produced only in thunderstorms, but may be ejected
from the top or sides of a thunderstorm to fall and strike
the ground without a cumulonimbus cloud directly
Ice crystalsTiny unbranched crystals of ice in
the form of needles, hexagonal columns, or plates.
They are often so small that they may be suspended in
air and are sometimes referred to as diamond dust. Ice
crystals are visible mainly when they glitter in the
sunlight or in spotlights at night. Although common in
polar regions, this phenomenon occurs only during very
cold temperatures in stable air masses. Ice crystals may
fall from any type of cloud or from clear air. As moist
air cools below -40°F, the water vapor may sublime
directly to form ice crystals, and precipitate, without
ever forming a cloud.
tion intensity is an approximation of the rate of fall or
the rate of accumulation of precipitation. During an
observation, intensity for each type of precipitation
(other than hail and ice crystals) must be determined.
NAVMETOCCOMINST 3141.2 and NAVMET-
OCCOMINST 3144.1 provide valuable information on
determining intensity by visibility, accumulation rate,
size of the rain drops, sound on the roof, height of
splashes, and the rate at which puddles form. The
primary indicator for snow and drizzle is visibility.
Table 1-4 summarizes the indicators to aid in your
understanding of the term precipitation intensity.
Direct observation is the best method of
determining the type of precipitation occurring.
However, in chapter 2, you will study observation
equipment that provides valuable indicators of
tion character is a term used to describe how
precipitation falls. Three terms are used to describe
character: continuous, intermittent, and showery. The
term continuous precipitation means that the
precipitation falls for a long period of time over a
specific area. When the system producing the
precipitation is moving, use of the term implies that the
area covered by the rain is extensive. Continuous
precipitation falls from stratiform clouds, especially
Continuous precipitation changes
intensity only slowly, and may be of light or moderate
intensity, rarely heavy. When used alone, the terms
rain, drizzle, and snow refer to either continuous or
The term intermittent precipitation is used to
describe precipitation that occurs for brief periods of
time (lasting less than 1 hour). Intermittent
precipitation changes intensity slowly, and is usually
light. Although the overall area affected by intermittent
precipitation is usually very large, at any given time
only a portion of the area is actually receiving
precipitation. Like continuous precipitation,
intermittent precipitation usually falls from stratiform
clouds, especially nimbostratus.
Sudden starting or stopping ofprecipitation or rapid
changes in the intensity of precipitation indicate
Showery precipitation, or
showers, fall from cumuliform clouds, especially
cumulonimbus. Showers cover only a relatively small
area at a given time, and, unless the cumuliform cloud is
stationary, showers last only a brief time before moving
on. Rain falling from cumuliform clouds is called a
"rain shower," and a cumuliform shower of snow is
called a "snow shower." The public popularly calls a
very light snow shower "snow flurries."
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 a combined form.
It is believed that most precipitation in the mid- and
high-latitudes starts as ice crystals. The crystals melt
and fall as liquid precipitation only when it passes
through an above-freezing stratum of air. Due to the
low freezing level in these regions, the abundance of
water vapor in the atmosphere is found at, near, or
below freezing temperatures. In clouds below freezing
temperatures, water coexists in all three states: solid,
liquid, and gas.
Both solid and liquid particles are
present within most clouds. The higher vapor pressure
for the liquid droplets compared to the low vapor
pressure for the solid ice crystals tends to cause a net
evaporation of gaseous water from the liquid droplets.
In turn, there is a corresponding net sublimation of the
gaseous vapor on to the solid ice crystals. This tends to
retard the growth of the liquid drops while aiding the
growth of the crystals. When the ice crystals become
too large (too heavy) to remain suspended in the
atmosphere, they fall as precipitation.
In the low-latitudes (tropics) and much of the mid-
latitudes during the warmer months, the freezing level
in the atmosphere is generally much higher. The
abundance of moisture in the above freezing portions of
the lower atmosphere allows the majority of the
precipitation to form initially as liquid water droplets.