AbstractHitherto, for synoptic and climatic purpose, means of temperature at the surface, and in the upper air, have been obtained without regard to the different classes of air masses, the means being for all masses combined.With the object of obtaining a greater insight into the structure of the atmosphere, it is pertinent to inquire what are the difference between (i) the vertical distribution of temperature, and (ii) the temperature near the surface of the earth, in the different classes of air which cross the British Isles.This investigation gives, for the winter season (December, January, and February combined), for 12 different classes of air over the British Isles: (i) the average and extreme temperatures, the lapse rates and the degree of thermal stability at a number of levels in the atmosphere, and the heights of the 32°F. and 8°F. isotherms; (ii) the daily maximum and minimum temperatures, the mean temperature, and daily range of temperature at the surface at Kew during the 24‐hour period, 0‐24 h., on days when no important change of air mass occurred during this period. The levels in the atmosphere for which values have been obtained are 1,000, 2,000, 3,000, 5,000, 10,000, 15,000, and 20,000 feet. Over 600 upper air ascents, chosen for the most part during the winters of 1930‐31 to 1943‐44, and 1,000 surface maximum and minimum temperatures at Kew during the winters 1924‐25 to 1943‐44 have been used. For comparison, and to obtain an estimate of the changes of temperature and stability, means of temperature at the same levels in the atmosphere were obtained over western Russia in Siberian air, and over the Icelandic and Azores regions in direct polar air and in tropical air. The reality of the difference of temperature between the various classes of air chosen has been examined. A discussion of the physical problems raised in this paper is reserved for another paper.