The influence of density inversions and free convection heat transfer of air-water layers in a horizontal tube with uniformly decreased wall temperature is investigated. Holographic interferometry is adopted to determine the time-dependent temperature distribution in the tube. The temperature and the flow patterns are markedly influenced by the cooling rate of the tube. The heat transfer characteristics along the tube wall are also determined. It is found that there are three characteristic dendritic ice growth patterns starting from the air-water interface, and also pure ice growth along the tube wall.