Manchester Memoirs, Vol. lix. (191 5), No. {>. 



VI. Note on the Monthly Variation of Sunshine. 

 By Professor W. W. Haldane Gee, B.Sc, M.Sc.Tech. 



{Read January 26th, igij. Received for publication, January 29th, 1Q15.) 



In connection with the work of the Air Pollution 

 Advisory Board of the Manchester City Council, I have 

 examined the records of the Sunshine taken with the 

 Campbell-Stokes glass sphere, which concentrates the 

 sun's rays and gives a charred trace on a graduated card 

 placed at the focus of the sphere. These records have 

 been obtained at a number of stations for many years. 

 I find that by averaging the figures over a number of 

 years, the percentage of hours of sunshine, calculated on 

 the number of hours from sunrise to sunset, reaches a 

 maximum value in the month of May. I am not aware 

 if this has been previously noted. It is clearly shown 

 graphically in the Figs, i and 2. 1 All these exhibit a 

 maximum in May and a lower one in August. 



That it is not a coincidence for these years is shown 

 by an inspection of Fig. 3, which applies to Stonyhurst 

 for the 33 years from 1880 to 19 12. 2 



The observations made at the Godlee Observatory by 

 Mr. Jenkins for 1906-1910 (see Fig. 4) show the same law, 

 but not in so marked a manner. 



It cannot be expected that each year will follow the 

 rule. It is followed for the year 19 10 for a number of 

 stations, as shown in Fig. 5, but in 191 1 the maximum 

 value is either in July or in August. 



Sunlight in May must have a powerful influence on 



1 From the records given l>y R. IT. Scott, Quarterly Journal Royal 

 Meteorological Society, p. 205, vol. xi., 18S5. 



- From results of observations at Stonyhurst by the Rev. \Y. Sidgreaves, 

 I9I3- 

 March /jlli, rgif. 



