A cold front with showers had just passed through Tom’s location and he probably felt compelled to paint since the rain would have prevented him from doing so.
What we know. The sequential point form layout of these facts reveal the iterative process of analysis and diagnosis applied during CSI.
• The low horizon identifies this painting as another skyscape.
• The deciduous trees were in full autumn colour so the timing of this sketch in Algonquin must be in late September or early October.
• The clouds were towering cumuli that were very vertically developed and more than tall enough (4000 feet thickness of cloud) to produce locally heavy rain showers.
• The tops of the towering cumulus clouds were tilted from right to left consistent with a westerly or northwesterly wind.
• Otherwise there was only patchy altocumulus in the sky. Daytime heating in an unstable air mass is the most likely cause of this. As a result we can rule out a warm frontal system which would have been accompanied by cirrus and other warm frontal layered cloud.
• It would take most of the day to develop this scale of instability from daytime heating in the autumn so the timing of this sketch was mid to late afternoon.
• The towering cumulus appeared to be organized along a line which could be a simple surface trough or a cold front. Given the time of year, I would expect that a cold front would be required to produce organized cloud like this that was so fully developed.