This photo was taken in the early morning on Rime Island in Jilin, China. Those white “snow” on the branches is known as hard rime ice. There are 2 types of rime, hard rime and soft rime. Hard rime is often very dense, less milky than soft rime, and can cover the whole tree evenly as the photo shows. It is caused by freezing fog with a low temperature between 17-28°F, and a high wind velocity.
Freezing fog forms out of the supercooled water droplet, which is the droplet that can remain liquid at or below the freezing point and can also suspend in the air as liquid. When there are objects that are colder than the fog, the water droplet will freeze immediately and cling to them. The quantity of the water droplets in the freezing fog determines the type of ice it’s going to form. The more droplets found in rime, the denser it will be. The hard rime has fewer water droplets but more ice crystals in it. High wind velocity can blow the droplets in one particular direction. It can accumulate water droplets along the windward sides of the trees. As time goes by, it would form the homogeneous and dense hard rime in the picture.