Cubic ice (Ic) is a metastable form of ice with very similar physical properties but different crystal symmetry than normal, hexagonal ice (Ih). Below 200 K, water preferentially nucleates to Ic, then transforms to the more stable Ih in minutes to days. As a metastable phase, Ic will necessarily have a higher vapor pressure than Ih. A cloud parcel model shows that nucleation to Ic, conversion to Ih, and the vapor pressure differential together produce larger ice crystals that can more effectively dehydrate air. The modelled effect is largest at 180 to 200 K, just the range most important for the tropical tropopause and polar stratospheric clouds. The cloud model also produces a wider size distribution when Ic is included, in agreement with observations.