The thing to realize about dew points in this range is that the difference in water vapor between 10F and 0F is between bupkis, and slightly less than bupkis. Once you get down to negatives you're in "absolute bupkis" territory.
That is to say that radiational cooling (~200 W/M^2) will absolutely overwhelm whatever tiny vapor pressure is being released by sublimination. Long-Wave radiational cooling will tank the temperatures (and therefore dew points) if given the chance to do so. The only thing that will stop radiational cooling is a blockage of the long wave infrared energy to space (ie. Clouds or Fog).