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Unpredictable smaller-scale turbulent gusts can occur
anywhere near a thunderstorm, so recognizing and
avoiding the gust front does not mean safety from
severe turbulence.
Large and strong up and downdrafts accompany thunderstorms
in the mature stage. Updrafts under the Cb
base feeding into the cloud can easily exceed 1,000
fpm. Near the cloud base, the distance to the edge of
the cloud can be deceptive; trying to avoid being
inhaled into the cloud by strong updrafts can be difficult.
In the later cumulus and early mature stage,
updrafts feeding the cloud can cover many square
miles. As the storm enters its mature stage, strong
downdrafts, called downbursts or microbursts, can be
encountered, even without very heavy precipitation
present. Downbursts can also cover many square miles
with descending air of 2,000 fpm or more. A pilot
unlucky enough to fly under a forming downburst,
which may not be visible, could encounter sink of
2,000 or 3,000 fpm, possibly more in extreme cases. If
such a downburst is encountered at pattern altitude, it
can cut the normal time available to the pilot for planning
the approach. For instance, a normal three-minute
pattern from 800 feet AGL to the ground happens in a
mere 19 seconds in 2,500 fpm sink!
When a downburst or microburst hits the ground, the
downdraft spreads out leading to strong surface winds,
that is, thunderstorm outflow referred to earlier.
Typically, the winds strike quickly and give little warning
of their approach. While soaring, pilots should
keep a sharp lookout between the storm and the
intended landing spot for signs of a wind shift.
Blowing dust, smoke, or wind streaks on a lake indicating
wind from the storm are clues that a gust front is
rapidly approaching. Thunderstorm outflow winds are
usually 20 to 40 knots for a period of 5 to 10 minutes
before diminishing. However, winds can easily exceed
60 knots, and in some cases, with a slow-moving thunderstorm,
strong winds can last substantially longer.
Although damaging outflow winds usually do not
extend more than 5 or 10 miles from the Cb, winds of
20 or 30 knots can extend 50 miles or more from large
thunderstorms.
0 to -2
-3 to -5
< = -6
Weak
Moderate
Strong
Lifted Index Chance of Severe
Thunderstorm
Figure 9-20. Lifted Index (LI) vs. thunderstorm severity.
near 0
15
15
to 20 20
21 to 25 20 to 40
26 to 30 40 to 60
31 to 35 60 to 80
36 to 40 80 to 90
>40 near 100
K Index Thunderstorm
Probability (%)
<
Figure 9-21. K-Index (KI) vs. probability of thunderstorm
occurrence.
9-18
Hail is possible with any thunderstorm and can exist as
part of the main rain shaft. Hail can also occur many
miles from the main rain shaft, especially under the
thunderstorm anvil. Pea-sized hail usually will not
damage a glider, but hail with a severe storm (3/4 inch
diameter or larger) can dent metal gliders or damage
the gelcoat on composite gliders, whether on the
ground or in the air.
Icing is generally only a problem within a cloud, especially
at levels where the outside temperature is around
–10°C. Under these conditions, super-cooled water
droplets (that is, water droplets existing in a liquid state
at below 0°C) can rapidly freeze onto wings and other
surfaces. At the beginning of the mature stage, early
precipitation below cloud base may be difficult to see.
At times, precipitation can even be falling through an
updraft feeding the cloud. Snow, graupel, or ice pellets
falling from the forming storm above can stick to the
leading edge of the wing, causing degradation in performance.
Rain on the wings can be a problem since
some airfoils can be adversely affected by water.
Poor visibility due to precipitation and possible low
ceilings as the air below the thunderstorm is cooled is
yet another concern. Even light or moderate precipitation
can reduce visibility dramatically. Often, under a
precipitating Cb, there is no distinction between precipitation
and actual cloud.
Lightning in a thunderstorm occurs in-cloud, cloud-tocloud
(in the case of other nearby storms, such as a
multicell storm), or cloud-to-ground. Lightning strikes
are completely unpredictable, and cloud-to-ground
strikes are not limited to areas below the cloud. Some
strikes emanate from the side of the Cb and travel horizontally
for miles before turning abruptly towards the
ground. In-flight damage to gliders has included burnt
control cables and blown off canopies. In some cases,
　
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