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The value of learning to read
a Skew-T Log (p) diagram 
In a couple of months, flight planning around
convective SIGMETs will be just a faint memory. AIRMET Zulu
will begin to get our attention as structural icing becomes the biggest
concern, especially for those pilots flying IFR.
This past February, I had the pleasure
of helping an online student plan a flight from Naples, Florida to
Aurora, Illinois near Chicago. Based on our analysis the
prior evening, it didn't appear that structural icing would be an issue
as long as the departure was early the next morning with an altitude
below 10,000 feet.
The flight plan required a fuel stop
around Nashville, Tennessee. Based on the Winds/Temperatures
aloft (FBWinds), the freezing level was forecast to be near 10,000 feet
at 1200 UTC in the Nashville area. This does not mean that
10,000 feet was the lowest
freezing level. Since the temperature is not provided in the
FBWinds below 6,000 feet, it was time to consult the Skew-T log (p)
diagram to see a continuous
forecast of the temperature aloft.
We took a look at the 12-hour Rapid Update Cycle
(RUC) forecast sounding for Nashville (KBNA). It became
apparent that the lowest freezing level was forecast to be just under
3,000 feet MSL with saturated conditions down to about 1,000 feet MSL.
In this overcast layer, the temperature was forecast to
remain below freezing down to the surface with -3°C at 2,000 feet being the coldest temperature - an
icing threat for sure.
The next morning I was a bit surprised not to see
AIRMET Zulu covering a good portion of Tennessee and Kentucky.
Instead, there were no AIRMETs for icing anywhere along the
proposed route From Naples to Aurora as shown in the upper right.
Click here to view a larger image.
Did this mean there was no threat of icing on a descent into
the Nashville area?
Not necessarily. The Current Icing
Product (CIP) (shown above left) wasn't much help either, since images
on the Aviation Digital Data Service (ADDS)
website are only shown at 1,000 feet and 3,000 feet MSL (yes, you can
view the icing at 2,000 feet using the Flight Path Tool, but that's not
always convenient).
Click here to view a larger image.
The
first clue that icing was possible came from a pilot of an SR22 that
reported moderate ice during a descent into Nashville as shown
on the right. Click here to view a larger image.
The pilot reporting moderate clear ice from 3,000 feet to the surface.
It was time to look again at the Skew-T log (p)
diagram for confirmation. The sounding analysis (below left)
from the RUC for Nashville clearly shows (see below) that a fairly deep
saturated layer from 5,000 feet down to 1,000 feet was present.
Click here to view a larger image.
This sounding analysis was very similar to what was forecast
the night before with the lowest freezing level around 3,000 feet with
saturated conditions to about 1,000 feet MSL. Even without a
pilot report, there was a clear indication of the potential for clear
icing from 1,000 feet through 3,000 feet.
None of the standard icing products
such as AIRMET Zulu and CIP did an adequate job showing the potential
for structural icing on a descent into Nashville. The
advantage of using the Skew-T log (p) diagram is that it will show a continuous profile
of both the temperature and dewpoint giving you the best opportunity to
identify hazardous weather, including icing.
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