Evaluation of NCAR Ice Nucleus Counter. Part I: Basic Operation

Abstract

A detailed examination was made of the variables affecting the operation of the National Center for Atmospheric Research (NCAR) ice nucleus counter. The design criteria for proper flow control to achieve consistent cloud formation were established. The loss of ice crystals by settling to the bottom of the cloud chamber instead of exiting to the ice crystal counter was found to be constant. The most important variable is the concentration of the cloud condensation nuclei added to the sample stream to control cloud supersaturation. The rate of activation for AgI can be increased considerably by increasing the super-saturation by the new method of combining the AgI particles with NaCl particles serving as low supersaturation condensation centers. This also gives a much faster nucleation rate of potential importance in cloud seeding. The response of kaolin and phloroglucinol under varying supersaturation was found to be in agreement with previous studies, the former being responsive to high and the latter to low supersaturation for activation. The counter provided normal temperature spectra for AgI and compared well to the Colorado State University (CSU) isothermal chamber.