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!