From SKYbrary Wiki
Rime ice is formed when small supercooled water droplets freeze on contact with a surface which is at a sub-zero centigrade (Celcius) temperature. Because the droplets are small, they freeze almost instantly creating a mixture of tiny ice particles and trapped air. The ice deposit formed is rough and crystalline and it is not possible to see through it. Because of its crystalline structure, rime ice is brittle.
Rime ice can form on aerofoil and engine inlet leading edges and thereby affect aerodynamic characteristics of wings and the airflow into engines. Accreted ice can also significantly increase the weight of an aircraft.
For information on other types of icing, the hazards associated with icing, the types of cloud that may be associated with In-Flight Icing, and solutions, see the main articles on In-Flight Icing and Cloud Types.