246 PHYSIOGRAPHY 



It has been noted already (p. 233) that the land is heated and 

 cooled more readily than the sea, and is therefore colder in winter 

 and warmer in summer. The January isotherm of 30 in the 

 northern hemisphere bends toward the equator in crossing the 

 northern continents, because the land is cooler than the water in 

 the same latitude at this time of year. In the southern hemi- 

 sphere, on the other hand, where it is summer, the isotherms bend 

 toward the pole on reaching the land, because the land is warmer 

 than the sea in the same latitude. 



The chart of the July isotherms leads to the same conclusion. 

 On this chart the isotherms crossing the northern continents bend 

 poleward on the land, while those crossing the southern continents 

 bend equatorward. This is the season when the lands of the 

 northern hemisphere are warmer than the seas of the same latitude, 

 and when the lands of the southern hemisphere are cooler than the 

 seas about them. 



The irregularities of the isotherms of the northern hemisphere 

 in July are much greater than those of the southern hemisphere 

 in January (summer in the southern hemisphere). This is prob- 

 ably because there is much more land in the northern hemisphere 

 than in the southern, and the larger land areas have a greater 

 effect on the isotherms than the smaller ones. 



3, There are some features of the isothermal lines which are 

 not explained by latitude, or by the distribution of continents and 

 oceans. Thus the bends of the isotherms are not as pronounced 

 on the east sides of the continents as on the west. This is shown 

 by Figs. 217 and 218. Again, traced eastward, the January 

 isotherm of 40 bends southward near the west coast of North 

 America on the land, while on the eastern side of the continent 

 it bends northward on the sea, not on the land. Such peculiarities 

 may be explained by the winds. The prevailing winds in the middle 

 latitudes of North America are from the west, and the westerly winds 

 tend to carry the temperature of the sea (warmer in winter) over 

 onto the land on the western side of the continent (Fig. 217), and 

 the temperature of the land (cooler in winter) over onto the sea, on 

 its eastern side. This explains the bends of the isotherm of 40, for 

 example, near the coasts in the northern hemisphere in January. 



