These are known as Rhime formations. Snow crystals accumulate rime when they collide with water droplets in the clouds. When the clouds are near to the ground you have fog, and sometimes the fog is made from supercooled water droplets -- water at a temperature below the freezing point. Then the droplets freeze on contact to anything they hit, yielding some bizarre, wind-driven rime formations.
Trees covered in Rime frost. View from my balcony at sunset (4:57 pm)