Tom Thomson's Squall Line Thunderstorms - Evening Summer-1915

 

Evening from the summer 1915

Tom painted what he saw! Here is what we know. The low horizon indicates this is another of Tom’s skyscapes. The art and the story are all about the weather. 

• This back lit cloud is clearly based in the low etage - near the ground. The air mass is very moist as the lifted condensation level is so low that the cloud bases are hidden by the landforms on the horizon. 
• There is a “white line” on the distant lake shore indicating that Tom was looking toward the light source – the sun. That bright line is caused by the sunlight glinting off the waves.
• The air mass is also very unstable with gusty surface winds from the left to right – southerly to southwesterly. These winds were fuelling the clouds with heat and moisture energy.
• White caps begin to appear at 8-10 knots and become more frequent above 15 knots. The absence of numerous white caps suggest a wind speed of perhaps 10 knots or around 18 kilometres per hour. That is enough wind to keep the mosquitoes from being too annoying. 

Evening, Alternate title: Evening Clouds,
Summer 1915, Oil on composite wood-pulp board,
8 1/2 x 10 1/2 in. (21.6 x 26.7 cm)

• The stronger illumination of the left flanks of the clouds indicates that Tom was turned a bit toward the northwest. 
• The clouds are towering cumuli also known as cumulus congestus that were transitioning into cumulonimbus – thunderstorms. Note the thunderstorm anvils developing as the updrafts near the stable tropopause and the top of the layer of the atmosphere that contains most of our weather. 
• The thunderstorm anvils were being directed by the northwesterly winds at the highest levels of the atmosphere. 
• The sketch is certainly late in the day due to the colouration of the atmosphere. The word “evening” in the title was not really needed to make this deduction. A squall line of developing thunderstorms such as this would be a rare event in the morning!
• There are perhaps some low cloud pieces between Tom and the line of developing thunderstorms which would indicate a wind direction along the elongated edges of these clouds. I could make a case for these clouds to be stratocumulus streets being drawn into the convection along a southerly breeze ahead of the approaching line of convection. If so, that would explain why the northern cell to the right was more developed than the southern cells. Note that there would be enhanced convergence, wind shear (helicity) and moisture advection along those stratocumulus streets that feed into the convection. The subtle cloud rolls in the painting appear to be pointed toward the more northern convective tower.  
• The rich orange colours in the clouds and sky were also influenced by the May 22nd, 1915 eruption of Lassen Peak in north central California. The Lassen Peak eruption was tiny compared to the major eruption of Mount St. Helens in 1980 but the effects on the sky were still noticed by the artists in Algonquin - and after all, they painted what they saw. 

  

  Sulphur dioxide spewed from volcanoes reacts in the atmosphere to form sulphate aerosols (aerosols are tiny, suspended particles in the air). Rayleigh scattering by volcanic ash and aerosols preferentially scatter the shorter blue wavelengths out of the sun's direct beam. The direct beam passes through a long path of atmosphere at sunrise and sunset leaving only
  longer wavelengths of orange and red to illuminate the scene.

PowerPoint slide from my presentation. The yellow circle is
intended to represent the sun's position. The red block arrows
denote the updraft locations and their relative strengths.
The black arrow is the atmospheric frame wind directing
the thunderstorm anvils at the tropopause.

The Creative Scene Investigation (CSI) Solution

Tom was looking west-northwesterly. A south to north line of thunderstorms was approaching Tom’s camp during the evening of a warm and moist summer day. Southerly breezes would shift dramatically to the west with the passage of these thunderstorms in about an hours time. Damaging downbursts and hail would be a distinct possibility and it would be getting dark when these arrived. 

The thunderstorms were probably associated with a cold front and the following day would be cooler and drier with brisk northwest winds likely. Angling would not be very good as it had been in the warm sector of the system (ahead of the cold front). Fish tend to become sluggish and stop biting behind a cold front. Tom's time would be better spent using the paint box on the day following the passage of this squall line. 

The accompanying photo of a cold frontal squall line compares very closely to Tom’s painting. The setting sun was in the photo so the west-northwest direction of view for that summer evening was well established. The multiple thunderstorm anvils guided by the northwesterly upper level winds are almost identical to Tom’s anvils. Those pouch-like cloud structures on the underside of the anvils are mammatus clouds. Mammatus clouds are an example of clouds in sinking air. They require strong updrafts to carry large concentrations of precipitation particles into the highest levels of the atmosphere. The moisture laden air is heavier than the surrounding air and must sink back to earth. Over time, the cloud droplets eventually evaporate and the mammatus dissolve. The slow vertical motion of mammatus cloud droplets results from a balancing of warming in sinking air and evapourative cooling. The mammatus pouches just hang around... The photo proves that squall lines similar to the one that Tom recorded certainly do exist. 

Two Very Different Photographs of the Tom's "Evening Clouds"

Photographing art can be challenging. Even the professionals at the major galleries must find it difficult to accurately represent the actual art. The differences in these two images are striking. The bold brush strokes are more crisp on the left image. The colours are dramatically different. The tone in the right image is far darker while some colours are more vivid. Perhaps the official image of the painting comes down to the personal choices and preferences of the photographer. Nothing is ever quite as good as the original. 

The right hand image does a better job of showing the scraping that occurs when the panel is slid into the sketch box upon completion. Tom was probably canoeing and certainly not staying at Mowat Lodge when he painted "Evening". 

I take great effort to match the official, high resolution  image
of my art with the original. Professional Studio lighting and even 
covering the reflective camera tripod makes the image the best it
can be... but each painting is different so careful experimentation
 with the lighting umbrellas is also important. My computer controls
the camera remotely so there is not even light bouncing off my body
that could influence the image of the painting... The pegboard grid  
ensures that the camera is aligned perfectly with the centre of each
painting. The goal is to honestly match the image with the original.

This fine painting was in the Laidlaw family of Toronto from 1922 to 1965 when it was gifted to the McMichael Canadian Art Collection, Kleinburg (1966.15.20). Tom Thomson did not autograph this weather observation either. 

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”