NO. 17.] 



PRECIPITATION. 



527 



The diurnal period comes out somewhat different in the different months. 

 The period of observation is rather short to give a trustworthy average for 

 a single month; but the means for the seasons seem to deserve fair reliance. 

 PI. IX. 1 mm. = 1 per cent. 



In Winter there are minima at 3 a. m. and at 2 p. m., and maxima at 

 8 a. m. and at 10 p. m. The range is 0.024. 



In Spring the minimum occurs at 4 a. m., and the maximum at 2 p. m. 

 The range is 0-092. 



In Summer the minimum occurs at 6 a. m., and the maximum at 10 

 p. m. The range is 0.075. 



In Autumn we have two minima, at 2 a. m., and at 5 p. m., and two 

 maxima, at 9 a. m. and at 10 p. m. The range is 0'028. 



The early morning hours in all seasons have the least precipitation, and 

 as shown above, p. 513, generally the least amount of cloud. At 10 p. m. 

 winter, summer and autumn have a maximum. Winter and autumn have 

 minima in the afternoon, spring has a maximum at the same time. The 

 diurnal periods of the amount of cloud and of the probability of precipitation 

 are not coincident. 



THE ANNUAL PERIOD. 



The following Table shows the mean values of the probability of preci- 

 pitation for the different months. 



