Potential temperature, specific humidity and wind profiles measured by radiosondes under unstable but windy conditions during FIFE in northeastern Kansas were analyzed within the framework of Monin-Obukhov similarity. Around 86% of these profiles were found to have a height range over which the similarity, formulated in terms of the Businger-Dyer functions, is valid and for which the resulting surface fluxes are in good agreement with independent measurements at ground stations. When scaled with the surface roughness z. = 1.05 m and the displacement height d,, = 26.9 m, for the potential temperature this height range was 45 (?31) 5 (z -d,,)lzo 5 104 (254) and the comparison of the profile-derived surface fluxes with the independent measurements gave a correlation coefficient of r = 0.96. For the specific humidity these values are 42 (*29) 5 (z -d,)lzo 5 96 ( 538) and r = 0.94. In terms of the height of the bottom of the inversion H,, in the morning hours the upper limit of (z -do) in the Monin-Obukhov layer is approximately 0.3H,, whereas for a fully developed ABL it is closer to O.lH,. Probably, as a result of the short sampling times and perhaps also of the small gradients under the windy conditions, the exact height range of validity was difficult to establish from a mere inspection of these profiles.