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point, at which time the air in the parcel begins to condense.
During the process of condensation, heat
(referred to as latent heat) is released to the surrounding
air. Once saturated, the parcel continues to cool, but
since heat is now added, it cools at a rate slower than
the DALR. The rate at which saturated air-cools with
height is known as the saturated adiabatic lapse rate
(SALR). Unlike the DALR, the SALR varies substantially
with altitude. At lower altitudes, it is on the order
Figure 9-8. Life cycle of a typical thermal with cumulus cloud.
Figure 9-9. Stable and unstable parcels of air.
9° °C 3,000
11° ft. °C ft.
9-8
of 1.2°C per 1,000 feet, whereas in mid levels it
increases to 2.2°C per 1,000 feet. Very high up, above
about 30,000 feet, little water vapor exists to condense,
and the SALR approaches the DALR.
UNDERSTANDING SOUNDINGS
The so-called Skew-T/Log-P (or simply Skew-T for
short) is an example of the thermodynamic diagram
most commonly used in the United States. The
Skew-T part of the name comes from the fact that
temperature lines on the chart are slanted, while the
Log-P is a reminder that pressure does not decrease
linearly in the atmosphere. A temperature and dewpoint
sounding presented on a Skew-T shows a
record of the current atmospheric stability, moisture
content, and winds versus altitude. Given surface forecast
temperatures, the potential for thermal soaring,
including the likelihood of cumulus and/or overdevelopment
can then be forecast. Using Skew-T diagrams
to their fullest potential requires practice.
[Figure 9-10]
There are five sets of lines on a standard Skew-T.
Other types of thermodynamic charts, for instance
the Tephigram often used in Great Britain, have the
same lines, but with a somewhat different presentation.
The colors and actual number of lines vary, but
the main diagram components should always be
present. The following discussion refers to the
Skew-T in Figure 9-10.
Horizontal blue lines indicate pressure levels and are
labeled every 100 millibars (mb) along the left side of
the diagram. On this diagram, the approximate height
(in feet) of each pressure level in the standard atmosphere
is shown on the right. The actual height of each
pressure level varies from day to day. Slanted
(skewed) blue lines indicate temperature and are
labeled every 10°C along the right side of the
isotherm. Thin red lines slanted at an angle almost
perpendicular to the temperature lines indicate dry
adiabats. (An air parcel following a dry adiabat is
changing temperature with height at the DALR.) Thin
green lines curving in the same direction as, but at a
different angle to, the dry adiabats represent saturated
adiabats. (An air parcel that is saturated follows a saturated
adiabat is changing temperature with height at
the SALR.) The thin orange lines slanting in the same
.4 1 2 3 5 8
12 16 20
300
400
500
600
700
800
900
1000
18300
23600
30100
38700
360
3200
6400
9900
13800
-10-10
0
1010
2020
3030
Figure 9-10. Skew-T from an actual sounding.
9-9
direction, but at an angle to the temperature lines represent
the ratio of water vapor to dry air, called the
mixing ratio. Lines of constant mixing ratio are
labeled in grams of water vapor per kilogram of dry
air, abbreviated g/kg.
Over the continental United States, several dozen
sounding balloons are launched twice daily, at 00 and
12 Universal Coordinated Time (UTC). These sounding
balloons record temperature, humidity, and winds
at several mandatory levels, as well as “significant”
levels, where notable changes with height occur. In
Figure 9-10, the actual temperature sounding (shown
in bold red), the actual dew-point temperature (shown
in bold green), and the winds aloft (shown in wind
barbs on the right side) for this day are shown.
The basic analysis for forecasting the potential for
dry thermals based on a sounding is achieved by
answering the question, “At what levels is a parcel of
air rising from the surface warmer than the ambient
air?” Assume on this day, the surface temperature is
forecast to reach 23°C. This point is marked on the
Skew-T; the parcel of air at 23°C is warmer than the
surrounding air and starts to rise at the DALR. When
the parcel has risen along a dry adiabat (parallel to the
slanted red line) to 900 mb (3,200 feet), it has cooled
　
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