A Note on the RelationsMj) hetween Cloud and Sunshine. 
141 
Table III shows, for each grade of sunshine from less than 66 per cent, to 
greater than 91 per cent., the variations month by month in the percentages 
of sunshine plus cloud. In each month there is a tendency to smaller numbers 
as the sunshine increases ; but the annual means (especially when they are 
smoothed in threes, as in the last column), although they establish a gradual 
fall, do not give the same rate of fall ^s Table I. Thus in Table III 70 per 
cent, of sunshine corresponds to 11 7-5 per cent, of sunshine plus cloud, while 
89 per cent, of the one corresponds to 104-8 per cent, of the other — that is, 
a rising range of 19 per cent, of the one corresponds to a falling range of 12 '7 
per cent, of the other. As against this a rising range of about 11 per cent, 
of sunshine in Table I corresponds to a falling range of somewhere about 15 
per cent, of sunshine plus cloud. 
We may judge from these results that a sunshine recorder is not (as it 
is sometimes asserted to be) an automatic device for determining the average 
cloudiness of the sky. Both sunshine and cloudiness have common elements 
it is true, yet they are no more convertible terms than are, say, the tem- 
peratures of the air and of radiation, or, better still, than are evaporation 
and rainfall. 
Greatest and least monthly percentages of sunshine recorded and 
cloudiness observed, 1900 to 1919, are given in Table IV. Here the cloudi- 
ness is deduced from six observations a day, two being night hours. 
Table IV. — Monthly Maxima and Minima of Sunshine and Cloud. 
Sunshine. 
Cloud. 
Max. 
Min. 
Eange. 
Max. 
Min. 
Range. 
(%)• 
(%)• 
(%). 
(%)• 
(%). 
(%). 
January 
83 
63 
20 
59 
26 
33 
February . 
83 
59 
24 
65 
31 
34 
March 
84 
62 
22 
52 
26 
26 
April 
91 
69 
22 
43 
13 
30 
May . 
94 
68 
26 
43 
5 
38 
June . 
90 
74 
16 
31 
9 
22 
July . 
92 
68 
24 
35 
3 
32 
August 
95 
72 
23 
33 
5 
28 
September . 
89 
65 
24 
45 
11 
34 
October 
90 
67 
23 
48 
22 
26 
November . 
92 
66 
26 
55 
16 
39 
December . 
84 
71 
13 
48 
25 
23 
Thus there has been an August (1906) with almost uninterrupted 
sunshine, and a July (1914) with only 3 per cent, of cloud. June has 
never had more and February never less than 31 per !cent. of sky clouded — 
the June maximum in 1913, the February minimum in 1919. 
