52 Introduction to the Study of Science 



Hail. In some localities hail may fall during the summer. 

 Examination of large hailstones shows that they are made up 

 of alternate layers of clear and of opaque ice. This suggests 

 the manner of their formation. Raindrops formed very high 

 and falling through air currents with a temperature below freez- 

 ing become frozen. Then falling through warm, moist air or 

 through a rain cloud, they accumulate a covering of water, are 

 caught and carried by a strong updraft into colder air currents, 

 where the water is frozen into clear ice. They acquire here a 

 coating of snow-like ice, and again falling into a rain cloud 

 receive another coating of water. Thus they are whirled 

 many times through alternate warm and cold air currents be- 

 fore they become heavy enough to fall through the updraft or 

 are thrown beyond its control and fall to the earth. 



Snow. When water vapor condenses at a temperature 

 below the freezing point, it forms small ice particles which 

 combine to form snow crystals. Snow crystals frequently 

 have six rays from a common center, as you may find by ex- 

 amination ; but they may take any of a large number of hexag- 

 onal shapes. Snow crystals unite, forming snowflakes which, 

 if the temperature is favorable, fall to the earth. 



Snowflakes imprison in their feathery masses a great deal of 

 air. Air and snow together make up a blanket that protects 

 from freezing the soil and the plant and animal life below its 

 surface. It does this by checking the escape of heat from the 

 soil. It also keeps a fairly uniform soil temperature by retard- 

 ing the escape of heat and protecting the soil from the direct 

 rays of the sun. Among other benefits of the snow may be 

 mentioned its influence upon the humidity of the atmosphere, 

 upon the conservation of water in the soil, upon the stream 

 supply, and upon the lumbering industry. Explain how these 

 are influenced. 



Dew. Often in clear weather the temperature of the land 

 surface and of many kinds of objects on the land is lowered at 

 night by radiation of the heat, which was received from the sun 



