Seasonal Variation of Atmospheric Temperature, &c. 61 



" On the Seasonal Variation of Atmospheric Temperature in the 

 British Isles and its Relation to Wind-direction, with a Note 

 on the Effect of Sea Temperature on the Seasonal Variation 

 of Air Temperature." By W. N. SHAW, M.A., F.R.S., 

 Secretary to the Meteorological Council, and R. WALEY 

 COHEN, B.A. Received June 11, Read June 20, 1901. 



The following paper is mainly concerned with the analysis of the 

 seasonal variation of temperature of the British Islands into a series of 

 simple harmonic curves. The variation of temperature is so irregular 

 that the use of this method of analysis for the investigation of the 

 subject may seem to be arbitrary and inappropriate, and a few words 

 of introduction are accordingly necessary to indicate the circumstances 

 under which this mode of dealing with the subject showed itself to be 

 specially adapted for the purpose. 



Observations have been made for the Meteorological Office since 1871 

 with self-recording instruments at the four observatories, Kew, Aber- 

 deen, Falmouth, and Valencia, and the daily means of pressure and 

 temperature from the twenty-four hourly readings of the curves of the 

 barometer and dry-bulb thermometer have been worked out for twenty- 

 five years, but have not yet been published. The means for temperature 

 are represented in Diagram 1 (p. 62), by curves whose co-ordinates are 

 respectively proportional to the number of days since the beginning 

 of the year, and the corresponding twenty-five-year mean of the day's 

 temperature at the several observatories. 



The following points may be noticed upon an inspection of the 

 curves : 



(1.) The summer portions of the curves have a larger amplitude and 

 a shorter duration than the winter portions. 



(2.) The curves do not exhibit a smooth run, but show a number of 

 irregularities which are, in some instances, particularly in May and 

 December, of considerable magnitude. 



Under these circumstances it is difficult to assign any specific number 

 as the normal mean temperature for the particular day to which any 

 actual observed temperature can be referred. To put the same point 

 in other words, it is difficult to say whether a depression between two 

 elevations is to be regarded as a period of abnormally low temperature- 

 between two normal periods, or as a period of normal temperature 

 between two abnormally high periods. 



It will be noticed that on the diagram smoothed curves have been 

 drawn, which represent with remarkable fidelity the general sweep of 

 the curve, cleared of the irregularities of small period. The smoothed 



