On some Meteorological Conditions controlling Nocturnal Radiation. 385 
TABLE 5. 
Frequency of Dew. 
Eelative 
Humidity. 
On Clear 
Nights. 
On Cloudy Nights. 
On Very Cloudy Nights. 
% 
90 and over... 
No. 
39 
42 
46 
47 
57 
33 
8 
20 
32 
29 
19 
6 
2 
No. 
Cloud %. 
29 
22 
24 
14 
3 
10 
No. 
26 
9 
6 
3 
5 
2 
4 
Cloud %. 
82 
80 
65 
67 
100 
95 
83 
85-89 
80-84 
75-79 
70-74 
65-69 
60-64 
272 
108 
55 
The above are hourly values. Thus of the whole number of nights 
considered, 190 were clear, and these comprised 950 separate hours of 
observation. Of these 950 hours 272 had dew. Again, out of 890 hours 
at which cloud was noted, 163 (= 108 + 55) had dew. Eoughly speaking, 
29 per cent, of the clear nights, 25 per cent, of the cloudy, and 12 per cent, 
of the very cloudy, had dew before midnight. On pretty well every clear 
night dew will be deposited as soon as the relative humidity at a height 
of 4 feet falls below 75 per cent. ; and also on pretty well every moderately 
clouded night as soon as it falls below 80 per cent. There would, no 
doubt, be a greater number of dewy evenings noted under very clouded 
skies were it not that the rain often intervenes to mask it. 
As might have been expected in accordance with current theories, the 
averages of Tables 2, 3, and 4, show that the radiation-temperature 
gradient is less as the sky is more clouded. But the quantity of water 
vapour in the air, and also the relative humidity, increase with increase 
of cloud. Hence, still following current theories, the quantity of water 
vapour might be regarded as contributing to the result. If, however, we 
examine these three Tables in detail we see that D gradually diminishes 
as the hour becomes later, the state of the sky, clouded or otherwise, 
apparently making very little difference in the rate of its decrease. 
Concurrently the vapour pressure — as indicated by the dew-point tem- 
perature — falls, though here this fall is greatest when the nights are 
clear; also the relative humidity increases, and, curiously enough, faster 
when the sky is less than one-half obscured ; while the wind rises on the 
whole both when the sky is clear and when it is very clouded, remaining 
fairly constant for a small amount of cloud. 
