OK, now I'm confused! I had always thought when it was very cold then to use carb heat could help form rather than remove carb ice.
Here's a scenario I had yesterday:
Crusing at 3500ft about 1000ft above a layer of building cumulus (plenty of moisture in the air), OAT of -6 deg C. Carb temp gauge with cruise power shows -15deg C (seems to tally with the +/- 20 deg drop in the venturi). This is just in the yellow 'avoid' zone from +5 to -20 on the gauge. Upon applying carb heat, the carb temp rose to approx +8 deg C, with the occasional indication of some ice having been present (slight splutter and a subsequent rise in MAP to above previous levels once carb heat deselected).
So far so good, but if you extrapolate the line you can see where if the OAT had been -9 deg C then the carb heat would have been insufficient to raise the temp out of the yellow zone, and if the OAT had been -14 deg C it would not have raised it above 0 deg C. Meanwhile the no carb heat temperature would have been -23 deg C and out of the icing zone. To apply it would seem to encourage the formation not removal of ice.
Or are you saying that at -14 or -9 deg C there is insufficient moisture to form ice once inside a venturi warmed by carb heat to around freezing? How come that yesterday there was a layer of cloud at 4300 ft from which snow was falling? Lapse rates would give an estimated temperature up there of +/- -9 deg C.
Any further elucidation much appreciated.
