76 DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 



the Netherlands Atlas of air- and sea-temperatures gives an average 

 difference of only 0. 1 degree. The reason for this discrepancy is that, on 

 an average, the air temperatures as determined on commercial vessels 

 are about 0.7° too high, owing to the ships^ heat. The Meteor result as 

 to the average value of the difference, ^^ — ??a, is in good agreement with 

 results obtained on other expeditions that took special precautions for 

 obtaining correct air temperatures. Present atlases of air- and sea- 

 surface temperatures have been prepared from the directly observed 

 values on board commercial vessels without application of a correction 

 to the air temperatures. This correction is so small that it is of minor 

 importance when the atlases are used for climatological studies, but, in 

 any studies which require knowledge of the exact difference between air 

 and surface temperatures, it is necessary to be aware of the systematic 

 error in the air temperature. 



The difference of 0.8° between air and surface temperatures, as derived 

 from the Meteor observations, is based on measurements of air tempera- 

 ture at a height of 8 m above sea level. At the very sea surface the air 

 temperature must coincide with that of the water, and consequently the 

 air temperature decreases within the layers directly above the sea. 

 The most rapid decrease takes place, however, very close to the sea 

 surface, and at distances greater than a few meters the decrease is so 

 slow that it is immaterial whether the temperature is measured at 6, 8, or 

 10 m above the surface. The height at which the air temperature has 

 been observed on board a ship exercises therefore a minor influence 

 upon the accuracy of the result, and discrepancies due to differences 

 in the height of observations are negligible compared to the errors due to 

 inadequate exposure of the thermometer. 



The statement that the air temperature is lower than the water 

 temperature is correct only when dealing with average conditions. In 

 middle latitudes the difference, ^^ — t^a, generally varies during the year 

 in such a manner that in winter the air temperature is lower than the sea- 

 surface temperature, whereas in summer the difference is reduced or the 

 sign is reversed. The difference also varies from one region to another 

 according to the character of the circulation of the atmosphere and of the 

 ocean currents. These variations are of great importance to the local 

 heat budget of the sea, because the exchange of heat and vapor between 

 the atmosphere and the ocean depends greatly upon the temperature 

 difference. 



It was shown that in middle latitudes the amount of heat given off 

 from the ocean to the atmosphere is, in general, great in winter and 

 probably negligible in summer. Owing to this annual variation in the 

 heat exchange, one must expect the air over the oceans in winter to be 

 much warmer than the air over the continents, but in summer the reverse 

 conditions should be expected. That such is the case is evident from a 



