64 THE HEAT BUDGET OF THE OCEANS 



This ratio was derived in a different manner by Bowen, and is often 

 referred to as the ''Bowen ratio." 



Values of the ratio R can be computed from cHmatological charts of 

 the oceans, but a comprehensive study has not been made. Calculations 

 based on data contained in the Atlas of Climatic Charts of the Oceans, 

 published by the United States Weather Bureau in 1938, show that the 

 ratio varies from one part of the ocean to the other. As a rule, the ratio 

 is small in low latitudes, where it remains nearly constant throughout 

 the year, but is greater in middle latitudes, where it reaches values up 

 to 0.5 in winter and in some areas drops to —0.2 in summer. A negative 

 value indicates that heat is conducted from the atmosphere to the sea. 

 On an average, the value for all oceans appears to lie at about 0.1, meaning 

 that about 10 per cent of the heat surplus that the oceans receive by 

 radiation processes is given off as sensible heat, whereas about 90 per cent 

 is used for evaporation. 



There are certain points regarding the character of the evaporation 

 that need to be emphasized. If the water is warmer than the air, the 

 vapor pressure at the sea surface remains greater than that in the air. 

 Under these circumstances, evaporation can always take place and will 

 be greatly facilitated, because, owing to the unstable stratification of the 

 very lowest layers, the turbulence of the air will be fully developed. It 

 must therefore be expected that the greatest evaporation occurs when cold 

 air flows over warm water. If the air is much colder than the water, the 

 air may become saturated with water vapor, and fog or mist may form 

 over the water surfaces. Such fog (steam fog) is common in the fall over 

 ponds and small lakes during calm, clear nights. When a wind blows, 

 the moisture will be carried upward, but streaks and columns of fog are 

 often visible over lakes or rivers and are commonly described as ''smoke." 

 The process can occasionally be observed near the coast, but it does not 

 occur over the open ocean because the necessarily very great temperature 

 differences are rapidly eliminated as the distance from the coast increases. 



When the sea surface is colder than the air, evaporation can take 

 place only if the air is not saturated with water vapor. In this case, 

 turbulence is reduced and the evaporation must stop when the vapor 

 content of the lowest layer of the atmosphere has reached such a value 

 that the vapor pressure equals that at the sea surface. If warm, moist air 

 passes over a colder sea surface, the direction of transport of water vapor 

 is reversed and condensation takes place on the sea surface in such a way 

 that heat is brought to the surface and not carried away from it. Owing 

 to the fact that this process occurs only when the air is warmer than the 

 sea and that then turbulence is greatly reduced, one can expect that 

 condensation of water vapor on the sea will not be of great importance 

 to the heat budget, but it should be borne in mind that this process can, 

 and frequently does, take place when conditions are favorable. In these 



