Seeding of tropical cumulus clouds, and
indeed any clouds, requires that they contain supercooled
water--that is, liquid water colder than zero Celsius.
Introduction of a substance, such as silver iodide, that has a
crystalline structure similar to that of ice will induce
freezing. In mid-latitude clouds, the usual seeding strategy has
been based upon the vapor pressure being lower over water than
over ice. When ice particles form in supercooled clouds, they
grow at the expense of liquid droplets and become heavy enough
to fall as rain from clouds that otherwise would produce none.
Seeding of tropical cumuli sought to exploit the latent
heat released by freezing as well. This strategy of "dynamic
seeding" assumed that the additional latent heat would add
buoyancy, strengthen the updrafts, ensure more low-level
convergence, and ultimately cause explosive growth
of properly selected cumuli.
The above sketch illustrates an aircraft dispensing
pyrotechnics doped with silver iodide into a supercooled cloud
that is invigorated by the latent heat released as the boundary
between liquid and frozen hydrometeors (blue horizontal line in
the cloud on the left) moves down to the zero Celsius isotherm
(green horizontal line). It was this transformation that the
experimenters hoped to use for construction of an artificial
outer eyewall during Project STORMFURY. References
Malkus, J. S., and R. H. Simpson, 1964: Modification
experiments on tropical cumulus clouds.
Science, 145, 541--548.
Willoughby, H. E., D. P. Jorgensen, R. A. Black, and S. L.
Rosenthal, 1985: Project STORMFURY, A Scientific Chronicle,
1962-1983, Bull. Amer. Meteor. Soc.,
66, 505-514.
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