Clear Ice

Clear ice has a smooth and glassy appearance and typically forms in cumulous clouds when there is freezing drizzle or freezing rain. Water droplets strike the surface of the wing and flow backward, freezing slowly. The ice conforms to the shape of the airfoil and can extend backward 14 inches or more. Eventually, it starts to build forward at a 45-degree angle upward and a 45-degree angle downward in a ram’s horn shape. Clear ice is the most dangerous form of icing—It can build up quickly and adhere firmly to the aircraft, making it difficult to remove even with de-icing equipment. It is also very heavy—an inch of clear ice on the frontal surfaces of a Cessna 182 can weigh as much as 800 pounds.

Rime Ice

Rime ice has a milky color and grainy texture, similar to the ice that can form on the surface of a freezer compartment. It is usually found in stratiform clouds which contain smaller water droplets that instantly freeze upon impact with the aircraft. Although rime ice is lighter than clear ice because of the air pockets between the frozen droplets, it has a rougher texture that disrupts the smooth aerodynamic flow of air over the wings and tail surfaces.

Mixed Ice

Mixed ice has the white color and rough texture of rime ice, but also contains patches of clear ice. It can form from accumulations of snow, sleet, or small hail and sometimes forms a ram’s horn or mushroom shape on the leading edges of the airfoils.

Frost

Frost forms when moisture from the air sublimates as ice crystals on the surface of the aircraft—usually when it is parked on the ground. Frost gives the airfoils a sandpaper-like texture which disrupts the smooth flow of air—drag can increase as much as 40%, lift can be reduced by 30%, and the stall speed can increase as much as 10%. Takeoff should not be attempted until the frost is removed.