Precipitation includes all forms of moisture that fall
to the earths surface, such as rain, drizzle, snow, and
hail. Precipitation is observed and classified by form,
type, intensity, and character.
PRECIPITATION FORM.Precipitation form
is the state that the moisture is in: liquid, freezing, or
frozen. Liquid precipitation is any precipitation that
falls as a liquid and remains liquid after striking an
object, such as the earths surface or the skin of an
aircraft. Rain and drizzle are the only two types of
Freezing precipitation is any precipitation that falls
as a liquid and freezes upon contact with an object, such
as freezing rain or freezing drizzle. In this form of
precipitation, the liquid water may be a super-cooled
liquid and freeze upon contact with an object, or the
water droplet may have an above freezing temperature
and freeze upon contact with an object that has a
temperature below freezing. (Super-cooled liquids
have a temperature below their normal freezing
temperature, but still exist in the liquid state.) Small
freezing drizzle particles form a milky white ice
coating, typically referred to as rime ice, especially on
aircraft in flight. Larger freezing drizzle and freezing
rain drops form a transparent ice coating known as clear
ice on aircraft in flight or as glaze ice on the ground,
power lines, or trees.
Frozen precipitation is any precipitation of water
that falls in its solid state, such as snow, hail, or ice
pellets. Different forms of precipitation may occur
together, such as mixed rain and snow; but such an
occurrence is simply a mixture of forms, not a separate
form of precipitation.
PRECIPITATION TYPE.Precipitation type is
the term used to identify various precipitation.
Discussion of the types of precipitation follows:
RainLiquid precipitation that has a water
droplet diameter of 0.02 inch (0.5 mm) or larger. If the
water droplets freeze upon contact with a surface, the
phenomenon is called freezing rain.
DrizzleLiquid precipitation that consists of
very small and uniformly dispersed droplets of liquid
water that appear to "float" while following air currents.
Drizzle usually falls from low stratus clouds and is
frequently accompanied by fog. A slow rate of fall and
the small size of the droplets (less than 0.02 inch)
distinguish drizzle from rain. When drizzle freezes on
contact with the ground or other objects, it is referred to
as freezing drizzle. Drizzle usually restricts visibility.
SnowPrecipitation that consists of white or
translucent ice crystals. In their pure form, the ice
crystals are highly complex, hexagonal, branched
structures. Snow falls as a combination of individual
crystals, fragments of crystals, or clusters of crystals.
Warmer conditions tend to favor larger crystal sizes and
clusters of crystals. Snow must form in cloud
temperatures below freezing, though it may fall through
air at above freezing temperatures for a short period of
time before melting.
Snow Pellets/Small HailWhite, opaque, round
(or occasionally conical) kernels of snowlike
consistency, 0.08 to 0.2 inch in diameter. They are
crisp, easily compressible, and may rebound or burst
upon striking a hard surface. Snow pellets occur almost
exclusively in snow showers.
Snow GrainsVery small, white, opaque grains
of ice similar in crystal structure to snow. Whereas the
crystal structure of snow has very fine, needlelike
branches, the crystal structure of snow grains has
thicker, denser elements, with the space between
hexagonal branched commonly completely filled.
Snow grains do not bounce or shatter on hard surfaces.
They usually fall in small quantities, mostly from
stratus clouds and never as showers.
Ice PelletsTransparent or translucent particles
of ice that are either round or irregular (rarely conical)
and have a diameter of 0.2 inch or less. They usually
rebound upon striking hard surfaces and make a sound
upon impact. The term ice pellets describes two
different types of similar looking solid precipitation.
One type is composed of hard grains of ice formed from
freezing rain or the refreezing of melted snowflakes. It
falls as continuous precipitation and is sometimes
referred to as sleet. Another type is composed of pellets
of snow encased in a thin layer of ice. It is formed from
the freezing of water droplets intercepted by snow
pellets or by the refreezing of a partially melted snow
pellet. This type falls as showery precipitation and is
usually associated with thunderstorms.
HailA clear to opaque ball of hard ice, ranging
in diameter from 1/8 inch or so to 5 inches or larger.
Hailstone size is measured and reported in inches, but
hailstones are usually compared to common objects
when reported to the public by television or radio, such
as pea size, walnut size, golf-ball size, baseball size, or
Hail frequently displays a layered
appearance of alternate opaque and clear ice. It is