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abbreviation “BF” is used to indicate a station that is
reporting a temperature below freezing. As the freezing
level increases in height, it is depicted on the chart as a
solid line. These isotherms, lines of equal temperature,
are given in 4,000-foot intervals but are labeled in hundreds
of feet MSL. For example, an isotherm labeled
40 indicates that the freezing level is at 4,000 feet MSL.
Since the freezing level panel plots an overall view of
the isotherms, it is easy to determine at which altitude
structural icing is probable. An inversion, with warm
air above the freezing level is indicated by multiple
crossings of 0° Celsius isotherms.
The precipitable water panel, located in the upper right
corner of the composite moisture stability chart, graphically
depicts the atmospheric water vapor available for
condensation. The coverage for this panel is from the
surface to the 500-millibars level. The top number in
the station model represents the amount of precipitable
water in hundredths of an inch. The lower number is the
percent of normal value for the month. For example,
when the value is .68/205, there is 68 hundredths of an
inch of precipitable water, which is 205 percent of the
normal (above average) for any day during the month.
When a station symbol is darkened, the precipitable
water value at the station is 1 inch or more. Isopleths
are also plotted on the chart for every 1/4-inch. To differentiate
the isopleths, a heavier line is used to indicate
1/2-inch of precipitable water. [Figure 9-46]
0 to -2
-3 to -5
< -6
Weak
Moderate
Strong
<15
15-19
20-25
26-30
31-35
36-40
>40
near 0%
20%
21-40%
41-60%
61-80%
81-90%
near 100%
AIRMASS
THUNDERSTORM
PROBABILITY
K INDEX
SEVERE
POTENTIAL
LIFTED INDEX (LI)
THUNDERSTORM POTENTIAL
Figure 9-44.Thunderstorm Potential.
Freezing level at the surface
79
57
BF
0 iostherm crosses at
5,700 feet and 7,900 feet
Temperature is below
freezing at the surface
PLOTTED
DATA
Figure 9-45. Freezing Level Panel. Figure 9-46. Precipitable Water Panel.
9-33
This panel is used primarily by meteorologists who are
concerned with predicting localized flooding. However,
when used with reference to the other panels it adds
credibility to the prediction and probability of severe
weather.
In the lower right corner of the composite moisture stability
chart is the average relative humidity panel. The
values for each station are plotted as a percentage and
are valid from the surface to the 500-millibar level. For
quick reference isohumes are drawn and labeled for
every 10 percent, with heavier isohumes drawn for 10,
50, and 90 percent. When the stations is reporting
humidity higher than 50 percent, the station symbol is
darkened. If relative humidity data is missing, an “M”
is placed above the station symbol. [Figure 9-47]
For flight planning, this chart is useful for determining
average air saturation at altitudes from the surface to
18,000 feet MSL. Average relative humidity of 50 percent
or higher is frequently associated with areas of
clouds and possible precipitation. However, an area
with high humidity may or may not be indicative of
high water vapor content (precipitable water). For
example, Kansas City may have the same relative
humidity as New Orleans, but if the precipitable water
value were .13 inches in Kansas City and .66 inches in
New Orleans, greater precipitation would be expected
in New Orleans.
While each panel of the composite moisture stability
chart provides important information for predicting
weather over a wide area of the United States, it is
more important to reference all four panels to develop
the best weather picture.
PRINTED REPORTS AND FORECASTS
Weather reports and forecasts are beneficial in numerous
ways. For example, predictions of warm temperatures
signal the beginning of thermal soaring in the northern
climates, if it were only that simple. Printed reports and
forecasts provide much more information to help pilots
decide if a flight can be conducted safely. To that end,
wide varieties of weather products are available to assist
pilots in that decision-making process.
PRINTED WEATHER REPORTS
In the simplest of terms, a weather report is a record of
observed weather conditions at a particular location
and time. Weather information gathered by trained
observers, radar systems, and pilots is disseminated in
　
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