How do I select a sub-freezing temperature for my experiment that will allow the formation of frost, but not supercooled liquid water (rime) I once conducted a set of experiments in which I had to form ice by direct vapor deposition (not condensation of liquid water followed by freezing, aka riming) onto a Peltier-cooled glass-coated copper rod.

The experiments were conducted outside at night within a small wind tunnel, with ambient air temperatures ranging from -2 oC to -25 oC and the glass substrate was cooled from 1 oC to 10 oC below the ambient air temperature.  Occasionally supercooled water droplets would condense, followed by freezing (forming rime ice) rather than direct deposition of ice crystals from the vapor phase.  These events would ruin my time-consuming experiments.

The only relevant parameter under my control was the temperature of the glass-coated copper rod.  The only relevant parameters I could measure during the experiment were the air temperature and humidity. I also introduced trace-levels (single to hundreds of ppb) of several C1 to C4 alcohols in order to later measure their incorporation into ice during ice deposition from the vapor phase.  However, the low levels of these compounds did not affect whether riming or direct vapor-ice deposition would occur, as based on a series of 'blank' experiments without the trace gases.

If I were having trouble with liquid water condensation, in what way would I want to alter the glass substrate temperature so that only the direct deposition of solid ice could occur?
Surprisingly, at a glance, I had to run my experiments with the glass substrate at higher temperatures in order to ensure ice would form and not liquid water.  When you think in terms of the difference between the saturation vapor pressure of liquid water and that of ice at the same temperature, this solution is obvious.  The temperature has to be above the dew point but remain below the frost point, which is always higher than the dew point at temperatures below 0 oC.  The reason for this is that at any given temperature, liquid water has a higher vapor pressure than ice.

As a real-world example of the ice-growth mechanisms observed in my trace-gas uptake experiments, what was a problem for my experimental conditions actually has important implications for ice-particle growth in clouds, in which context it’s called the “Wegener-Bergeron-Findeisen Process”.  Here, when there is a mixture of ice and supercooled water droplets, water vapor condenses onto the ice particles so that the humidity with respect to liquid water decreases enough for the liquid water to further evaporate.  Thus there is a net transfer of water mass from liquid droplets to solid particles and the solid particles may then grow large enough to precipitate (as in fall to the ground in this context).