Seeding single clouds using pyrotechnic cartridges, 1973-74

Experiments conducted in 1968-69, which involved seeding single cumulus clouds using silver iodide burners mounted on an Air Force Dakota, indicated that the rainfall could be increased by seeding suitable cells at about 14,000 ft a.s. l., just below their centres. However, in an operation in which a large number of clouds are dosed, it is much easier if the seeding can be carried out from near cloud-top. This is because, firstly, the turbulence inside a growing cloud is much less near the top than it is lower down. Secondly, it is important to seed only those clouds which have attained sufficient height for the treatment to work, the height being best estimated by flying at cloud-top level. The top-seeding technique is different from that used lower down in the cloud, and separate confirmation of its effectiveness is therefore necessary; this was the prime objective of the 1973-74 experiments…..