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[chap. 4 



clouds. The atmosphere is thus fuelled mainly from below, with 80% (global 

 average) of its fuel initially latent in the form of water vapor. The latter figure 

 is the heart of air-sea interaction, and the key to the importance of the oceans 

 to meteorology. Of the water- vapor fuel, considerably more than half (see 

 Fig. 1) is supplied to the lowest air by the tropical oceans between 30° North 

 and 30° South latitude. 



Intervening between the input, by evaporation, of this fuel and the eventual 

 conversion of a tiny fraction into winds and ocean currents are multifold and 

 complex transportations and transformations. The fluid engine is highly in- 

 efficient, converting one or two per cent of its thermal energy input into 



Fig. 2. Typical photograph of oceanic trade cumulus clouds. Row is parallel to easterly 

 wind (blowing from left to right). Tallest clouds reach about 6000 ft. 



motion ; the vast majority of the calories received by the atmosphere are con- 

 sumed in balancing its radiation losses to space. The free air is everywhere a 

 radiation sink and would begin to cool off by 1-2°C per day if its heat input 

 from the earth below were suddenly to terminate. The overall radiation balance 

 (energy received minus that lost) of the earth-atmosphere system is positive 

 between about 38° North and South latitudes, and negative poleward. The 

 circulations of sea and air must, therefore, operate in such a fashion as to 

 convey heat energy from regions of positive balance to those of deficit. Heat 

 energy must be carried poleward, and yet, we know that in the tropical input 

 regions the prevailing low-level winds all blow toward the equator. How is this 

 apparent paradox resolved? To find out, we must briefly explore tropical 

 meteorology. 



