Abstract
This paper presents the results of an analysis of thermometric records at Greenwich Observatory over the period 1841‐1936 undertaken with the object of ascertaining the number of occasions on which each day of the year gave a mean temperature above and below the smoothed average value to specified extents. Days with positive departures of at least 10°F. and 15°F. are classed as “warm” and “very warm,” and those with corresponding negative departures as “cold” and “very cold,” respectively. Diagrams and tables referring these several categories to the seasons and calendar months, as well as to pentads and individual dates, are given and discussed. It is shown that the average year contains 17·5 “warm” days, 0·7 “very warm” days, 11 “cold” days, and 1·3 “very cold” days; that the winter months December to February provide 43 per cent of the “warm” days, 53 per cent of the “cold days,” and 86 per cent of the “very cold” days; that there is an entire absence of “very cold” days from March 17 to November 15; and that only three “cold” days are on record for August. Instances, 15 in all, of very exceptional heat and cold, represented by daily mean temperatures at least 20°F. above and below the average, are also listed, special reference being made to the single extant case of a positive departure so great as 20°F. (May 22, 1922). Explanations are advanced for anomalies brought to light by the investigation.