Refer to figures.
Structural Icing

It builds up on any exposed surface of an aircraft, causing loss of lift, an increase in weight, and control problems.

There are 2 general types of structural icing:

RIME ICE

• Normally encountered in stratiform clouds. In these clouds, continuous icing can be expected in the temperature range from 0°C to -30°C, and results from instantaneous freezing of small supercooled water droplets striking the aircraft’s surface.
• It has an opaque appearance caused by air being trapped in water droplets as they freeze. Because it freezes instantly, it builds up on the leading edge of airfoils and it does not flow back over the wing.

CLEAR ICE

• Can develop in areas of large water droplets that are found in cumulus clouds or in freezing rain beneath a warm front inversion. In this case, the droplets flow over the aircraft structure and slowly freeze (they can glaze the aircraft’s surface).
• Clear icing is most heavily concentrated in cumuliform clouds (dense clouds) in the range of temperature from 0°C to -10°C. However, you can encounter clear icing in Cumulonimbus clouds with temperatures as low as -25°C. In addition, supercooled water and icing have been encountered in thunderstorms as high as 40 000 ft, with temperatures of -40°C.
• Clear ice is the most serious of the various forms of ice because it has the fastest rate of accumulation and is more difficult to remove than rime ice.

A hard and heavy layer of transparent ice is formed that spreads from the point of impact to areas further away on the structure. It may be difficult or sometimes impossible to completely remove this ice accretion using the aircraft de-icing equipment.