gradient throughout the year. A less intense temperature gradient is observed 

 east of Greenland. In low latitudes the temperature gradient is always small. 



Figure 6 shows a minor temperature minimum along the west coast of Africa. 

 This is due to the presence of northerly winds which produce an upwelling of cold 

 water near the coast. As a result of the upwelling, we find that at 30° N in the 

 summer the water on the western side of the Atlantic is about 8° C warmer than 

 the water on the eastern side. 



5. Mean Air Temperature. 



The mean air temperatures for four months of the year are illustrated in 

 Figures 8-11. The data from which the charts were constructed was obtained 

 from the Climatic Charts of the Oceans^ (up to 60° N), Schott^ (north of 60° N) 

 and Brooks* (North America) . 



These figures show that over the ocean the isotherms are similar to those of 

 sea surface temperature in Figures 4-7. This agreement would be expected, since 

 the properties of any air mass are greatly modified by the underlying sea surface, 

 and consequently the air temperature will always approach the sea surface tempera- 

 ture. The principal features of Figures 8-11 are summarized below: . — — , ^^,, 



(XX in all seasons of the year the Gulf Stream system c auses , warmer air to a^'-*^"^ "^ 

 be*%long the northeast side^F the 7)cean than alcmg TheTTorthwest sideT In low 

 latitudes in winter there is practically no contrast between the eastern and western 

 Atlantic, whereas in summer the eastern side is colder than the western side. This 

 is due to the upwelling of cold water which is produced by the northerly winds in 

 summer. 



(2) In winter the air over the oceans is considerably warmer than the air 

 over the adjacent land masses. Conversely, in summer the air over the land is 

 generally slightly warmer than the air over the ocean. This results from the small 



variation in ocean temperature with season as compared to the pronounced tempera- 



ture change from winter to summer over the land. >»,^// 



(3) The largest'^temperature gradients are found in winter, particularly along w^^a^-*;^^. 

 the eastern coastline of North America and Greenland. The sharp temperature 



contrast along the east coast of North America is due to the frequent presence of 

 cold polar continental air over the land, while over the ocean we have warmer 

 maritime air. As the cold continental air is swept over the ocean by the winds (see 

 Figure 1), it is rapidly heated by the warmer underlying surface. The resultant 

 turbulent and convective mixing will produce an unstable stratification. After 

 the air has been over the ocean for a short time, its temperature will be only a few 

 degrees cooler than the ocean temperature. The maximum temperature gradient, 

 therefore, is found where the heating is greatest, viz., near the east coast of North 

 America. The very steep gradient shown east of Greenland can be similarly 

 explained. The extreme gradient is also due to the presence of the high Greenland 

 plateau close to the ocean. The air over the plateau is generally very cold during 

 the winter months. 



(4) From the above discussion we should expect to find the maximum 

 difference between air and sea temperature in winter along the northwestern edge 

 of the Atlantic. This expectation is borne out by a comparison of Figures 4 and 8. 

 Because the air over the eastern Atlantic, south of 60° N, will have had a long 

 trajectory over the ocean, we find that the air temperature is approximately the 

 same as the ocean temperature in this region. 



(5) Figure 10 shows that the temperature gradients are much less in summer 

 than in winter. This results from the absence of vast differences between the 



