CLOUDS AND CLOUD FOEMS 



79 



at a slightly lower level, is not a tall tower 

 cloud, and has in it an admixture of the nim- 

 bus or rain cloud. It is in fact a form of rain 

 cloud and is responsible for the "sun-shower " as 

 well as for others of greater force, like the thun- 

 der-shower. It is also a cloud that shows a sil- 

 ver lining when seen against the sun, and at 

 night it reflects heat-lightning very brilliantly. 

 In the daytime its base appears dark, its top 

 light ; and at twilight, when lying off in the 

 east, it banks up at times like a table mountain 

 in layers and terraces that reflect the pinks and 

 violets of the sunset. Its usual altitude is about 

 four thousand feet, and its movement is more 

 rapid than that of the cumulus. 



THE NIMBUS (4) is the rain cloud, and every 

 cloud from which rain falls is some form or 

 combination of the nimbus, though the nimbus 

 proper is the flat, sheet-like or rolled rain cloud. 

 It is the closest to the earth of all the clouds 

 and is consequently the first one to receive the 

 smoke, dust, and heat arising from the earth. 

 By comparison it is a foul cloud, and is for that 

 reason a rain cloud the formation of vapor 

 spherules being, perhaps, dependent upon the 

 presence of dust-particles in the air. The nim- 

 bus takes all forms according to its density and 



