Tom Thomson recorded every type of thunderstorm with his oils before meteorologists even understood that there were different types! We started "Tom Thomson Was A Weatherman" blog-book with the most severe type of thunderstorm, "supercells" in Tom’s Tornado! Now we will step back to what people like to refer to as "garden variety thunderstorms". That condescending term always bothered me as a meteorologist. Every thunderstorm packs lightning and potentially very heavy rain so even a pulse type thunderstorm can ruin your garden or light you up for the very last time...
The pulse type thunderstorm is the composed of a single convective cell. It is characterized by the formation of a single updraft (as seen to the left in the graphic) which grows and billows upward into a single towering cumulus cloud. We discussed cumulus clouds in "Tom's Summer Clouds". At the mature stage of the pulse thunderstorm, the updraft is accompanied by a developing downdraft. Dissipation follows as the downdraft intensifies at the expense of the updraft. Pulse thunderstorms dissipate within one hour. Even this more ordinary and common thunderstorm can produce severe weather such as high winds or hail. Tornadoes are rare with pulse thunderstorms although brief, non-supercell tornadoes can occur.
Lightning processes start when the cloud top temperature reaches minus 20 to minus 25 Celsius. At these colder temperatures, one can be certain that the ice crystals needed to exchange electrical charge are present in great numbers within the cell. More on these processes is coming as Tom painted lightning.
The backlit pulse thunderstorm that I photographed was very similar to Tom’s pink thunderstorm. My view was toward the shadowed side of the thunderstorm so my colours were quite different. My pulse thunderstorm also had several anvils at different levels. The updraft of the Schomberg thunderstorm was a bit stronger than Tom’s at the moment I took the picture. Note the updraft overshooting the tropopause and penetrating into the stratosphere as an “updraft pileus” structure.
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| Pulse Thunderstorm Stages |
Lightning processes start when the cloud top temperature reaches minus 20 to minus 25 Celsius. At these colder temperatures, one can be certain that the ice crystals needed to exchange electrical charge are present in great numbers within the cell. More on these processes is coming as Tom painted lightning.
What Tom recorded in the summer of 1916 was certainly a cumulonimbus cloud. Was it severe? Perhaps but not likely. This cloud was precipitating! But there was little wind shear in the vertical. The updraft was certainly strong but I believe it was simply an example of strong pulse convection. Pulse type convection is most likely to occur in mid to late summer when the atmospheric winds are less dynamic. In addition the cumulonimbus probably formed on a boundary related to a lake breeze convergence line generated from the lake in the foreground.
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| A Pulse Thunderstorm I Witnessed in Schomberg, Ontario Looking very Similar to Tom's Pulse Thunderstorm |
The backlit pulse thunderstorm that I photographed was very similar to Tom’s pink thunderstorm. My view was toward the shadowed side of the thunderstorm so my colours were quite different. My pulse thunderstorm also had several anvils at different levels. The updraft of the Schomberg thunderstorm was a bit stronger than Tom’s at the moment I took the picture. Note the updraft overshooting the tropopause and penetrating into the stratosphere as an “updraft pileus” structure.
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| Explanatory Pulse Thunderstorm Graphics as Overlaid during the Powerpoint |
Tom’s pink thunderstorm was probably a mature phase pulse thunderstorm developing late in the afternoon. The spreading anvils were disorganized. The upper winds were weak and not overly directional. Two of the three anvils spread to the right while one spread to the left. There was minimal wind shear in that environment. As a result this was not a supercell with a back building anvil. The anvils were simply there as a result of the strong air mass convection and the presence of the stable layers aloft with the most notable one being the tropopause. This pulse storm was at its maximum intensity when Tom immortalized it in oils and it was soon to be gone. The life cycle of a pulse type thunderstorm is in the order of an hour.
The fact that two anvils were headed westward suggests that the overall air mass pattern must be that of a blocking stagnant high pressure area. This pattern is very stable in the interior of the high and only unstable on the outer edges. I call it the “ring of fire” or the Johnny Cash pattern. The center of the high would be behind Tom’s back. So that the anticyclonic flow around the high would direct the anvils mainly left to right. It would have been a hot and humid day even over Canoe Lake.
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| Thunderhead: Pink Cloud over a Lake, Summer 1916, Oil on composite wood-pulp board, 8.5 X 10.5 inches more or less (21.5 x 26.8 cm), National Gallery of Canada, Ottawa (4688), Bequest of Dr. J.M. MacCallum, Toronto |
The fact that two anvils were headed westward suggests that the overall air mass pattern must be that of a blocking stagnant high pressure area. This pattern is very stable in the interior of the high and only unstable on the outer edges. I call it the “ring of fire” or the Johnny Cash pattern. The center of the high would be behind Tom’s back. So that the anticyclonic flow around the high would direct the anvils mainly left to right. It would have been a hot and humid day even over Canoe Lake.
The pink/orange colour of the ambient light was another clue to the later afternoon timing of this painting. The right side of the cloud was illuminated a bit stronger so given the late afternoon timing of the thunderstorm Tom easterly view with the setting sun on his back might have been even more south-easterly. Remember that the sun sets more to the northwest in early summer.
The thunderstorm was nearly stationary or moving away slowly from Tom’s position. Tom was in nil threat of getting struck by lightning.
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| Portrait of Dr. J.M. MacCallum, “A Cynic”, 1917 by A. Curtis Williamson, National Gallery of Canada, Ottawa (4734). Bequest of Dr. J.M. MacCallum, Toronto |
Warmest regards and keep your paddle in the water,
Phil the Forecaster Chadwick
PS: For the Blog Version of my Tom Thomson catalogue raisonné, Google Search Naturally Curious "Tom Thomson Was A Weatherman - Summary As of Now" or follow this link “http://philtheforecaster.blogspot.com/2022/10/tom-thomson-was-weatherman-summary-as.html”
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