The photolysis of NO2 by solar UV radiation produces ozone in the troposphere. Radiative transfer models predict an increase in actinic UV flux with altitude, but this has never been well documented experimentally. This paper presents direct airborne measurements of both upwelling and downwelling flux (2&pgr; sr each) from 0.2 to 7.6 km for clear-sky conditions and 58¿ solar zenith angle. The downwelling flux increases by 33% and upwelling flux increases by a factor of four going from the surface to 7.6 km. Excellent agreement was found with models using detailed solutions to the radiative transfer equation or the six-stream approximatiaon, while a number of two-stream computer models agreed within 10 to 25%. Compared to clear-sky measurements, in-cloud enhancement of up to 58% was also observed. ¿ American Geophysical Union 1995.