Wave number spectra from Nimbus 7 temperature-humidity infrared radiometer 6.7-μm water vapor data are analyzed using series 4800 km long, in regions free of high clouds and frontal zones. In these regions, the brightness temperatures approximate temperatures on a water vapor isosteric (constant density) surface, rather than averages over a broad vertical layer. Power above the noise can be extracted down to wavelengths of about 60 km. Fitting the power spectrum versus horizontal wave number k to a k-n power law for wavelength from 60 to a few hundred kilometers gives slopes of n=2.7 to 3.0 depending on the exact wave numbers that are fitted. Thunderstorms and convective cloud systems may contribute an energy source for the reverse energy cascae which produces a -5/3 spectral slope. Our results suggest that when these features are not present, the enstrophy-cascading process that gives a -3 slope may govern the motion at scales smaller than it has heretofore been obseved. ¿ American Geophysical Union 1990