Ice Pellets
Ice pellets are transparent or translucent pellets of
ice that are round or irregular (rarely conical) and have
a diameter of .02 inch (.5 mm) or less. They usually
rebound upon striking hard ground and make a sound
on impact. Ice pellets are generally subdivided into two
groups, sleet and small hail. Sleet is composed of hard
grains of ice, which has formed from the freezing of
raindrops or the refreezing of largely melted
snowflakes; it falls as continuous precipitation. Small
hail is composed of pellets of snow encased in a thin
layer of ice that has formed from the freezing of either
droplets intercepted by the pellets or water resulting
from the partial melting of the pellets; small hail falls as
showery precipitation.
Hail
Ice balls or stones, ranging in diameter from that of
a medium-size raindrop to two inches or more are
referred to as hail. They may fall detached or frozen
together into irregular, lumpy masses. Hail is composed
either of clear ice or of alternating clear and opaque
snowflake layers. Hail forms in cumulonimbus cloud,
and is normally associated with thunderstorm activity
and
surface
temperatures
above
freezing.
Determination of size is based on the diameter (in
inches) of normally shaped hailstones.
Ice Crystals (Ice Prisms)
Ice crystals fall as unbranched crystals in the form
of needles, columns, or plates. They are often so tiny
they seem to be suspended in the air. They may fall
from a cloud or from clear air. In a synoptic
observation, ice crystals are called ice prisms. They are
visible mainly when they glitter in the sunlight or other
bright light; they may even produce a luminous pillar or
other optical phenomenon. This hydrometeor is
common in Polar Regions and occurs only at low
temperatures in stable air masses.
PRECIPITATION THEORY
Several valid theories have been formulated in
regard to the growth of raindrops. The theories most
widely accepted today are treated here in combined
form.
Raindrops grow in size primarily because water
exists in all three phases in the atmosphere and because
the air is supersaturated at times (especially with
respect to ice) because of adiabatic expansion and
radiation cooling. This means that ice crystals coexist
with liquid water droplets in the same cloud. The
difference in the vapor pressure between the water
droplets and the ice crystals causes water droplets to
evaporate and then to sublimate directly onto the ice
crystals. Sublimation is the process whereby water
vapor changes into ice without passing through the
liquid stage. Condensation alone does not cause
droplets of water to grow in size. The turbulence in
cloud permits and aids this droplet growth processes.
After the droplets become larger, they start to descend
and are tossed up again in turbulent updrafts within the
cloud. The repetition of ascension and descension
causes the ice crystals to grow larger (by water vapor
sublimating onto the ice crystals) until finally they are
heavy enough to fall out of the cloud as some form of
precipitation. It is believed that most precipitation in
the mid-latitudes starts as ice crystals and that most
liquid precipitation results from melting during descent
through a stratum of warmer air. It is generally believed
that most rain in the tropics forms without going
through the ice phase.
In addition to the above process of droplet growth,
simple accretion is important. In this process, the
collision of ice crystals with super-cooled water
droplets causes the droplets to freeze on contact with
the ice crystals. As the droplets freeze on the ice
crystals, more layers accumulate. This process is
especially effective in the formation of hail. There are
other factors that explain, in part, the growth of
precipitation, but the above processes are the primary
ones.
OTHER HYDROMETEORS
The
hydrometeors
that
follow,
are
not
precipitation; however, they are equally important.
Clouds
A cloud is a visible mass of minute water droplets
(or ice particles) suspended in the atmosphere. It differs
from fog in that it does not reach the surface of Earth.
Clouds are a direct expression of the physical processes
taking place in the atmosphere. An accurate description
of both type and amount plays an important part in the
analysis of the weather and in forecasting the changes
that take place in the weather.
CLOUD
FORMATION.To
be
able
to
thoroughly understand clouds, the Aerographers Mate
must know the physical processes that form clouds.
Three conditions must be met before clouds can form as
5-2