260 PHYSIOGRAPHY 



without rising; (3) when it rises by convection, for it is then cooled 

 both (a) by being brought to cooler air, and (&) because it expands; 

 and (4) when cooler air is brought to warmer air. Rains due to (1) 

 are not rare in mountain regions, and rains due to (3) are common 

 where convection currents are strong, as in the region of tropical 

 calms, where they occur almost daily during the hot season. 



The distribution of rainfall is dependent, in large measure, on 

 the winds, and will be considered later. 



Dew and frost. It sometimes happens that the temperature 

 of the surface of the land becomes lower than the dew-point of the 

 air. This is likely to be the case in the clear nights of late summer 

 and autumn. If the temperature of the grass blades, for example, 

 becomes lower than the dew-point of the surrounding air, moisture 

 from the surrounding air will be condensed on them. Such mois- 

 ture is dew if the temperature of condensation is above 32. Dew 

 does not fall, but condenses on the surface of solid objects. A 

 good illustration of dew is often furnished by the moisture which 

 gathers on the outside of a pitcher of ice-water in a summer day. 

 The temperature of the pitcher is below the dew-point of its sur- 

 roundings, and moisture from the air therefore condenses on it. 

 Dew forms on still nights rather than windy ones, because the wind 

 tends to move away the air which is approaching its dew-point, 

 supplying other air in its place, and the incoming air is often wanner 

 than that which moved on. Dew is more likely to form on clear 

 nights than on cloudy ones, because radiation and cooling are 

 greater when there are no clouds. 



When the temperature of the dew-point is below 32 F., the 

 moisture which condenses on solid objects condenses as frost in- 

 stead of dew. Frost is not frozen dew any more than snow is frozen 

 rain. It stands in the same relation to dew that snow does to rain. 

 In the autumn, frost is more likely to occur in valleys and on low 

 flats than on adjacent hills, because the colder air settles to the 

 lower levels. 



Dew, and sometimes frost, may form on the undersides of ob- 

 jects. If a pan is placed bottom up on the ground, there will often 

 be dew on the inside of it in the morning. There is often dew on 

 the underside of a flat stone when there is none on its top. This 



