Formation process of branches of needle ice crystals grown from vapor

In-situ observations of needle ice crystals growing in air at 10^5 Pa at -7℃ and at about water saturation and above water saturation were performed to study the formation process of the branches of needle ice crystals. At about water saturation, six branches with boomerang-like facets are formed after the formation of a thin wall with basal facet along the edge of the basal plane by bunching of steps formed by two-dimensional nucleation at each corner on the basal plane. As time elapses, these branches change into sheath-like branches. Above water saturation, six branches with round tips grow along the c-axis because adhesive growth occurs at each corner on the basal plane. As the ice crystal grows further, these branches change into sheath-like branches through the same process as the branches forming at about water saturation