ThunderheadSupercell Thunderstorm StructureThe Supercell and Tom's Plein Air Sketch

Thunderhead -Summer 1912 or 1913 Click for the CSI - Creative Scene Investigation Text will appear in a new window and I suggest that you drag this window to the side so that you can read it while looking at the image. This simple approach works but not as well as Flash! It works best if you have two monitors.

Tom Thomson painted “Thunderhead” in the summer of 1912 or 1913. Tom didn’t name his paintings. He was simply painting “records” of what he saw. In fact, he didn’t think his art was at all special enough to warrant a title. As a result Tom’s art was typically named and looked after by his artist friends and benefactors. I am not certain who titled this sketch. Dr. James MacCallum possibly named it as he was the first owner and inscribed on the back of the canvasboard. In any event I am quite certain that it was not named by a severe weather meteorologist. Dr. James MacCallum was an ophthalmologist.

The title is accurate as far as it goes. This is indeed an event associated with a thunderstorm or as a meteorologist would prefer, a “cumulonimbus” type cloud. However, this particular storm is a very special “cumulonimbus”.   It is certainly a supercell producing a tornado.  Most people probably would like to see a tornado before the die, but not just before the die. Happily, this tornado was not lethal for Tom who passed away suspiciously four years later on July 8th. But that’s another even longer story.

Tom recorded this particular event for the very simple reason that it was awesome. The jet-like roar of the tornado associated with the towering supercell would have captured anyone’s attention.  The violent winds in the core of the tornado would have been churning the forest into instant debris. What is amazing is that Tom managed to record the characteristics of a tornado in paint decades before meteorologists understood even the simplest aspects of a supercell and how tornadoes are formed.

Tom was focused on the tornado and wall cloud. No one would have time to paint the entire supercell and in any case he was too close to see the entire cloud. It is always safer to spot severe weather from a distance!

Tom painted the wall cloud underneath the rain free base of a large supercell. Notice the gray of the circular wall cloud as contrasted with the darker blues under the wall cloud. The darker blues identify the forest debris lifted by the tornado. The gray of the wall cloud is distinctly different and is composed of moisture dropped by heavy rain in the forward flank downdraft of the supercell. The base of the wall cloud is more than half way below the rain free base of the supercell to the ground. Even without the debris filled air of the tornado, this is more than enough using a “meteorologist’s rule of thumb” to produce damaging winds on the ground. Remember that the tornado is the damaging winds and not the cloud.

The striations in the edge of the wall cloud are pronounced and just scream out “rotation”.  The SCUD (scattered cumulus under deck) in front of the wall cloud will be drawn into the wall cloud but for the moment is being stretched by stronger winds closer to the ground which is typical for strong tornadoes.

The thunderstorm updraft extends from the wall cloud vertically through the depth of the supercell perhaps reaching 40 thousand feet about the forest floor of Algonquin Park. The updraft and wall cloud was moving from the left to the right.   In this interpretation, the brightening on the left is the rear flank downdraft while the bright area on the right is the space between the updraft and the precipitation and hail shaft which is not painted.  The left brightening should be brighter than the right as Tom painted it. There are some cloud pieces on the right that could be tail cloud being drawn up into the updraft and condensing at a lower level since the air parcels have been moistened by the precipitation.  These cloud pieces combine to form the wall cloud and can be used to estimate the probable base of the wall cloud. However these could also be secondary vortices orbiting the large central circulation. These secondary vortices around a main circulation (tornado) are called “sisters”.

The beaver tail cloud that represents the rain moistened air being drawn upward into the rotating updraft is certainly the pronounced cloud to the right.

Tom was much too close.  He was too close to even include much of the lifted condensation level base of the thunderstorm in the painting. His “south of tornado track” location was probably the best for viewing the tornado while avoiding being a casualty.  Tom would have had to sketch in the cloud colours and basic shapes in just minutes! The landscape that remained after the passing of the tornado could be done later. The horizon remained but I’m certain that many of the trees were pretty chewed up. I’m guessing that the entire sketch would not have taken more than a couple of hours to complete and it was probably even much shorter. Art historians have spent much longer talking about this painting than Tom took to complete it. I am just as guilty as it’s taken me a couple of hours to write this article. Thanks Tom!