REPORT ON ATMOSPHERIC CIRCULATION. 29 



Two maxima are here indicated, the one about or shortly after sunrise, and the 

 other in the early part of the afternoon ; and two minima, the one at noon and the 

 other from sunset to midnight. But the difference between the daily extremes is only 

 6 per cent, of the sky. The diurnal variations in the amount of cloud are among the 

 less satisfactorily observed phenomena of meteorology. From what has been done, 

 however, a few general deductions may be made. A maximum occurs in the morning 

 and continues till a little after sunrise, and this maximum is more pronounced over the 

 open sea than over land. Its appearance may be regarded as due to the general cooling 

 of the atmosphere through its whole height by terrestrial radiation, and its disappear- 

 ance by the heating of the air by the returning sun. The first of the two minima 

 extends from this time to about noon, this relatively greater clearness of sky occurring 

 thus while temperature is most rapidly increasing and before the ascending current has set 

 in in any considerable volume. The period of this ascending current, or the time of the 

 afternoon minimum of atmospheric pressure, marks the afternoon maximum of cloud, 

 which over the land surfaces of the globe is much larger than the morning maximum, 

 being thus the reverse of what the Challenger observations disclose. 



Of this maximum the cumulus is the characteristic cloud. These are but the 

 summits of the ascending currents that rise from the heated land, in which the 

 aqueous vapour is condensed into cloud during the expansion and consequent cooling 

 that takes place with increase of height. Cumulus clouds cast an instructive light 

 on the behaviour of the ascending currents rising from the more highly-heated lower- 

 most strata of the atmosphere, inasmuch as they indicate that the current ascending 

 from the surface is broken up into subdivisions that are thereafter grouped into 

 separate well-defined ascending currents, each of which is marked off and topped by the 

 cumulus cloud. It is highly probable, considering the clearly-defined positions of 

 these clouds, that the air composing the ascending currents is not only warmer but that 

 it is also moister than the air in and beneath the clear interspaces ; and, further, it may 

 be regarded as probable that it is down through these clear interspaces that the 

 descending air filaments shape their course in their way downwards to take the place 

 of the air molecules that ascend from the heated surface of the earth. 



The secondary minimum of cloud occurs from about sunset onwards during the 

 time occupied by the evening maximum of atmospheric pressure. The frequent 

 dissolving and final disappearance of cloud from about sunset onwards as the evening 

 advances is familiar to all, occurring in those types of weather, principally, when the 

 evening maximum of pressure for the day is most distinctly marked. 



It is to be noted here that in a highly -saturated atmosphere, which is so 

 characteristic a feature of many tropical cbmates at certain seasons, this time of 

 the day is remarkable for the amount of cloud ; and it is in those seasons, and during 

 those hours, that heat-lightning, or lightning without thunder, attains its annual 



