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Using the visible satellite
image to determine wind flow and stability 
Seasoned glider pilots learn how to read the
clouds. Certain clouds imply the potential for lift and they
can also tell the glider pilot about the direction of flow of the air
aloft. Cloud streets shown here on the right (click here to see a larger image)
are perfect examples of clouds that tell you a lot about the
environment in the planetary boundary layer (PBL).
Cloud streets (yes, you read that
correct - streets, not streaks) are lines or rows of small cumulus
clouds (shallow, moist convection) that hang out near the top of the
mixed layer. The mixed layer (shown in the diagram below) is
the turbulent unstable layer near the surface with an adiabatic lapse
rate (decrease of 3 degrees Celsius for every 1,000 feet gain in
altitude). Bases of the cumuliform clouds will typically
form just above the lifted condensation level or LCL which is normally
right near the top of the mixed layer.
Cloud streets immediately tell you that there is a
layer just above the mixed layer that is very stable - also referred to
as a cap or lid. This is best seen on a thermodynamic chart
called a Skew-T log (p) diagram such as the one on the left.
A cap is normally marked by a very small positive lapse
rate or negative lapse rate (inversion) just above the top of the mixed
layer. This generally limits the vertical development of the clouds
keeping them to only a few thousand feet thick - although this depends
on the strength of the cap. Weaker caps can be
broken allowing for deep, moist convection including
thunderstorms. In the example above, the cap is fairly
strong, but can be broken when clouds start to build through about
10,000 feet or at the level of free convection (LFC).
Cloud streets also idenify the wind
direction within the planetary boundary layer. The wind from
the surface through the top of the cumulus clouds is typically oriented
along
the rows  of
clouds. That is, if the rows are oriented south to north (as
they are in the
visible satellite image above), the wind in the boundary layer will
also be south to north. The winds above the clouds often
shift to a more perpendicular flow to the cloud streets as can also be
seen by the Skew-T log (p) diagram above.
Looking at the 850 mb constant pressure
chart analysis (click here for a larger image)
shown on the right, the winds are indeed oriented in a south to north
flow. The 850 mb constant pressure chart depicted the
conditions at roughly 5,000 feet MSL. Also notice how the
winds shift to a more southeasterly flow over Georgia and South
Carolina. This causes the cloud streets in this area to shift
to a more southeast to northwest orientation. |
