Meteorology: Understanding the Atmosphere            Ackerman and Knox



Hydrometer Types

The term hydrometeors encompasses water and ice particles suspended, or falling, in the air as well as those formed at the surface, such as dew. A precipitation particle is a type of hydrometeor. Different precipitation types are listed in the accompanying table.

Liquid water drops less than 0.5 millimeters (0.02 inches) in diameter
Drops with diameters greater than 0.5 millimeters (0.02 inches). Also widely scatter drops of smaller size.
Freezing Drizzle
Drizzle that freezes on contact with the ground
Freezing Rain
Rain that freezes on contact with the ground.
Precipitation in the form of ice crystals. They can be of various sizes. Snowflakes are aggregates of ice crystals, often of the dendritic form, and can reaches several inches in size.
Snow pellet
or graupel
White, opaque grains of ice with diameters between about 2-5 millimeters (0.1 to .2 inches). Can be spherical or conical. Snow pellets are brittle.
Snow grain
Small, white opaque grains of ice. Sizes are generally less than 1 millimeter (.0.4 inches). The ice equivalent of drizzle, they are fairly flat or elongated..
Ice pellet
Transparent or translucent pellets of ice that are spherical or irregular in shape, but rarely conical. Diameters are less than 5 millimeters (0.2 inches). Ice pellets bounce when the hit ground. Sleet is composed of ice pellets.
Precipitation in the form of pieces of ice with diameters greater than 5 millimeters (0.2 inches). Hailstones are either transparent, or have concentric layers of transparent and opaque ice. Observed during intense thunderstorms.