The datasheet specifies humidity. Sensirion’s documents say that prolonged exposure to high humidity will induce the reversible high bias which has been discussed ad nauseam here, but it also mentions extreme low humidity for prolonged periods will introduce a low bias, which hasn’t been discussed because most of us probably don’t experience it. Finally, the sensor outputs relative humidity with reference to saturated water vapor pressure over liquid water even for temperatures below freezing. The result is as temperatures get colder and colder below 0C, maximum humidity output by the sensor decreases. So at 0F (-18C) in your example, maximum humidity output from the sensor would be about 85% and that reading would indicate fully saturated air.