CLOUDS, RAIN, SNOW, AND HAIL. 277 



What is the TliG Stratus,* or stratified cloud, consists 

 stratus cloud ? ^£ horizontal streaks, or layers of vapor, which 

 float like a veil at no very great elevation from the surface 

 of the earth. They frequently appear with extraordinary 

 brilliancy of color at sunset. 



The appearance of the stratus is represented at c, Fig. 225. 



•What is the ^^^^ Nimbus, OF the cloud of rain, has no 



Nimbus? characteristic form. It generally covers the 



whole horizon, imparting to it a bluish black appearance. 



The various forms of clouds gradually pass into each other, so that it ia 

 often difficult to decide whether the appearance of a cloud approaches more 

 to one t^-pe than another. The intermediate forms are sometimes designated 

 as cirro-stratus, cirro-cumulus, and cumulo-stratus. 



609. Eain is the vapor of the clouds or air 

 condensed and precipitated to the earth in drops. 

 How is rain Raiu is generally occasioned by the union of 

 occasioned? ^^^ ^j. morc volumcs of humid air, differing 

 considerably in temperature. Under such circumstances, 

 the several portions in union are incapable ot absorbing the 

 same amount of moisture that each could retain if they 

 had not united. The excess, if very great, falls as rain ; 

 if of slight amount, it appears as cloud. 

 Upon what law ^1^- ^hc law upou which the condensation 

 uon^of rliHel of vapor and the formation of rain depends is, 

 P^°**^ that the capacity of the air for moisture de- 



creases in a greater ratio than the temperature. 



611. Rain falls in drops, because the vesicles of A-apor, in 

 ^oTin'dro'mi? *'^®"' <iescent, attract each other and merge together, thus 

 forming drops of water. The size of the drop is increased in 

 proportion to the rapidity with which the vapors are condensed. 



In rainy weather the clouds foil toward the earth, for the reason that they 

 are heavy with partially-condensed vapors, and the air, on account of its 

 diminished density, io less able to buoy them up. 



612. The quantity of rain falling at any one time or 

 place, is measured by means of an instrument called a 

 " Kain-Guage." 



This usually consists of a tin cylindrical vessel, M, Fig. 

 ^^^Gua<'*^.^ 226, the upper part of which is closed by a cover, B, in the 

 shape of a funnel, with an aperture in its center. The water 

 * Stratus, from the Latin stratus — that which lies low in the form of a bed or layer. 



