This is a coating of tiny ice crystals you might see coating a tree or car in winter. Rime ice forms when water droplets in a cloud, fog or mist freeze very quickly onto a surface. This type of ice can be very pretty. But sometimes it can be dangerous. Rime ice can form on the wings of an airplane when it flies through very cold clouds. It can make the plane heavier and harder to fly. It can even damage propellers and engines.