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and velocity, and temperatures for the altitude of a
planned cross-country flight. The chart contains eight
panels that correspond to forecast levels6,000;
9,000; 12,000; 18,000; 24,000; 30,000; 34,000; and
39,000 feet MSL. Soaring pilots planning to attempt a
proficiency award for altitude should be aware that the
levels below 18,000 feet are in true altitude, and levels
18,000 feet and above are reported in pressure altitude.
The predicted winds are depicted using an arrow from the
station circle pointing in the direction of the wind. The
second digit of the wind direction is given near the outer
end of the arrow. Pennants and barbs are used to depict
wind speed in much the same manner as the surface
analysis chart. When calm winds are expected the arrow
is eliminated and 99 is entered below the station circle.
Forecast temperatures are shown as whole degrees
Celsius near the station circle. In the example, the temperature
is 3° Celsius and the wind is 160° at 25 knots.
[Figure 9-41]
COMPOSITE MOISTURE STABILITY CHART
The composite moisture stability chart is a four-panel
chart, which depicts stability, precipitable water,
freezing level, and average relative humidity. It is a
computer-generated chart derived from upper-air
observation data and is available twice daily with a
valid time of 0000Z and 1200Z. This chart is useful
for determining the characteristics of a particular
weather system with regard to atmospheric stability,
moisture content and possible hazards to aviation hazards,
such as thunderstorms and icing. [Figure 9-42]
The stability panel located in the upper left corner of
the chart outlines areas of stable and unstable air.
[Figure 9-43] The numbers on this panel resemble
fractions, the top number is the lifted index (LI), and
the lower number is the K index (KI). The lifted index
is the difference between the temperature of a parcel
of air being lifted from the surface to the 500-millibar
level (approximately 18,000 feet MSL) and the actual
temperature at the 500-millibar level. If the number is
positive, the air is considered stable. For example, a
lifted index of +8 is very stable, and the likelihood of
severe thunderstorms is weak. Conversely, an index of
Figure 9-39. U.S. Low-Level Significant Weather Prognostic Chart.
9-31
SYMBOL MEANING SYMBOL MEANING SYMBOL MEANING
Showery precipitation
(thunderstorms/rainshowers)
covering half
or more of the area.
Continuous precipitation
(rain) covering half or
more of the area.
Showery precipitation
(snow showers) covering
less than half of the
area.
Intermittent precipitation
(drizzle) covering less
than half of the area.
Rain Shower
Snow Shower
Thunderstorms
Freezing Rain
Tropical Storm
Hurricane (Typhoon)
Moderate Turbulence
Severe Turbulence
Moderate Icing
Severe Icing
Rain
Snow
Drizzle
Figure 9-40. Prognostic Chart Symbology.
3
Figure 9-41. Winds and Temperatures Aloft (FD) chart.
Figure 9-42. Composite Moisture Stability Chart.
-4
22
Figure 9-43. Stability Panel.
9-32
–6 or less is considered very unstable, and severe thunderstorms
are likely to occur. A zero index is neutrally
stable. [Figure 9-44]
The K index indicates whether the conditions are
favorable for airmass thunderstorms. The K index is
based on temperature, low-level moisture, and saturation.
A K index of 15 or less would be forecast as a 0
percent probability for airmass thunderstorms, and an
index of 40 or more would be forecast as 100 percent
probability.
The chart shows relative instability in two ways. First,
the station circle is darkened when the lift index is zero
or less. Second, solid lines are used to delineate areas
which have an index of +4 or less at intervals of 4 (+4,
0, -4, -8). The stability panel is an important preflight
planning tool because the relative stability of an airmass
is indicative of the type of clouds that can be
found in a given area. For example, if the airmass is
stable, a pilot can expect smooth air and given sufficient
moisture, steady precipitation. On the other hand,
if the airmass is unstable convective turbulence and
showery precipitation can be expected.
The lower left panel of the chart is the freezing level
panel. [Figure 9-45] This panel plots the observed
freezing level data gathered from upper air observations.
When the freezing level (0° Celsius isotherm) is
at the surface, it is shown as a dashed contour line. The
　
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