TEMPERATURE OF THE ATMOSPHERE 31 



land is warmer than the air, and this warmer air will be distributed 

 by the setting in of convection currents. The heated air expands 

 and rises, while the heavier, colder air from above sinks down to 

 take the place of that which has risen, pushing upward and out- 

 ward, at the same tiitie, the lighter air which still remains. Vertical 

 and horizontal convection currents thus come into existence, and 

 distribute the heated air in the atmosphere. The surface of the land 

 is heated four or five times as fast as the surface of the water, due 

 partly to the greater specific heat of the latter, the more ready ab- 

 sorption of heat by the land and its reflection by the water. The 

 land radiates its heat more readily than the water, in which convec- 

 tion currents tend to distribute it, while at the same time the heat 

 of the sun's rays penetrates more deeply into it than into the land, 

 and a given amount of heat is distributed through a thicker layer of 

 water than of land. All this shows that the temperature of the air 

 over the land is heated more highly than that over the water, and 

 conversely it is cooled more quickly than that over the water. Hence 

 continental climates show greater annual extremes than those of the 

 sea. As an example, two points in 62° N. latitude are given, one at 

 Thorshaven on the Faroe Islands, where the mean annual range is 

 7.9° C. (+3.0° C. in March, -f 10.9° C. in July) the other at Ya- 

 kutsk, Siberia, where the range is 61.6° ( — 42.8° C. in January, 

 -fi8.8° C. in July). 



Geological Work of Heat and Cold — Changes in Temperature. 



This may be grouped under the heads of (a) insolation, or the 

 exposure to the influence of the sun's rays by day; (b) radiation, 

 or the loss of heat at night through cooling; and (c) congelation, or 

 the work of frost. 



Insolation and Radiation. Heating in general causes expansion 

 of rock masses and decreasing density, while cooling causes contrac- 

 tion and an increase of density. Even water follows this rule down 

 to a temperature of -[-4° C, when it reaches its maximum density, 

 after which it expands. Naked rock surfaces may be highly heated 

 by exposure to the sun during the day, especially in the rarefied air 

 of high altitudes. This heating will aiifect the outer part only, 

 owing to the poor conductivity of rock, and the resulting expansion 

 will cause a surface layer to scale off from the cooler, less expanded 

 mass below. On cooling at night, the outer layer loses its heat most 

 rapidly, and the accompanying contraction causes the rock to break. 

 Where changes of temperature are great and regular, as on exposed 

 mountain summits, the surface may be characterized by scaling-off 



