A cool Arctic air mass my be clear first thing in the morning but it will
likely cloud over by midday morning especially on the first day following the
cold front. The rain associated with the cold frontal passage provided the
moisture which was still in the top layers of the ground. The September sun
provides the heat to float hot air balloon parcels of air skyward. The air mass
was convectively unstable especially in the low levels thus accelerating the air
parcels upward on their way to become cloud droplets. The result was cumulus
clouds with flat bases that were generally about the same height above the
ground throughout the air mass. These clouds get organized into streets parallel
to the average wind through the boundary layer. The distance between the cloud
streets depends on the height of the boundary layer - both of these distances
are about the same. My hands even got cool while I painted although the sun was
pleasantly warm on my back!
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