Rime ice forms on the leading edges of the wings from -10°C to -20°C.
Clear ice is formed in dense cloud or freezing rain from 0°C to -10°C. It is spread unevenly due to the droplet spreading out before it freezes. The wing shape becomes altered, decreasing lift and increasing drag from 300% to 500%. The best conditions for clear ice are cumulus clouds, large super-cooled water droplets, and a high catch rate.
Light — The rate of accumulation may create a problem if flight is prolonged in this environment.
Moderate — The rate of accumulation is such that even short encounters become potentially hazardous, and use of de-icing equipment or diversion is required.
Severe — The rate of accumulation is such that de-icing or anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.