270 EARTH FEATURES AND THEIR MEANING 



move horizontally, just as rising vapor spreads out in a room be- 

 neath the ceiling. Above this level, as far as exploration has gone, 

 or to a height of more than twelve miles, the temperature remains 

 nearly constant, and this upper zone is, therefore, called the iso- 

 thermal or the advective zone the uniform temperature zone 

 of the lower atmosphere. Beneath the convective ceiling the 

 process of convection is characteristic, and this zone is therefore 

 described as the convective zone (Fig. 298). 



A large part of the moisture which rises from the ocean's sur- 

 face is condensed to vapor before it has ascended three miles, and in 

 this form it makes its transit over land as fleecy or stratiform 

 clouds the so-called cumulus and stratus clouds and their many 

 intermediate varieties (see Frontispiece). This lower layer within 

 the convective zone is, therefore, a moist one overlaid by a rela- 

 tively drier middle layer of the convective zone. That mois- 

 ture which rises above the lower cloud layer is congealed by adia- 

 batic cooling to fine ice needles visible as the so-called cirrus 

 clouds which float as feathery fronds beneath the convective 

 ceiling (see frontispiece at right upper corner of picture). T^s 

 we have within the convective zone an upper layer more or less 

 charged with water in the form of ice needles. It is the clouds 

 of the lower zone whose moisture in the form of vapor supplies 

 the nourishment of mountain glaciers, and the high cirrus clouds 

 whose congealed moisture, after interesting transformations, is 

 responsible for the continued existence of continental glaciers. 



As we are to see, there are other noteworthy differences be- 

 tween continental and mountain glaciers, in the manner of their 

 sculpture of the lithosphere, so that long after they have disap- 

 peared the characters of each are easily identified in their handi- 

 work. How the lower clouds are forced upward and so compelled 

 to give up their moisture to feed the mountain glaciers, and how 

 the upper clouds are pulled downward to nourish the glaciers of 

 continents, can be best understood after the characteristics of 

 each glacier class have been studied. 



