156 FIRST TEAR SCIENCE 



On examining the map of the mean annual rainfall, we 

 see that there are large areas where it is not sufficient for 

 agriculture without irrigation. Such areas are within the 

 belts of dry winds or in continental interiors far from 

 large bodies of water. The rain-bearing winds coming 

 from the water are forced to rise and cool so that their 

 moisture is deposited before reaching these interior regions. 

 The rainfall of a place depends largely : (1) upon its 

 elevation, since most of the rain -bearing clouds lie at low 

 altitudes ; (2) upon the direction and kind of winds that 

 blow over it ; and (3) upon the elevation of the land about 

 it. The sides of mountains toward the direction from 

 which the rain-bearing winds approach will be well 

 watered, while the opposite side may be a dry desert. 

 Explain the cause of the dryness of five of the great dry 

 regions as found on the map on page 155. 



A cylindrical vessel having vertical sides, called a rain 

 gauge (Fig. 74), is used to determine the amount of rain. 

 It is placed in an open space away from all trees 

 and buildings and after each rain the amount col- 

 lected is measured. Snow is melted before it is 

 measured. As a rule eight or ten inches of snow 

 make an inch of rain. 



If the temperature is below the freezing point, 

 32 F., when condensation takes place, the mois- 

 ture of the air will form into a wonderful variety 

 of beautiful six-rayed snowflakes. These float 

 downward through the air and often cover the 

 ground with thick layers of snow. Although 

 snow is itself cold, yet it keeps in the heat of the ground 

 which it covers, so that in cold regions soil which is snow- 

 covered does not freeze as deeply as that without* snow. 

 Therefore, to keep water pipes from freezing, it is not 

 necessary to bury them as deeply in localities where 



