The Range of Variation of the British Climate

To many of us the virtues of our day-to-day changeable weather are manifest. Seventy years of standardized meteorological records give us a good idea of the limits within which any day's events are likely to fall. In the southern English lowlands a fall of two feet of snow, a shade temperature of 970, or a month with 70 per cent. of the possible sunshine, represent extremes unlikely to be surpassed. Superimposed upon our day-to-day weather how? ever we now recognize that small but definite fluctuations of climate exist and that since about 1925 an appreciable amelioration has set in round the North Atlantic. Professor Ahlmann has recently reviewed the many associated events.1 It remains for us to consider the possible amplitude of the variation of our climate in Britain, as distinct from the weather. I shall first comment on the magnitude of the present fluctuation in Britain, in terms of overall mean temperature. I shall follow with some evidence for past fluctuations provided by the instrumental records since 1700, relating them to the earlier climatic recessions demanded by the pollenanalysts in historic and prehistoric time. In the second part of the paper I shall address myself more to those technical experts who are acquainted with the geological and botanical background, and I shall give estimates for the temperature of the various phases since the last Glacial maximum in order to compare the range of variation over which our climate appears at intervals to have changed. In my view it is high time such estimates of the mean tem? perature of summer and winter were made, and an opportunity of doing this for Late Glacial times is now provided by using Professor Ahlmann's relationships between the height of the snow-line, the amount of accumulated pre? cipitation and the mean temperature of the ablation season. Comparisons between present-day fluctuations and past changes may throw light on the possible causes of climatic variation; some will be led to consider whether the present amelioration in north-west Europe will con? tinue, and for how long. Throughout this paper I shall largely confine atten? tion to the mean temperature of January and July as a sufficient initial indication of winter and summer characteristics. Periods of disturbed weather when the oceanic influence is greater are characterized by a diminished January-July range; in quieter and more "continental" phases the range increases. Meteorological aspects of this "varying vigour of the atmospheric circulation" are discussed by Angstrom * and Willett.3 1 H. W:son Ahlmann, "The present climatic fluctuation," Geogr.J. 112 (1948) 165-95. 3 A. Angstrom, "The change in the temperature climate in present time," Geogr. Ann. 21 (1939) 119. 3 H. C. Willett, "Long-period fluctuations of the general circulation of the atmo? sphere," Journ. Met., 6, 1949, pp. 34-50; "Solar-variability as a factor in the fluctua? tions of climate," Geogr. Ann. 31 (1949) 295. See also Q. J. R. Met. S., Centenary Proceedings, 1950.