Seeding convective type of clouds with hygroscopic particles produced by flares have received a lot of attention in the past decade, several field projects around the world are actively exploring the possibility of enhancing rainfall using this technique. This renewed interest in cloud seeding and the increasing number of field projects, has led to the development of new and improved hygroscopic flares. The hypothesis is that the larger particles form the flares broaden the droplet size distribution and enhances the drizzle formation that then mixes through a larger part of the cloud and increases the precipitation efficiency of the convective clouds. Numerical model studies have shown that particle sizes around 1 µm in diameter are most effective for drizzle formation. Based on these studies several flare formulations have been developed during the past year. This paper will describe the design and testing of a hygroscopic cloud seeding flare test facility, highlighting the theory and effect that the different formulations have in the flare produced particle size distributions.

The test facility was designed to mimic the aircraft flight environment with the flare burning in a wind tunnel with wind speeds up to 45-50 m/s. While somewhat slower than typical aircraft flight speeds, it provides adequate air for dilution and mixing of the smoke. Two optical particle counters, the PCASP (Particle Cavity Aerosol Spectrometer Probe, size range 0.1-3µm) and the FSSP (SPP-100 Forward Scattered Spectrometer Probe, size range 0.5-16µm) provided real-time sizing of the smoke particle produced.

Measurements and the associated theory and numerical modeling will be presented in the paper.