CLOUDS AND CLOUD FORMS 
719 
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 usnal altitude is about 
four thousand feet, and its movement is more 
rapid than that of the cumulus. 
THE Nrmpvs (4) is the rain clond, 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 
The cumu- 
lo-ninmbus. 
nimbus. 
