Geoscience Reference
In-Depth Information
Table 11.2
Recognized forms of precipitation.
Precipitation form
Description
Rain
Drops of water with diameter >0.5 mm but smaller if widely scattered
Liquid precipitation
Drizzle
Fine drops of water with diameter <0.5 mm but close together
Freezing rain or drizzle
Rain or drizzle the drops of which freeze on impact with a solid surface
Frozen precipitation
Snowflakes
Loose aggregate of ice crystals, often adopting a hexagonal form, most
of which are branched
Sleet
Partly melted snow flakes, or snow and rain falling together
Snow pellets, soft hail, graupel
White, opaque grains of ice or conical with diameters of 2-5 mm
Snow grains, granular snow, graupel
Very small, white, opaque grains of ice which are flat or elongated with a
diameter generally <1mm
Ice pellets
Transparent or translucent pellets of ice, spherical or irregular, with
diameter <5 mm
Hail
Small pieces of ice with diameters >5 mm commonly comprising
alternate layers of clear and opaque ice
Ice prisms
Unbranched ice crystals in the form of needles, column, or plates
have significant depth
, implying that there is greater opportunity for cloud
particle growth, especially if the cloud has appreciable thickness between the
0°C to -40°C isotherms where all the particle growth processes described
above can operate;
●
have high liquid-water content
, this being a critical requirement in the case
that the cloud is below the 0°C isotherm.
●
Precipitation form
Meteorologists have defined many different classes of precipitation, see Table 11.2.
The most basic distinction between precipitation forms is between (a) liquid pre-
cipitation falling as rain or drizzle, which are distinguished mainly by drop size,
and (b) frozen precipitation, which is distinguished by morphology and by whether
it is melting or otherwise when it reaches the ground. In fact the distinction between
rain and drizzle is somewhat confused because although mainly based on drop
size, with rain having drops greater than 0.5 mm, this distinction may be colored
by a subjective measure of how widespread the precipitation is. Meteorologists
sometimes speak of 'widespread drizzle' but isolated 'light rain, for example. Rain,
which is the most common form of liquid precipitation, can result from several
different processes, but mainly results either from coalescence in shallow, low
clouds at warm latitudes, or elsewhere as the melted remnant of ice particles fall-
ing from clouds. Drizzle is most common in temperate latitudes, near coasts or on
high mountains as precipitation from stratified clouds, but it can also result from
coalescence in comparatively warm shallow clouds.
