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Using the Skew-T log (p)
diagram to find those stratus cloud tops 
You are about to depart Greensboro, North Carolina
early
one morning. The morning sun is just beginning to
show a
thick overcast sky. Not wanting to fly through thick IMC for
the
next couple of hours, you are interested in knowing where the cloud
tops might be located. You are also concerned that you want
to be
able to reach clear air before you reach the freezing level.
Of
course, early in the morning, pilot reports (PIREPs) are usually few
and far between. Other than PIREPs, what can you
use to
determine the potential cloud tops?
Often the enhanced (colorized) infrared satellite
image
(left) can help identify the tops of stratiform clouds. Any
body
with a temperature emits radiation. The satellite measures this
radiation and from that, a temperature can be determined. The
colors on this image are directly related to the temperature emitted by
the top of the cloud. Simply find the color on the chart and
compare this against the scale to determine the cloud top temperature.
Ah, but how do you know what cloud height is associated with
that
temperature? This is where knowing how to read a
thermodynamic
chart called a Skew-T log (p) diagram is priceless. Pull up a
recent Rapid Update Cycle (RUC) analysis for an airport in the vicinity
of the tops you want to identify and locate the height that
corresponds to the temperature. As my geometry teacher used
to
say, QED (quite easily done).
However, there may be a high cirrus deck aloft
that will
ulimately obscure your view of the cloud top temperature for the lower
cloud deck. All is not lost. The Skew-T log (p)
diagram
comes to the rescue once again. Let's go back to our planned
departure from Greensboro (KGSO). In this case, there just
happened to be two PIREPs over GSO. One is at 1138 UTC and
the
other is at 1225 UTC.
Both PIREPs confirm the bases of the cloud deck
are at
1,500 feet MSL. However, the tops vary a bit at 7,500 feet
MSL
with the earlier report and 6,600 feet with the later report.
Let's look at the RUC analysis valid at 1200 UTC, nearly in
the
middle of these two PIREPs from a time perspective.
On this diagram, I have combined the
cursor location corresponding to two different points on this chart. Click here
or on the image to the right to see the complete sounding analysis for
GSO. Note that adding two cursor locations is for
illustrative
purposes only and cannot be accomplished using the tool. I
chose
these two points since they represent two locations where the
temperature and dewpoint temperature distinctly diverge.
Since
this analysis is valid at 1200 UTC, there's a good chance the tops were
decreasing and this analysis seems to do an excellent job showing this
transition.
The higher point is located at
8,000 feet MSL and the lower point is located at 6,500 feet MSL.
These two locations definitely match the pilot reports and
show a
definite trend toward tops that are indeed lowering with time.
Also, on this diagram, you can identify that the freezing
level
is approximately 12,500 feet. On the climb, you won't enter
icing
conditions and you can feel comfortable getting on top of this thick
cloud deck certainly by 8,000 feet. |
