110 EXPERIMENTAL GENERAL SCIENCE 



noted that this latter phase of condensation is a form of 

 sublimation. 



97. Cloud Forms. A considerable number of cloud forms 

 have been distinguished and named, the principal ones being 

 easily recognized. Highest of all are certain thin wisps of 

 cloud commonly called "mares' tails" and known to the 

 meteorologists as cirrus clouds. They may be nearly ten miles 

 above the earth and are supposed to consist of small ice 

 crystals. The cirro-cumulus clouds produce the familiar 

 "mackerel sky." They are in the form of short, often curved, 

 sections and are nearly as high in the air as the cirrus clouds. 

 Cumulus clouds are the fleecy day clouds which appear like 

 great piles of wool in the sky. They are much nearer the 

 earth seldom more than a mile high. Cumulus clouds may 

 continue to enlarge until they form the cumulo-nimbus clouds 

 called "thunder heads" which produce most of our thunder 

 storms. The nimbus cloud-, found at an altitude of less than 

 a mile, is the ordinary rain cloud which produces the all-day 

 rains. Lowest of all are the stratus clouds flat, level clouds 

 of no distinct form, seen most frequently in the early part of 

 the day. They are seldom more than half a mile high, and 

 usually disappear as the day advances. Fog may be called 

 a stratus cloud near the ground. 



Practical Exercises 



1. Into a metal cup or can with a brightly polished outer surface put 

 a quantity of cold water. Then drop pieces of ice into the water stirring 

 it with a thermometer until a film of moisture begins to appear on the 

 outside of the vessel. The reading of the thermometer at this instant 

 will give the approximate temperature of the dew-point in the place 

 where the experiment is made. What was the temperature of the 

 dew-point in your experiment? 



2. Account for the formation of frost on window panes in cold weather 

 (93). 



