8S2 



Handbook of Nature-Study 



Snow crystal formed in high clouds. 

 Photomicrograph by W. A. Bentley. 



The high clouds are composed of 

 ice crystals formed from the cloud mists; 

 such ice clouds form a halo when veiling 

 the sun or the moon. 



When the water changes to vapor and 

 is absorbed into the atmosphere, we call 

 the process evaporation. The water 

 left in an open saucer will evaporate more 

 rapidly than that in a covered saucer, 

 because it comes in contact with more 

 air. The clothes which are hung on the 

 line wet, dry more rapidly if the air is dry 

 and not damp ; for if the air is damp, it 

 means that it already has almost as much 

 water in it as it can hold. The clothes 

 will dry more rapidly when the air is hot, 



because hot air takes up moisture more 



readily and holds more of it than does 



cold air. The clothes will dry more 



rapidly on a windy day, because more air 



moves over them and comes in contact 



with them than on a still day. 



If we observe a boiling teakettle, we 



can see a clear space of perhaps an inch 



or less in front of the spout. This space 



is filled with steam, which is hot air 



saturated with hot water vapor. But 



what we call "steam" from a kettle, is 



this same water vapor condensed back 



into thin drops of water or mist by 



coming into contact with the cooler air Blizzard type of snow crystal formed 



of the room. When the atmosphere is in low cloud. 



dry, water will boil away much more Photomicrograph by W. a. Bentley. 



rapidly than when the air is damp. 



The breath of a horse, or our own breath, is invisible during a warm day; 

 but during a cold day, it is condensed to mist as soon as it is expelled from 

 the nostrils and comes in contact with the cold air. The one who wears 

 spectacles finds them unclouded during warm days; but in winter the 

 glasses become cold out of doors, and as soon as they are brought into con- 

 tact with the warmer, damp atmosphere of a room, they are covered 

 with a mist. In a like manner, the window-pane in winter, cooled by 

 the outside temperature, condenses on its inner surface the mist from the 

 damp air of the room. 



The water vapor in the atmosphere is invisible, and it moves with the 

 air currents until it is wrung out by coming into contact with the cold. 

 The air thus filled with water vapor may be entirely clear near the surface 

 of the earth; but, as it rises, it comes in contact with cooler air and dis- 

 charges its vapor in the form of mist, which we call clouds; and if there is 

 enough vapor in the air when it meets a cold current, it is discharged as rain 

 and falls back to the earth. Thus, when it is very cloudy, we think it will 

 rain, because clouds consist of mist or fog; and if they are subjected to a 

 colder temperatiire, the mist is condensed to rain. Thus, often in moun- 



