20 



SCIENTIFIC SURVEY OF YORK AND DISTRICT 



temperature ; (b) that the diurnal range of absolute humidity (vapour 

 pressure) is very small. Inspection of observatory statistics shows that 

 the vapour pressure at 21 h. is, on the average, within about 0-2 mb. 

 of the vapour pressure at 14 h. or 15 h., the normal time of occurrence of 

 the maximum temperature. We may, therefore, calculate approximate 

 mean minimum values of relative humidity from the relationship : 



Minimum relative humidity = 



Vapour pressure at 21 h. 



X 100. 



Saturation pressure corresponding to maximum temperature 



The data given in the last column of Table III and plotted in Fig. i 

 have been calculated in this way. Data for other stations are not avail- 

 able for comparison, but it has been thought desirable to give the figures 

 for York, partly because of their intrinsic interest and partly to illustrate 

 the fact that more information can be obtained from humidity data based 

 on readings at morning and evening hours than is sometimes supposed. 



Sunshine. 



The normal daily duration of bright sunshine is given in the second 

 column of figures in Table IV. The third column gives the percentage 

 of the astronomically possible sunshine, the figures being obtained by 



Table IV. — Sunshine and Miscellaneous Phenomena. 



