[Manuscript received M a y 13-rend June 21, 1099.1 W h a t is the greatest diurnal range of air temperature likely to occur anywhere in the British Isles under existing climatic conditions? This is a question t o which published statistics can give no satisfactory answer. At the present time even the maximum thermal variation actually registered within a single day a t any official observing station during the last half-century could only be determined a t the cost of a search through many thousands of manuscript monthly returns. If this laborious task were to be undertaken, the figure so obtained for extreme diurnal range would not necessarily represent an approximation to the probable limiting value, since the climatological exploration of our islands is still far from complete. It is well known that the widest vicissitudes of temperature are ordinarily found a t the bottom of valleys, the reason being that in such a situation a maximum contact area of land surface is available for the transfer of heat to the lowest layers of the atmosphere during sunshine, while a t night drainage of cold air from the adjacent higher ground, with consequent stratification and stagnation, produces the maximum cooling effect during the prevalence of quiet, clear weather.Unfortunately for the study of local climatology, there exists a strong prejudice against living a t the bottom of a valley, the belief being, apparently, that such a position is ordinarily damp and unhealthy. Estate agents can testify that '
Records of temperature over a 13-year period in a Hertfordshire \alley which has been described as possessing the most " continental " thermal climate yet investigated instrumentally in the British Isles are sumniarised, discussed, and compared with data from other
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