Earth Study 



853 



tainous regions, the fog may be seen streaming and boiling over a mountain 



peak, and yet always disappears at a cer- __ 



tain distance below it. This is because the 

 temperature around the peak is cold and 

 condenses the water vapor as fast as the 

 wind brings it along, but the mist passes over 

 and soon meets a warm current below and, 

 presto, it disappears! It is then taken 

 back into the atmosphere. The level base 

 of a cumulus cloud has a stratum of warmer 

 air below it, and marks the level of con- 

 densation. 



At the end of the day, the surface of the 

 ground cools more quickly than the air 

 above it. If it becomes sufficiently cold 

 and the air is damp, then the water from it 

 is condensed and dew is formed during the 

 night. However, all dew is not always 

 condensed from the atmosphere, since some 

 of it is moisture pumped up by the plants, 

 which could not evaporate in the cold night 

 air. On windy nights, the stratum of air 

 cooled by the surface of the earth is moved 

 along and more air takes its place, and it 

 therefore does not become cold enough to 

 be obliged to yield up its water vapor as 

 dew. If the weather during a dewy night 

 becomes very cold, the dew becomes 

 crystallized into hoar frost. The crystals 

 of hoar frost are often very beautiful and 

 are well worth our study. 



The ice on the surface of a still pond 

 begins to form usually around the edges 

 first, and fine, lancelike needles of ice are 

 sent out across the surface. It is a very 

 interesting experience to watch the ice 

 crystals form on a shallow pond of water. 

 This may easily be seen during cold winter 

 weather. It is equally interesting to 

 watch the formation of the ice crystals in a 

 glass bottle or jar. Water, in crystallizing, 

 expands, and requires more room than it 

 does as a fluid; therefore, as the water 

 changes to ice it must have more room, and 



Dew on spider's web; Dewdrops on 

 strawberry leaf; Hoarfrost on 



strawberry leaf. 

 Photographs by W. A. Bentley 



often presses so hard against the sides of the bottle as to break it. The ice 

 in the surface soil of the wheat fields expands and buckles, holding fast in 

 its grip the leaves of the young wheat and tearing them loose from their 

 roots; this "heaving" is one cause for the winter-killing of wheat. Sleet 

 consists of rain crystallized in the form of sharp needles. Hail consists of 

 ice and snow compacted together, making the hard, more or less globular 

 hailstones. 



