The characteristics of water waves produced by flowing air in closed channels were studied to uncover the effects of surface pressure variation. From theoretical considerations, it is proposed that the point of onset of the Kelvin-Helmholtz instability for such waves may be found from  
1.35ρaρwVc2ghc=1
Photographs of internal flow patterns and wave shapes confirm the occurrence of this instability, but the theoretically predicted reduction in wave celerity does not occur. The wave celerity for high waves was found to be predictable by the formula  
C=0.191gLtanh2πhwL1/2
The measured height to length ratios for the highest observed waves are of the order of 0.1.
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