Icing Types MET
There are three main types of ice, with different causes.
Hoar Frost (which you know as frost) is caused when water vapour sublimates directly to solid ice without ever becoming water. This is exactly what happens on your car during cold nights (as the car cools quicker than the surrounds.) On aircraft, this is caused either by aircraft being parked outside overnight or by an aircraft that is "cold soaked" (that has spend a long time in freezing temperatures at high altitudes) descending into warmer air. In either case, the cold aircraft cools the air and frost forms on the outside.
Rime Ice and Clear Ice are caused by the same thing: supercooled water droplets freezing when they hit the aircraft. However, for clear ice these droplets are large and spread out slowly over the aircraft (because freezing releases latent heat, which keeps the rest of the droplet from freezing instantly). For rime ice the droplets are **small and freeze instantly.
Note that freezing rain consists of large supercooled droplets and therefore gives rapidly forming clear ice.