Ice Pellets
Ice pellets (PL) are small, transparent or translucent pellets of ice that form when raindrops or partially melted snowflakes fall through a sub-freezing layer and refreeze before reaching the ground. They bounce on impact and produce a distinctive clicking sound. Reported in METARs as PL.
Formation
Ice pellets form through the same warm-layer / cold-layer structure that produces freezing rain, but with a key difference: the rain drop re-enters a deeper, colder sub-freezing layer and has time to completely refreeze before impact. In freezing rain, the sub-freezing layer is shallow and the drop remains liquid (supercooled); in ice pellets, the layer is deep enough to complete the refreezing.
Aviation Hazards
- Ice pellets indicate a classic warm-layer structure aloft — freezing rain likely exists above the refreeze layer
- Ground surfaces become extremely slippery
- Ice accumulation on aircraft parked outside
- In-flight ice pellet encounter may indicate the presence of freezing rain at a slightly different altitude
Diagnostic Value
Ice pellets on the surface are a significant weather indicator — they confirm a warm layer aloft with freezing rain above the surface, and a sufficient cold layer below to refreeze the drops. Pilots encountering ice pellets in flight should be alert to the possibility of freezing rain at slightly higher altitudes.
On the TC Written Exam
Know how ice pellets form and how to distinguish them from freezing rain (frozen vs. still-liquid on impact). Know the METAR code (PL) and the aviation hazard implications.
Related Terms
See also: Freezing Rain, Freezing Drizzle, Icing, Warm Front.