WMOD00298

• Title: Quantifying Ice Nucleation by Silver Iodide Aerosols
• Date: 5/1/1990
• State: CO
• Weather Modification - Doc Type: Report

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nucleation. The existence of a rate constant at sustained Tl and Sw1

could not be examined here. Aerosols or unactivated droplets may also

nucleate ice by this mechanism at colder cloud temperatures than the

initial formation point where lower water supersaturations may be

necessary. If nucleation does not occur rapidly, but a macroscopic

droplet forms and freezes upon c;ooUng to a lower cloud temperature

(T2), nucleation no longer bears any relationship to water

supersaturation and is rightly a form of immersion-freezing.

Whenever cloud is present, transport processes (brownian motion,

differential aerodynamic fall speeds, phoretic forces) will cause some

fraction (F3) of the aerosol to be collected by cloud droplets. At some

supercooled cloud temperature (Tl), some fraction of these collected

particles (F1f) will cause ice nucleation instantly after collision by

contact-freezing nucleation (top pathway in Figure 4.1). If freezing

does not occur instantaneously, some fraction (F lif) of the immersed

fraction (FU) may freeze at a colder temperature (T2). This latter
mechanism is again a form of immersion-freezing.

Some processes are not represented in Fig 4.1 and were not within

the scope of this study. These include the potential dissolution of

nuclei over time within droplets, the nature of renucleation following

the evaporation of droplets or ice crystals containing AgI aerosols,

and photolytic effects on the aerosol character and its ability to

nucleate ice. These processes should not have been at play in the

experiments performed due to the short time scales and the nature of

the experiments themselves.