Here is a quote from a declassified U.S. Army Missile Command, Redstone Arsenal, AL, document by Dorathy Stewart dated 27 June 1990 and entitled -
"Occurrence and Detection of Supercooled Water in the Atmosphere"
- with reference to - "Supercooled Water; Aircraft Icing; Clouds; Climatology".
The atmosphere typically contains particles which act as freezing nuclei between -15°C and -20°C. In one study (Jursa, 1985), 90 percent of clouds with supercooled water were warmer than -20°C. A few studies Indicate that water in the atmosphere may occasionally contain impurities which have the opposite effect and permit it to supercool well below -40°C. Seagraves (1981) reported supercooling to -46°C In the atmosphere,
and Curran and Wu (1982) found -47°C. Laboratory experiments by Hoffer (1961) demonstrated that droplets saturated with soluble salts commonly found in the atmosphere froze at much lower temperatures than the freezing point of pure water drops.
and another;
It must be reiterated that a low temperature does not guarantee the absence of supercooled drops. According to Sassen, Starr, and Uttal (1989), airframe icing at temperatures from -42°C to -51°C has been reported. Impurities in drops can permit excessive supercooling. It is also possible for icing to occur in updrafts of air with droplets which have not yet reached a temperature in equilibrium with the air temperature at the new level.
The last sentence of each quote is important, as we are dealing with events that had their beginnings in relatively clean air above an expansive maritime environment.
So it would appear that the military had at their disposal some definitive data on this form of icing over 20 years ago.