AIR PRESSURE 139 



68. Land and Water Temperatures. As was seen in Ex- 

 periment 27, water has the power to hold a great amount 

 of heat. During the summer, water is heated less rapidly 

 than the air above it, so it continually extracts heat from 

 the air, making the air cooler than it otherwise would be. 

 In the winter, water loses its heat less rapidly, so, being 

 warmer than the air above it, it constantly gives heat to 

 the air. Consequently the air over large bodies of water 

 changes its temperature less rapidly than does the air 

 over the land. 



When air moves in wind from the ocean to the land, it 

 cools the land in summer and warms it in winter. It is 

 found therefore that lands which border on the ocean 

 usually have a smaller range of temperature than those 

 which are far from the sea. On some islands the range 

 of temperature throughout the year is almost impercep- 

 tible, whereas in the interiors of continents the average 

 temperature of some of the summer months is more than 

 a hundred degrees higher than that of some of the winter 

 months. 



69. Distribution of Air Pressure over the Earth. An exam- 

 ination of the isobar maps for January and July (pages 140 

 and 141) shows that atmospheric pressure, like tempera- 

 ture, is greatly affected by land masses. In the southern 

 hemisphere, south of 40 latitude where there is little land, 

 the isobars are very regular in their directions and nearly 

 parallel to the parallels of latitude. North of this in the 

 same hemisphere they are somewhat affected by the land, 

 but the sea is still the predominant influence. 



In the northern hemisphere, however, the land and 

 water are much more equally divided and here the effect 

 of the land masses is at once apparent. In the winter the 

 high-pressure areas and the low-temperature areas are 

 found over the land, but in the summer, the low-pressure 



