212 



[chap. 4 



In that portion of the oceanic trade-wind belt from 10°-20°N, covering an 

 area of 32 x lO^^ cm2, the evaporation is roughly 1.1 x IQis cal/sec. Of this 

 latent heat supply, about 84%, or 1.5 x IO12 g/sec, of water vapor is being 

 transported upward through the lower cloud layer. We shall use Fig. 52 to 

 deduce what the trade-wind clouds would have to be like to effect this trans- 

 port, and Fig. 54 to check the prediction. 



6-28-52 



U 





T^ =20.5 



r„ = 19 6 .g = 15.4, 



^ -f. =21.0 



'1:'^^- ="^ " -'— ' — '— "- - " "'" % - 22.: 



•I 200 mi- 

 Distance (m) 

 Fig. 54. Aircraft profile made through the large, active trade cumulus photographed in 

 Fig. 55. (After Malkus, 1954, Fig. 3. By courtesy of the American Meteorological 

 Society.) 



Cloud profile constructed from photographs with individual aircraft runs measuring 

 vertical motion (m/sec), solid curves; temperature (°C), dashed curves; and specific 

 humidity (g/kg), x-ed curves superposed to scale. Environment values of wet-bulb 

 temperature Ty^, specific humidity q, and dry-bulb temperature T^i given to right. 

 Cloud was studied in active phase over western Atlantic trade-wind ocean under 

 normal trade conditions on June 28, 1952. Series of six horizontal passes, in the plane 

 of the wind from top down, completed within twenty minutes. 



The equation for the transfer of water vapor across a given reference level, 

 say at 1400 m (or ~ 850 mb), is 



F = 2a WapaQa' ^a - 2c Wcpcqc' ^c - 2d WdRaqd ^d, (46) 



where F is the vertical water-vapor flux in g/sec, iv is vertical velocity in cm/sec, 

 p is air density in g/cm^, q is the specific humidity in g vapor per g air and A is 

 area in cm^. The subscript a refers to the actively ascending cloud portion, c to 

 stationary or descending cloud portions, and d to the more weakly descending 

 clear air between clouds. The superscript s indicates that the in-cloud air is 

 saturated. Since this is an order-of-magnitude calculation, the liquid-water 

 content (~10% of the vapor), the vertical component of the mean motion 



