A PHOTOELECTRIC DETECTOR SUSPENDED UNDER A BALLOON FOR ACTINIC FLUX MEASUREMENTS

An instrument to measure ultraviolet actinic flux in and outside cloud s was constructed and calibrated. Characteristics of the instrument ar e: 1) it is equipped with gallium phosphide photodiodes, 2) its isotro pic directional response is almost perfect due to the special design o f the optical diffusers, and 3) it is a lightweight construction (150 g) allowing the use of a balloon or a kite as an observation platform. Ground-level observations and vertical profiles, measured with this U V photometer in clear air and in stratocumulus during the Atlantic Str atocumulus Transition Experiment in the Azores, confirm that the Madro nich radiation transfer model is valid for clouds.