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cloud condensation nuclei counter, especially at low saturations, to obtain accurate data on droplet concentration. An electrostatic separator was improved to extend its operation to particles up to 0.5 m and to make the device portable. An aerosol particle classifier was used to extend previous work on the ice- nucleating characteristics of different chemical compounds including those capable of suppressing ice nucleation. A study of aerosols from silver iodide flares was done with a mobility analyzer complemented by laboratory investigations of aerosols with small flare samples. Findings indicate the particles below 0.1 m do not exhibit activity at temperatures above -10 oC. ~ Studies were initiated on the effectiveness of dry ice as a seeding agent and the NCAR ice nuclei counter was operated routinely at warm temperatures. Dry ice pieces were dropped through a special tube into the cloud chamber, exposing them to the supercooled cloud for a short time. Results indicated that the number of formed ice crystals may depend on the concentration of droplets. CONTRACTOR: Meteorology Research, Inc., Altadena, California CONTRACT NO. 14-06-D-7699 PRINCIPAL INVESTIGATOR: Theodore Smith CONTRACT PERIOD: May 15, 1975, to November 30, 1980 FY7Y FUNDING: $308,188 PURPOSE: Cloud-base airborne atmospheric research and data analyses This contract provided research personnel, an instrumented atmospheric research aircraft, and aircraft data reduction and analyses for HIPLEX. In 1975, an instrumentation package designed around a digital recording system was constructed for the MRI Piper Navajo. This aircraft was used at HIPLEX sites for the summer season of 1975-79, and the aircraft data set provided useful information on cloud characteristics at all three HIPLEX sites. In 1979, MRI operated the Piper Navajo to collect cloud droplet and raindrop spectra data, state parameters, and provide time-lapse film of clouds studied. Data were collected during nine study periods on 8 days from June 4 to July 17, 1979. An intensive case study of a convective complex in west Texas on July 3, 1978, showed that: . Cloud microphysics development of rainfall was controlled by large- scale surges of cloud dynamics. . Warm rain processes exist, but rapidly mixed ice and liquid phase rain processes were more efficient in rain production. . Enough supercooled liquid water exists in ice-free regions for seeding effects for brief periods (< 15 minutes). This study showed the importance of mesoscale controls affecting precipita- tion growth and the need for quick response to provide seeding agents at the appropriate time and location for successful results. More comprehensive analysis of cloud microphysical mechanisms of precipitation growth is continuing. 14