Chapter 2 - Theory and Aircraft Performance

Ice Pellets

13.  These are a type of precipitation consisting of transparent or translucent pellets of ice, 5 mm or less in diameter. They may be spherical, irregular, or (rarely) conical in shape. Ice pellets usually bounce when hitting hard ground, and make a sound upon impact. Now internationally recognized, ice pellets include two fundamentally different types of precipitation, which are known in the United States as (a) sleet, and (b) small hail. Thus a two-part definition is given:

• Sleet or grains of ice:  Generally transparent, globular, solid grains of ice which have formed from the freezing of raindrops or the refreezing of largely melted snowflakes when falling through a below-freezing layer of air near the earth's surface. Note that the term "sleet" in British terminology and in some parts of the U.S. refers to a mixture of rain and snow and therefore should not be used.
   
• Small hail:  Generally translucent particles, consisting of snow pellets encased in a thin layer of ice. The ice layer may form either by the accretion of droplets upon the snow pellet, or by the melting and refreezing of the surface of the snow pellet. It is believed that the ice pellets are capable of penetrating the fluid and have enough momentum to contact the aircraft's surface beneath the fluid. Additionally, the ice pellets are of significant mass and therefore local dilution of the fluid by the ice pellet would result in the very rapid failure of the fluid.