92 The Three-dimensional Temperature Distribution and its Variation in Time 



(d) Evaporation (see Chapter VII). 



A further debit item is the heat lost by evaporation. The amount of heat involved 

 can be easily found from the mean zonal values for evaporation (WiJST, 1922), since 

 for the evaporation of 1 mm of water from 1 cm^ of a water surface 60-65 g cal 

 are needed. 



(<?) Convection (heat exchange between the ocean and the atmosphere) 



Little is known of the transfer of heat from water to air by convection. From the 

 approximate calculations of Angstrom (1920) it can be concluded that for a difference 

 in temperature of TC between the water and the air (air temperature measured 

 60 cm above the surface) the mean heat transfer by convection is between 0-01 and 

 0-03 g cal/cm-2 min-^ For the mean temperature difference between the water 

 surface and the air which has been measured more accurately for the Atlantic Ocean 

 (KuHLBRODT, 1938 a, b) the convectional flux amounts on the average to about 

 0-014 gcal/cm-2 min"^ or the average heat loss from the surface of the sea results 

 to about 20 g cal/cm-2 per day. In warmer climates this value will be increased up to 

 about 0-030 g cal cm'^ min-^ which is about 45-50 g cal/cm^^ per day. These values 

 are only rough estimates of this heat loss which is too large to be neglected in the heat 

 budget of the ocean. 



The heat transport by convection follows from the equation 



Qh = -CpA (^ + r 



where Cp is the specific heat of the air at constant pressure, A the turbulent exchange 

 coefficient (eddy conductivity), —ddjdz is the vertical temperature gradient of the air 

 above the water (positive since the temperature decreases with height) and y is the 

 adiabatic lapse rate. CpA replaces the thermal conductivity coefficient (see p. 50); 

 y can be neglected in the above equation since it is much smaller than ddjdz. For 

 stationary conditions, that is with constant heat flux through a horizontal unit sur- 

 face, the temperature changes rapidly with height near to the sea surface and there A 

 is very small. For larger distances A increases and the temperature decreases so that 

 A(dOldz) can remain constant. 



If the surface of the sea is warmer than the air above, the air is heated from below. 

 The vertical stratification of the air is then unstable and as the air turbulence increases 

 the vertical heat transport becomes large. If the vertical temperature differences are 

 large this can lead to intensive atmospheric disturbances. On the other hand, heat is 

 transported from the atmosphere to the sea when the water is colder than the air above, 

 but the heat transferred by this process is not very large since it stabilizes the air. 

 The exchange A is then small and if the vertical stability is sufficiently large, turbulence 

 of the air and the corresponding downward heat flux then ceases. 



If mean values for the heat gain and loss, described in the above discussion, are 

 calculated for different latitudes, a heat budget for the ocean surface can be drawn 

 up as shown in Table 38. 



