Thanks, Paul...
Yeah, one of the WFO LMK guys and I've been experimenting a couple of years with 'visual', to catch on cam any Ice building... especially onset, and the 'depth'. It's tricky. The dowel doesn't seem to 'work' for that, at least with 'natural' wood color, but that dowel method has been experimented with to a greater degree, and more toward utilities and forest damage.
From my experience, I think I can tell more personally from the 'flat'... for general, It seems that a 'green shaded' surface, as opposed to other tones, indicates the 'accretion' better on a camera both in natural light, and typical IR. An black surface better simulates 'pavement' response, with it on ground. A green 'elevated' (or even the black) has proven to be 'freezing imminent', (related to ground surfaces /pavement -- even if no accretion there yet) since it 'cools quicker'. Melting response in sun is very similar in temps below freezing, and 'ground' only lags elevated slightly in time in temps >freezing... depending on sun and wind.... I guess what I'm saying, is an elevated, green, flat surface is both better 'predictor' and indicator, for all except trees. The Wood (dowel) would probably better show 'tree' response, but I'm doubtful about using natural wood color....(I think needs 'bark' of some shade rather than 'bare' natural) gonna experiment a bit more if old age, weak mind, will let me... assuming we have more events.
I wouldn't get out of the covers at -25°C.
Hope above not too confusing... not measurable science, just observations, and subjective.