226 INTERACTION BETWEEN THE ATMOSPHERE AND THE OCEANS 



The great range of 22.4° clearly demonstrates the continental character 

 of the climate of Yokohama. In San Francisco, on the other hand, the 

 coldest and warmest months have average temperatures of 9.7° and 15.2°, 

 respectively, and the small range of only 5.5° clearly demonstrates that 

 San Francisco, from this point of view, has a maritime climate. 



A comparison of the air temperature with the water temperatures off 

 the coasts is of further interest (fig. 63). Off Yokohama the water tem- 

 perature varies during the j^ear between 16.6° and 26.2°, and in nearly all 

 months the water is warmer than the air. The great difference in winter 

 is particularly striking, and is due to the fact that the northerly winds 

 bring air of low temperature. Off San Francisco, on the other hand, 

 the water temperature off the coast varies only between 11.3° and 14.6°. 

 The coldest water is found in April and May, because in these months the 

 up welling (p. 201) is most intense. The air temperature throughout the 

 year remains very close to the water temperature, but is generally some- 

 what higher. It is quite evident that the low spring temperatures in 

 San Francisco are closely associated with the cold waters off the coast, 

 and that there, in contrast to the conditions at Yokohama, the air tempera- 

 ture is to a great extent controlled by the temperature of the currents off 

 the coast. At Yokohama the warm water off the coast makes the winter 

 milder, but the air temperature is not completely controlled by the water 

 temperature. The difference is due to the fact that in Yokohama offshore 

 winds prevail during the winter, whereas in San Francisco onshore winds 

 prevail. The specific influence of the ocean currents gives San Francisco 

 an abnormally low summer temperature and a relatively high winter 

 temperature. Any number of examples could be added, but it is sufficient 

 to refer to the^Climatological Charts of the Oceans. These charts, 

 together with representations of the currents of the oceans, clearly 

 demonstrate the relation between the oceanic circulation and the climates 

 of the coasts. 



The Oceans and the Weather 



The relations between the ocean currents, temperatures, and the 

 weather are equally important, but, being more difficult to examine, these 

 relations have received little attention. For this reason we shall deal 

 mainly with one subject — namely, the regional difference in evaporation 

 and in heat exchange between the atmosphere and the ocean. 



It was emphasized that great evaporation takes place where and when 

 the surface temperature of the water is higher than the temperature of 

 the air a few meters above the water (p. 64). From the discussion 

 of the ocean currents we know that in middle latitudes the currents carry 

 warm water away from the Equator along the eastern coasts of the conti- 

 nents, and cold waters toward the Equator along the western coasts. In 

 winter the winds on an eastern coast in middle latitudes blow, in general, 



