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to 17.2°C, which is warmer than the surrounding air
at 15°C. Continuing upward along the dry adiabat,
at about 780 mb (7,100 feet) the air parcel and surrounding
air are at the same temperature, and the air
no longer rises due to its buoyancy. The Thermal
Index (TI) at each level is defined as the temperature
of the air parcel having risen at the DALR subtracted
from the ambient temperature. Experience has
shown that a TI should be -2 for thermals to form and
be sufficiently strong for soaring flight. Larger negative
numbers favor stronger conditions, while values
of 0 to –2 may produce few or no thermals. On this day,
with a surface temperature of 23°C, the TI is found to
be 15–17.2 or -2.2 at 900 mb, sufficient for at least
weak thermals to this level. At 780 mb, the TI is 0, and
as mentioned, the expectation is that this would be the
approximate top of thermals.
Thermal strength is difficult to quantify based on the
TI alone since many factors contribute to thermal
strength. For instance, in the above example, the TI at
800 mb (6,400 feet) was –1. The thermal may or may
not weaken at this level depending on the thermal size
and the amount of vertical wind shear. These factors
tend to mix in ambient air and can decrease the thermal
strength.
It is important to remember that the TI calculated as
above is based on a forecast temperature at the surface.
If the forecast temperature is incorrect, the
analysis above produces poor results. As a further
example, assume that on this day the temperature only
reached 20°C. From a point on the surface at 20°C and
following a dry adiabat upwards, the TI reaches 0 only
1,000 feet AGL, making the prospects for workable
thermals poor. On the other hand, if temperatures
reached 25°C on this day, thermals would reach about
730 mb (8,800 feet), be stronger, and have more negative
TI values.
The previous analysis of the morning sounding calculated
the TI and maximum thermal height based upon
a maximum afternoon temperature. In reality, the
sounding evolves during the day. It is not untypical
for a morning sounding to have an inversion as shown
in Figure 9-10. A weaker inversion on another day is
shown in Figure 9-11. This sounding was taken at 12
UTC, which is 05 local time (LT) at that location. The
surface temperature was 13°C at the time of the
sounding. A shallow inversion is seen near the surface
with a nearly isothermal (no temperature change)
layer above. Two hours after the sounding was taken
(07 LT), the surface temperature had risen to only
14°C. By 09 LT, the temperature had risen to 17°C.
The line labeled “09” shows how the sounding should
look at this time. It was drawn by taking the surface
temperature at that time, and following the DALR
until TI is 0, which is the same as intercepting the
ambient temperature. Lines at other times are drawn
in a similar fashion. At 10 LT, the TI becomes 0 at
about 2,200 feet AGL, so the first thermals may be
starting. By 12 LT, at 25°C, thermals should extend to
4,000 feet AGL. Because of the isothermal layer, thermal
heights increased steadily until about 16 LT, when
temperatures reached 31°C, at which time they
reached about 8,000 feet AGL. Understanding this
evolution of the convective layer can help predict
when thermals will first form, as well as if and when
they might reach a height satisfactory for an extended
or cross-country flight. [Figure 9-11, on next page]
The analysis presented thus far has neglected the possibility
of cumulus clouds, for which the orange
slanted mixing ratio lines on the Skew-T need to be
considered. The assumption that a rising parcel conserves
its mixing ratio is also needed. For instance, if
an air parcel has a mixing ratio of 8 g/kg at the surface,
it will maintain that value as it rises in a thermal.
Typically, this is true, though factors, such as mixing
with much drier air aloft can cause errors.
Refer to the sounding in Figure 9-12. The temperature
on this day reached 26°C during the afternoon. In order
to determine if cumulus clouds would be present, draw
a line from 26°C at the surface parallel to a red dry adiabat
as before. Draw a second line from the surface
9-10
dew point temperature parallel to the orange mixing
ratio lines. The two lines intersect at a point before the
parcel has a zero TI. This is the base of the cumulus,
called the convective condensation level (CCL). In
this case, cloudbase occurs at about 750 mb (8,100
　
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