Class: Precipitation Particles

Subclass	Shape	Place of formation	Physical process	Dependence on most important parameters	Common effect on strength
PPco j Columns	Prismatic crystal, solid or hollow	Cloud; temperature inversion layer (clear sky)	Growth from water vapour at -3 to -8C and below -30 C
PPnd k Needles	Needle-like, approximately cylindrical	Cloud	Growth from water vapour at high super-saturation at -3 to -5 C and below -60 C
PPpl l Plates	Plate-like, mostly hexagonal	Cloud; temperature inversion layer (clear sky)	Growth from water vapor at 0 to -3 C and -8 to -70 C
PPsd m Stellars, Dendrites	Six-fold star-like, planar or spatial	Cloud; temperature inversion layer (clear sky)	Growth from water vapor at high supesaturation at 0 to -3 C and at -12 to -16 C
PPir n Irregular crystals	Clusters of very small crystals	Cloud	Polycrystals growing in varying environmental conditions
PPgp o Graupel	Heavily rimed particles, spherical, conical, hexagonal or irregular in shape	Cloud	Heavy riming of particles by accretion of supercooled water droplets. Size: ≤5mm
PPhl p Hail	Laminar internal structure, translucent or milky glazed surface	Cloud	Growth by accretion of supercooled water. Size: >5mm
PPip q Ice pellets	Transparent, mostly small spheroids	Cloud	Freezing of raindrops or refreezing of largely melted snow crystals or snowflakes (sleet). Graupel or snow pellets encased in thin ice layer (small hail). Size: both ≤5 mm
PPrm r Rime	Irregular deposits or longer cones and needles pointing into the wind	Onto surface as well as on freely exposed objects	Accretion of small, supercooled fog droplets frozen in place. Thin breakable crust forms on snow surface if process continues long enough	Increase with fog density and exposure to wind