lowest individual night-time temperatures anywhere in the region and between 1940 and 1970 it provided the lowest annual minima in England on no less than six occasions (1941, 1947, 1951, 1953, 1955 and 1965).
The lowest of these were the -21.1°C recorded in February 1941 and March 1947.
But even on calm summer nights, temperatures can fall to unseasonally, chilly levels reminding the unwary of the oddities of our local weather.
|Hottest day|| 33.9||Hartburn Grange on 3rd August 1990|
|Coldest day|| -23.3||Haydon Bridge on 21st January 1881|