Weather causes this accumulation as do ground operational conditions conducive to icing. In either case, atmospheric conditions vary the type of accumulation, the amount, etc. Aircraft in flight experience a variety of atmospheric conditions which alone or together can produce ice formations on the aircraft and its components. Cloud liquid water content, ambient temperature, droplet size, and the aircraft's size, shape, and velocity all contribute to the rate of accretion and the shape of the ice formed. These clouds exist at very cold temperatures where their moisture has frozen to the solid or crystal state.
Freezing rain is different from freezing drizzle only by virtue of droplet size.
Aircraft on the ground are susceptible to many of the same conditions as in flight even when they are parked or when they are operating on the ground. There are also conditions specific to ground operations. There are areas of the aircraft other than the ones we've mentioned where frozen contamination can accumulate and not be detected except by careful, visual inspection. Some of these areas are not found in smaller, general aviation aircraft, but pilots of those aircraft can get an idea of how extensive a visual examination has to be. Anti-icing fluids may not reach areas under leading edge slats and portions of trailing edge flaps.