Abstract
Limitations on the accuracy of CCN counts obtained with thermal gradient diffusion cloud chambers (TGDCC) are discussed. Evidence presented here regarding 1) the absence of a well-defined plateau when the number of CCN in the TGDCC are plotted as a function of time, 2) the discrepancy between CCN counts obtained by instantaneous photography and the time-exposure method, and 3) the difficulty in reaching a sufficiently small minimum detectable size throughout the range of supersaturations, points to the possibility of considerable error in instantaneous CCN counts. The time-exposure method is demonstrated as a means of eliminating the problems associated with the instantaneous method and of extending the measurements to lower supersaturations.
