RAINFALL 



241 



272. Rainfall. Rain is caught for measurement in a 

 rain gauge (Fig. 220). This is really a funnel with a long 

 stem. The opening of the funnel has exactly 10 times the 

 area of a cross section of the stem, so that Yio of an inch 

 of rainfall gives a depth of 1 inch in the stem. The greater 

 depth of water in the stem makes the reading more accu- 

 rate. Snow is estimated as rain, the snow being allowed to 

 melt. About 10 inches of snowfall equals 

 1 inch of rain; but the relation is not 

 accurate for any particular case, since 

 snow falls in so many different degrees of 

 wetness. 



Scientists have found that there are 

 several rules, or laws, of rainfall. Among 

 them are the following : 



(1) Rainfall is less in the interior of 

 continents than on the coast. The 



amount for each continent depends on how far rain 

 clouds can penetrate before a mountain barrier or 

 plateau condenses their moisture. 



(2) The greatest rainfall occurs in the tropics. This is 

 due to the great capacity of hot air for moisture, or, better, 

 to the great evaporation that can occur at high tempera- 

 tures. 



(3) When rainfall takes place on mountain slopes, the 

 sides get more than the base. This is true for the reason 

 that the moist air currents that bring the rain must go up 

 a certain distance before they are chilled enough to give 

 their greatest deposits. 



Let us now consider some illustrations of these rules. No regions 

 are absolutely rainless, but half of the earth's land surface gets less than 



