A short time after the rain starts its initial fall, the
updraft reaches its maximum speed. The speed of the
updraft increases with altitude. Downdrafts are usually
strongest at the middle and lower levels, although the
variation in speed from one altitude to another is less
than in the case of updrafts. Downdrafts are not as
strong as updrafts; downdraft speeds range from a few
feet per second to 40 feet per second or more.
Significant downdrafts seldom extend to the top of the
cell because, in most cases, only ice crystals and
snowflakes are present, and their rate of fall is
insufficient to cause appreciable downdrafts.
The mature cell, then, generally extends far above
25,000 feet, and the lower levels consist of sharp
updrafts and downdrafts adjacent to each other. Large
water droplets are encountered suspended in the
updrafts and descending with the downdrafts as rain.
DISSIPATING (ANVIL) STAGE.Throughout
the life span of the mature cell, the falling raindrops are
dragging down more and more air aloft. Consequently,
the downdraft spreads out to take the place of the
dissipating updraft. As this process progresses, the
entire lower portion of the cell becomes an area of
downdraft. Since this is an unbalanced situation and
since the descending motion in the downdraft effects a
drying process, the entire structure begins to dissipate.
The high winds aloft have now carried the upper section
of the cloud into the anvil form, indicating that gradual
dissipation is overtaking the storm cell.
Thunderstorms have been accurately measured as
high as 67,000 feet and some severe thunderstorms
attain an even greater height. More often the maximum
height is from 40,000 to 45,000 feet. In general,
air-mass thunderstorms extend to greater heights than
do frontal storms.
Rising and descending drafts of air are, in effect,
the structural bases of the thunderstorm cell. A draft is a
large-scale vertical current of air that is continuous over
many thousands of feet of altitude. Downdraft speeds
are either relatively constant or gradually varying from
one altitude to the next. Gusts, on the other hand, are
smaller scaled discontinuities associated with the draft
proper. A draft may be compared to a great river
flowing at a fairly constant rate, whereas a gust is
comparable to an eddy or any other random motion of
water within the main current.
thunderstorm that we observe and record at the surface
thundercloud itself is another story. Visual observations
from aircraft are difficult because of the speed with
which they pass through the thunderclouds, and man
has yet to devise an instrument that will measure all
hydrometeors in the cloud. Let us look at those forms of
precipitation turbulence and icing occurring with and
within thunderclouds as we know them today.
RAIN.Liquid water in a storm may be ascending
if encountered in a strong updraft; it may be suspended,
seemingly without motion, yet in an extremely heavy
concentration; or it may be falling to the ground. Rain,
as normally measured by surface instruments, is
associated with the downdraft. This does not preclude
the possibility of a pilot entering a cloud and being
swamped, so to speak, even though rain has not been
observed from surface positions. Rain is found in
almost every case below the freezing level. In instances
in which no rain is encountered, the storm probably has
not developed into the mature stage. Statistics show
that although heavy rain is generally reported at all
levels of a mature storm, the greatest incidence of heavy
rain occurs in the middle and lower levels of a storm.
HAIL.Hail, if present, is most often found in the
mature stage. Very seldom is it found at more than one
or two levels within the same storm. When it is
occurrence is at middle levels for all intensities of hail.
SNOW.The maximum frequency of moderate
and heavy snow occurs several thousand feet above the
freezing level. Snow, mixed, in many cases, with
supercooled rain, may be encountered in updraft areas
at all altitudes above the freezing level. This presents a
unique icing problem: wet snow packed on the leading
edge of the wing of the aircraft resulting in the
formation of rime ice.
TURBULENCE.There is a definite correlation
between turbulence and precipitation. The intensity of
associated turbulence, in most cases, varies directly
with the intensity of the precipitation.
ICING.Icing may be encountered at any level
where the temperature is below freezing. Both rime and
clear ice occur, with rime predominating in the regions
of snow and mixed rain and snow. Since the freezing
level is also the zone of greatest frequency of heavy