Excerpt from:                                     
             Second Aviation Weather Workshop
         Forecaster Workshop IV:  Aircraft Icing
           Marcia Politovich and  Greg Thompson 
               Research Applications Program 
          National Center for Atmospheric Research 
                     17 November 1993
                    Boulder, Colorado 

In-flight icing is the accretion of supercooled liquid water (SLW) on the airframe. This SLW can be in the form of cloud droplets or freezing rain/drizzle. Generally, cloud ice and snow do not adhere to the airframe, and graupel and small hail may actually help to remove accreted ice.

We worry about icing because it can adversely affect the flight characteristics of an aircraft. Icing can increase drag, decrease lift, and cause control problems. The added weight of the accreted ice is generally only a factor in light aircraft.

The general agreement in the community is that severity is most dependent upon SLW content, temperature and droplet size.

Icing is currently classified into four severity categories:

TRACE: Ice becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. It is not hazardous even though deicing/anti-icing equipment is not utilized, unless encountered for an extended period of time - over one hour.
LIGHT: The rate of accumulation may create a problem if flight is prolonged in this environment (over one hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used.
MODERATE: The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or diversion is necessary.
SEVERE: The rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.

Note that these definitions are based on the pilot's perception of her or his aircraft's ability to deal with the accreted ice. They are not based on meteorology.

Types of icing encountered are:

RIME: Rough, milky, opaque ice formed by instantaneous freezing of small supercooled water droplets.
CLEAR: A glossy, clear or translucent ice formed by the relatively slow freezing of large supercooled water droplets.
MIXED: Mixture of rime and clear ice.

The presence of SLW represents a difference between production and depletion mechanisms.

Here are 3 photos of aircraft icing taken by collegues at NASA-Lewis in Cleveland as part of the NASA-Lewis/FAA/NCAR-RAP Supercooled Large Drop Experiment.
Photo1 Photo number 1 is of the wing of NASA's Twin Otter aircraft after landing. This looks to be clear icing or perhaps mixed. Notice the runback well past the leading edge and on the underside of the wing.
Photo2 Photo number 2 shows a close-up of a test wing section. A 16-inch portion of a wing being tested is lifted through a port in the Twin Otter's fuselage and exposed to supercooled liquid water then pulled back inside and photographed.
Photo3 Photo number 3 shows another close-up of the test wing section.

An example of a POSSIBLE aircraft icing situation is depicted below. This image shows relative humidity as the colored field and cloud liquid water as the contoured field as predicted by the MM5 mesoscale model. The model was run by Jim Bresch.

Prepared by
Greg Thompson