Thunder Cloud: Summer 1912

 

Tom Thomson and Thunder Cloud (from my PowerPoint)

Tom Thomson painted every type of thunderstorm - and several types more than once. This is a multi-cell thunderstorm which is the next step in severe potential from simple pulse convection. This painting was done looking northerly and there are several ways to deduce this. Most convective cells move from the southwest to the northeast over Ontario and specifically Georgian Bay and the shape of this cloud reveals that it is moving to the right. The upper winds of the westerly jet stream are also revealed by direction of the anvil drift. The cumulonimbus clouds are clearly front lit and there is some colour in the distant hills. The sun had to be on Tom's back and to the right. The lifted condensation level in Tom’s multicell thunderstorm is quite low indicating that the air mass was very moist and thus had abundant energy to fuel the thunderstorm. 

Thunder Cloud Summer 1912 or 1913 
National Gallery of Canada, Ottawa (4708). Bequest of Dr. J.M. MacCallum, Toronto, 1944

There is one overshooting updraft top on the anvil in Tom's painting with a second developing upstream along the flanking line - competing for dominance. Overshooting refers to an updraft climbing above the average level of the thunderstorm top. Typically the top of a thunderstorm is found at the stable top of the troposphere which is also known as the tropopause.  This second updraft in the multi-cell thunderstorm will take its turn as the star and eventually overshadow the first in strength. The new cells in the flanking line are still relatively weak but each will have its turn in the starring role of the thunderstorm updraft. This multi-cell can be expected to survive a few hours as the updraft pulses cycle from the flanking line through the downstream anvil. 

This multi-cell thunderstorm conceptual model is a good summary of what Tom saw and painted.
Multi-cell thunderstorms are characterized by multiple updrafts forming new mature cells as each downdraft (and precipitation) dissipates the previous cell. Cold air outflow from each dissipating cell triggers new cells along the leading edge of the outflow. This thunderstorm type is more long-lived than an ordinary, pulse-type, single cell thunderstorm.

Actual multi-cell thunderstorm similar to the one that Tom
observed and painted. 

The wind at Tom’s location is toward the thunderstorm updraft and is thus backed or turned counter-clockwise 60 degrees from the upper winds revealed by the west to east orientation of the anvil. Tom would have been experiencing a warm and moist southwesterly breeze on his back.  This wind was even strong enough to produce white caps on the lake implying a wind of at least 17 mph - almost 30 kilometres per hour! The same flow was delivering heat and moisture to fuel the multi-cell thunderstorm complex.

Tom would have experienced a beautiful afternoon plein air session with the wind and the sun on his back accompanied by the sounds of distant thunder and the flash of lightning.  The wind would also have been strong enough to keep the biting bugs from being much of a nuisance. It would have been a perfect day out painting while surrounded by nature. Paint on Tom!

Thunder Cloud 7x10 inches
in the Tom Thomson Sketch Box

Tom painted what he saw… Tom's patron Dr. J.M. MacCallum felt the truth and power of the multi-cell  thunderstorm and kept this painting on his walls until bequeathing it to the people of Canada in 1944. 

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”