Passage VI

In a rain cloud, small particles called cloud condensa­tion nuclei (CCN) attract nearby water droplets to form a cloud droplet. A cloud droplet grows by attracting more water droplets until it becomes a raindrop, which then falls from the cloud. Rainfall can be increased by seeding the cloud with additional small particles, such as NaCl parti­cles, to act as CCN.

Over a year, 2 studies of cloud-seeding were done at a subtropical location, using every cumulus cloud that was isolated from other clouds, that had a top at an altitude between 3,350 m and 4,900 m, and that had a liquid water content of at least 0.5 g/m³.

Study 1

Four types of flares (A-D) were used. Flares of each type, when ignited, released NaCl particles having a specific distribution of particle sizes (see Table 1).

A plane carrying all 4 types of flares was sent into the base of each cloud. As the plane entered the base of a cloud, a computer determined whether or not to immediately ignite at least 1 flare, seeding the cloud. The com­puter also selected which type of flare to ignite, and how many flares to ignite to introduce a concentration of 10, 100, 1,000, or 10,000 particles/cm³ into the cloud. The average amount of rainfall from the seeded clouds for each type of flare and at each concentration is shown in Figure 1.

Table 1 and Figure 1 adapted from Y. Segal et al., "Effects of hygroscopic seeding on raindrop formation as seen from simula­tions using a 2000-bin spectral cloud parcel model." ©2004 by Elsevier B. V.

Study 2

Radar was used to monitor how the mass of raindrops within each cloud changed over the 55 minutes following the time the plane entered the base of the cloud. The aver­aged results for all the unseeded clouds and for all the seeded clouds are shown in Figure 2.

Figure 2 adapted from G. K. Mather et al., "Results of the South African Cloud-Seeding Experiments Using Hygroscopic Flares." ©1997 by American Meteorological Society.