THE HEAT BUDGET OF THE OCEANS 



65 



circumstances, contact with the sea and conduction lower the air tem- 

 perature to the dew point for a considerable distance above the sea surface. 

 Condensation then takes place in the air and fog is formed. This fog 

 (advection fog) is the type that is commonly encountered over the sea. 

 The relation between the frequency of fog or mist and the differences 

 between sea-surface and air temperatures are well illustrated by charts 

 in the Atlas of Climatic Charts of the Oceans. As an example, fig. 12 shows 

 the frequency of fog, the difference between the air temperature and the 

 sea-surface temperature, and the prevailing wind direction over the Grand 

 Banks of Newfoundland in March, April, and Ma3^ It can be concluded 

 that in spring, when the water is colder than the air, no evaporation takes 

 place in this region, but in the fall and winter, when the water is warmer, 

 evaporation must be great. 



Fig. 12. Left: The difference, air temperature 

 minus sea surface temperature, and the prevailing wind 

 directions over the Grand Banks of Newfoundland in 

 March, April, and May. Right: Percentage frequency 

 of fog in the same months. 



In middle and higher latitudes the sea surface in winter is mostly 

 warmer than the air, and therefore one must expect the evaporation to 

 be at its maximum in winter and not in summer. This conclusion 

 appears contrary to common experience that evaporation from heated 

 water is greater than that from cold water, but there is no contradiction 

 here because greatest evaporation occurs when a water surface is warmer 

 than the air above it, and this is exactly what happens in winter. 



Observations and Computations of Evaporation. Present knowl- 

 edge of the amount of evaporation from the different parts of the oceans 

 is derived partly from observations and partly from computations based 

 on consideration of the heat balance. 



Observations have been made by means of pans on board ships, but 

 such observations give too high values of the evaporation from the sea 

 surface, partly because the wind velocity is higher at the level of the 

 pan than at the sea surface, and partly because the difference between 

 the vapor pressure in the air and that of the evaporating surface is 

 greater at the pan than at the sea surface. Analyzing the decrease of 

 the wind velocity and the increase of the vapor pressure between the 



