Roughening transition observed on the prism facet of ice

Single crystals of ice are grown from vapor, and viewed by means of an interference microscope. The basal facet persists until melting, but the prism facet disappears above about -2 ^∘{}C, the surface taking a dome shape. With the temperature dropping from near 0 ^∘{}C, a facet reappears at -1.35 ^∘{}C. The measured curvature just above the transition is consistent with the prediction of a ``universal curvature jump'' by Jayaprakash, Saam, and Teitel. The surface curvature near the facet edge is also measured.