June, 1949, Canadian Aviation
Experiments in artificial rainmaking, or induced precipitation, are in progress in many parts of the world. One of the largest systematic tests to date was carried out in Canada, starting in June last year. The National Research Council, the Defense Research Board, the Meteorological Service, the RCAF and the Ontario Dept. of Lands and Forests all took part.
A number of the tests had a direct application, being aimed at the quenching of forest fires and the supply of waterpower. The tests in progress at the Flight Research Section of NRC were purely investigatory. Some observations on these tests are made by D. Fraer in the latest NRC Quarterly Bulletin as follows:
“Well over 95% of observed cases of natural precipitation occur from ‘supercooled’ clouds, i.e., clouds which consist at least partly of liquid water droplets at temperatures below 0°C. Bergeron’s theory of precipitation from such clouds states that if ice crystals are introduced among the super-cooled droplets, they will grow by absorption at the expense of the liquid droplets until they eventually fall out of the clouds as snow, or having melted, as rain.
“It has been found by laboratory experiments on small super-cooled clouds that if points in the cloud are cooled to less than -39°C, or if certain crystallisation nuclei are inserted, or if certain shock waves pass through, ice crystals will form. The cooling can be performed by various means, of which the most convenient is sprinkling with pieces of dry ice.
“The intention of the Arnprior experiments was to begin by seeding clouds at random, whether super-cooled or not. It has been claimed that even if seeding does not produce precipitation, it can produce remarkable cloud modifications by changing water droplets into ice crystals. The quantities of the precipitation and whether it reached the ground, were treated as being of secondary importance.
“The procedure finally adopted for the tests was for aircraft to climb, taking temperature measurements, to the top of a suitable cloud deck where it would ‘seed’, using dry ice, in the pattern of a cross or circle under directions from the aerodrome. Subsequent results were observed visually from the aircraft and the ground and, where possible, photographs were taken. This way it was possible to observe modification of the cloud, precipitation inside the cloud (by flying through it) and precipitation issuing from the cloud base.
“When the 57 results from all four sets of Canadian experiments were combined, it was found that precipitation occurred from 70% of all super-cooled cumulous clouds that had been seeded and 45% of all super-cooled stratus clouds. Altogether there seems to have been developed a full-scale experimental method that will enable some basic research to be made with comparatively slight expenditure of effort and equipment.”
This article originally appeared in the June, 1949 edition of Canadian Aviation magazine.