^Evaporation, 67 



the make-up of the moisture globules of which 

 the clouds are formed. If these globules are 

 in an advanced stage of condensation the 

 cloud is darker and more opaque. In earlier 

 conditions of condensation the cloud will have 

 a bright look, which shows that it reflects most 

 of the light, whereas in the case of the dark 

 cloud the light is largely absorbed. 



There is a sort of notion prevailing that 

 clouds come up from the horizon, and in many 

 cases they do, but they may form directly over 

 our heads. There always has to be a begin- 

 ning, and that occurs wherever the conditions 

 are most favorable for condensation of vapor. 

 If the earth is wet and the sun is hot the 

 evaporation may be very rapid as well as the 

 ascent of the invisible moisture, which carries 

 with it the air, which in turn expands the 

 higher it rises, thus producing cold. This, 

 taken with the normal cold that exists in the 

 higher regions, may be sufficient to produce a 

 sudden condensation of this ascending vapor, 

 which is all that is necessary to form a cloud. 



The inquiry may arise, Why is the moisture 

 condensed, almost always, in the upper regions 

 of the air, where it is rare ? Because the more 

 rare and therefore expanded it is, the more 

 moisture it will hold. This, taken with the 

 fact that cold currents are encountered high 

 up, sufficiently answers the question. 



It is interesting to know that the processes 



