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of thunderstorms can be predicted. Figure 9-19
illustrates a late-spring morning sounding from the
southeast United States. The temperature and dew
point are within about a degree below a shallow surface
inversion, indicating the likelihood of haze and
the possibility of fog. In addition, a shallow layer
40
30
20
10
0
0 5 10 15
Distance (n.m.)
Gust Front
Shelf Cloud
Cumulus
Storm Motion
Mature
Dissipating
Altitude (Thousands of feet)
Figure 9-17. Multi-cell thunderstorm.
Anvil
Cumulonimbus
Flanking
Line
Wall Cloud
Precipitation
Rain-Free Cloud Base
Overshooting Tops
Figure 9-18. Supercell thunderstorm. Air enters on left into the bottom of the storm and exits at top towards the reader.
9-16
where temperature and dew point coincide are also
located at about 12,000 feet, indicating a thin, mid-level
cloud.
As the day warms to 28°C, cumulus should form at
about 2,500 feet AGL, using the same analysis as
before. By the time surface the temperature reaches
31°C, the CCL should be around 4,000 feet AGL.
Recall that once condensation occurs, the parcel follows
the SALR. Following parallel to the nearest saturated
adiabat from the CCL, the parcel does not
intersect the ambient temperature line again until
almost 40,000 feet. Thunderstorms are possible if the
surface temperature reaches 31°C. [Figure 9-19]
Two common indices are routinely reported using this
type of analysis, the Lifted Index (LI) and the K-Index
(KI). The LI is determined by subtracting the temperature
of a parcel that has been lifted (as in Figure 9-19)
to 500 mb from the temperature of the ambient air.
This index does not give the likelihood of occurrence;
rather it gives an indication of thunderstorm severity if
they occur. In the example above, LI would be given
by –9 – (-4) = -5. Looking at Figure 9-20, a LI of –5
indicates moderately severe thunderstorms if they
develop. [Figure 9-20]
The KI is used to determine the probability of thunderstorm
occurrence and uses information about temperature
and moisture at three levels. It is given by the
equation KI = (T850 – T500) + Td850 – (T700-
Td700). Here, T stands for temperature, Td is the dew
point, and 500, 700, or 850 indicate the level in mb.
All values are obtained from a morning sounding. In
the above example, using values from the sounding KI
= (16 –[-9]) + 12 – (6 –0) = 31. This indicates about a
4 1 2 3 5 8
12 16 20
300
400
500
600
700
800
900
1000
-10-10
-20-20
2020
-30-30
3030
0
1010
5 10 14 18 22 26 30
18300
23600
30100
38700
360
3200
6400
9900
13800
Figure 9-19. Skew-T from an actual sounding for a thunderstorm day.
9-17
60 percent probability that thunderstorms will occur.
[Figure 9-21] As discussed below, charts showing both
the LI and KI for all the sounding sites in the continental
United States are produced daily.
Thunderstorms have several hazards, including turbulence,
strong up and down drafts, strong shifting surface
winds, hail, icing, poor visibility and/or low
ceilings, lightning, and even tornadoes. Once a cloud
has grown to be a Cb, hazards are possible, whether or
not there are obvious signs. Since thermal soaring
weather can rapidly deteriorate into thunderstorm
weather, recognition of each hazard is important.
Knowledge of the many hazards may inspire the pilot
to land and secure the glider when early signs of thunderstorm
activity appear—the safest solution.
Moderate turbulence is common within several miles
of a thunderstorm and it should be expected. Severe or
even extreme turbulence (leading to possible structural
failure) can occur anywhere within the thunderstorm
itself. The inside of a thunderstorm is no place for
glider pilots of any experience level. Outside of the
storm, severe turbulence is common. One region of
expected turbulence is near the surface gust front as
cool outflow spreads from the storm. Violent updrafts
can be followed a second or two later by violent
downdrafts, with occasional side gusts adding to the
excitement–not a pleasant proposition while in the
landing pattern. At somewhat higher altitudes, but
below the base of the Cb, moderate to severe turbulence
can also be found along the boundary between
the cool outflow and warm air feeding the Cb.
　
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