viii Atmospheric Phenomena 117 



A great quantity of air is entangled between the spicules of 

 snowflakes, and this makes a covering of snow act as a non- 

 conductor of heat almost as perfectly as a covering of 

 feathers preventing radiation from the Earth at night, and 

 so keeping the ground from freezing in cold weather. 

 Under heavy pressure snow is compacted into solid ice. 



171. Hail. In winter there are often showers of tightly 

 packed little snowballs about the size of small shot or 

 rarely as large as peas. This is called soft hail, and it 

 appears to be formed by the larger ice particles in a deep ice 

 cloud overtaking and adhering to the smaller ones. True 

 hail is a different thing, which only occurs in warm weather 

 usually as an accompaniment of thunderstorms ( 173) or 

 tornadoes ( 209). True hailstones are lumps of ice which 

 sometimes weigh several ounces, and occasionally as much 

 as 3 Ibs. A shower of such masses is very destructive, 

 breaking windows, cutting down standing crops, and 

 often killing animals or even people. The, hailstone 

 when cut across usually shows alternate layers of clear 

 ice and of compact snow. According to Ferrel such a 

 hailstone is produced by an ordinary soft hailstone formed 

 at a great height falling into a rain-cloud, where it gets 

 a coating of water, and then being carried by an ascending 

 current into a high cold region, where the water is frozen 

 into clear ice and a deposit of snow takes place outside. 

 The same hailstones may be caught in ascending and 

 descending currents several times in succession, thus getting 

 alternate coats of ice and snow. This theory accounts 

 for true hailstones only occurring in summer, for it is 

 only in hot weather that powerful ascending currents of 

 air are formed. 



172. Electrification of the Atmosphere. Every 

 change in the atmosphere, particularly evaporation, con- 

 densation and wind, gives rise to some disturbance in the 

 distribution of electricity. As electricity resides on the 

 surface of a body, it follows that when the minute particles 

 of a cloud are uniting to form rain-drops, their electrical 

 potential ( 76) is rapidly rising, because the surface of a 

 large rain-drop is smaller than the total surfaces of the 



