Rime ice is formed from small super-cooled droplets and is rough, milky and opaque. The droplets freeze on impact and have air trapped within -- hence the white appearance. Rime ice is relatively light, but its shape significantly reduces the aerodynamics of the affected surfaces.
Clear ice (also known as Glaze ice) is formed from larger drops, which only partially freezes on impact. The remaining precipitation flows and freezes further along the airfoil, resulting in a smooth, solid sheet. Clear ice is very heavy.
FAA's Aviation Weather Service provides this guide to intensity (Table 3-4 on page 3-4):
|Aircraft Ice Accumulation
|Ice becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. It is not hazardous even though deicing/anti-icing equipment is not used unless encountered for an extended period of time (over 1 hour).
|The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the dicing/anti-icing equipment is used.
|The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or diversion is necessary.
|The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
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