Prepared by: Troy M. Kimmel, Jr.
Lecturer, Studies in Weather and Climate
Manager, Weather and Climate Resource Center
Department of Geography, University of Texas at Austin
Precipitation in the form of liquid water drops, which have diameters greater than 0.02 inch, or, if widely scattered, the drops may be smaller. Generally produced by nimbostratus or cumulunimbus clouds.
Intensity is classified as:
(a) "very light" when scattered drops do not completely wet an exposed surface regardless of duration
(b) "light" when the rate of fall varies between a trace and .10" per hour with the maximum rate of fall being no more than .01" in six minutes
(c) "moderate" when the rate of fall is between .11" and .30" per hour with the maximum rate of fall being no more than .03" in six minutes
(d) "heavy" with over .30" per hour or more than .03" in six minutes.
Very small, numerous, and uniformly dispersed water drops that appear to float while following air currents. Unlike cloud/fog droplets, drizzle actually falls to the ground. It usually falls from low stratus clouds and is frequently accompanied by low visibility and fog. By convention, drizzle drops are considered to be less than 0.5 millimeter/0.02 inch.
Intensity of drizzle is based upon rate of fall:
(a) "very light" is when exposed surface is never completely wet
(b) "light" is when rate of fall is trace to .01" per hour
(c) "moderate" is when rate of fall being .01" to .02" per hour
(d) "heavy" is when rate of fall is more than .02" per hour even though when rate is equal or exceeds .04" per hour, all or part of the precipitation is generally rain
Rain (see above) that falls in liquid form but freezes upon impact to form a coating of glaze on the ground and exposed objects. While the temperature of the ground surface and glazed objects initially must be near or below freezing, it is also necessary that the water drops be supercooled before striking. A shallow layer of sub-freezing air at the surface is common. Freezing rain frequently occurs as a transient condition between the occurrence of rain and ice pellets (sleet) as cold air advection and the deepening of the cold air layer occurs. When encountered by an aircraft in flight, freezing rain can cause a dangerous accretion of clear aircraft icing. Droplet size, as in rain, must be greater than 0.02 inch.
Drizzle (see above) that falls in liquid form but freezes upon impact to form a coating of glaze. The physical cause of this phenomena is the same as that for freezing rain (see above). Droplet size, as in drizzle, must be less than 0.02 inch.
Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds (cumulonimbus). By convention, hail has a diameter of 0.2 inch or more, while smaller particles of similar origin may be classified as ice pellets or snow pellets (graupel). Thunderstorms which are characterized by strong updrafts, large liquid water contents, large cloud droplet size and great vertical heights are favorable to hail formation. Hail size is important in determining the strength, of course, of the up and downdrafts and, therefore, the severity of thunderstorms. By definition, hail size of 0.75 inch (3/4 inch) or greater is one criteria that classifies a thunderstorm as being "severe."
Ice Pellets (Sleet)
A type of cold weather precipitation consisting of transparent or translucent pellets of ice, 0.2 inch or less in diameter. They form from the freezing of rain droplets or refreezing of largely melted snowflakes when falling through a fairly significant below-freezing layer of air at or near the earth's surface. The ice pellets may be spherical, irregular, or (rarely) conical in shape. Ice pellets usually bounce when hitting the ground and usually make a distinctive sound.
Cold weather precipitation form composed of white or translucent ice crystals, chiefly in complex branched hexagonal form and often agglomerated into snowflakes. Snow is produced in supercooled clouds where water vapor is deposited (deposition) as ice crystals that remain frozen during their entire descent, which usually requires a deep layer of sub-freezing air. Snowflakes can be up to 0.8 inch in diameter.
Intensity of snow is based upon visibility:
(a) "light" snow is when visibility is 5/8 statute mile or more
(b) "moderate" snow is when visibility is less than 5/8 but more than 5/16 statute mile
(c) "heavy" snow is when visibility is less than 5/16 statute mile