XVI INTRODUCTION. 



forced upwards, if it contains much aqueous vapor, a thin film of 

 cloud, as over the top of 2, (p. xii.) will be formed in it by the cold of 

 diminished pressure, entirely distinct from the great dense cumulus 

 below ; but as the cumulus rises faster than the air above it, (for 

 some of the air will roll off) the thin film and the top of the cu- 

 mulus will come in contact; and sometimes a second film or cap 

 may be formed in the same way, and perhaps a third and fourth. 

 When these caps form, there will probably be rain, as their form- 

 ation indicates a high degree of saturation in the upper air. (96.) 

 When the complement of the dew point is very great, (twenty 

 degrees and more) clouds can scarcely form ; for up-moving col- 

 umns will generally either come to an equilibrium with the sur- 

 rounding air, or be dispersed before they rise twenty hundred 

 yards, which they must do in this case, before they form clouds. 

 Sometimes, however, masses of air will rise high enough to form 

 clouds ; but they are generally detached from any up-moving col- 

 umn underneath, and of course cannot then form cumuli with flat 

 bases ; such clouds will be seen to dissolve as soon as they form, 

 and even while forming, they will generally appear ragged, thin, 

 and irregular. Moreover, if the ground should be colder during 

 the day, than the air in contact with it, as it sometimes happens 

 after a very cold spell of weather, then, as the air touching the 

 cold earth will be colder than the stratum above it, ascending col- 

 umns cannot exist, and of course no cumuli can be formed on that 

 day, even though the air may be saturated with vapor to such a 

 degree as to condense a portion of it on cold bodies at the surface 

 of the earth. Also, if during the whole winter, any part of Sibe- 

 ria, or the northern part of North America, should be so much 

 colder than surrounding regions, that no up-moving columns could 

 be produced, then neither clouds nor snow could be formed. 1 

 Neither can clouds form of any very great size, when there are 

 cross currents of air sufficiently strong to break in two an ascend- 

 ing current, for the ascensional power of the up-moving current 

 will thus be weakened and destroyed. Immediately after a great 

 rain, too,*when the upper air has yet in it a large quantity of cal- 

 oric, which it received from the condensation of the vapor, the 

 up-moving columns which may then occur, on reaching this upper 

 stratum, will not continue their motion in it far, from the want of 

 buoyancy ; therefore, they will not produce rain, nor clouds of 

 any kind, but broken cumuli. Besides, as the air at some distance 

 above the surface of the earth, and below the base of the cloud, 



1 There is a district in Siberia, mentioned by Erman, where, during winter, 

 snow never falls, and clouds are unknown. Report of Committee of Royal 

 Society on Physics and Meteorology, page 45. 



