This communiqu¿ describes a new and novel instrument for measuring concentrations of natural cloud condensation nuclei (CCN) from the free atmosphere. It simultaneously provides the size spectrum of nucleated particles, a useful parameter for gaining insight into the composition of the CCN. The supersaturation spectrum N(S), where N is the number concentration of droplets nucleated at supersaturation S, is readily obtained. This parameter is one of the keys to evaluating aerosol influence on cloud microphysics and on the possible modulations of cloud albedo. Laboratory evaluations show that the instrument has good measurement accuracy and precision, even at low CCN concentrations found in the clean troposphere. The instrument is rugged and reliable. ¿ 1998 American Geophysical Union