Tom Thomson's Approaching Snowstorm

The genius of Tom Thomson was that sometimes he painted in "verbs". "Approaching Snowstorm" is actually an action shot of real weather just as it happened back in the late autumn of 1915. Actually, Tom’s painting is an excellent capture of an approaching snowsquall as opposed to a synoptic scale snowstorm as suggested by the title. The cold Arctic winds blowing off the warm waters of the Great Lakes are a distinctly different weather phenomenon than a winter storm with a low-pressure area. 

With the bare ground in Tom’s painting, this would have likely been the first snowsquall outbreak of the winter. The winds would have been westerly off Georgian Bay for these snowsqualls to penetrate into the highlands of Algonquin Park. The low-pressure area generating these winds and weather patterns would have been far to the east.  

Approaching Snowstorm from 1915 oils on wood
21.3 x 26.6 cm (8.5 x 10.5 inches which was Tom's paint box size)
Bequest of Dr. J.M. MacCallum, Toronto, 1944
National Art Gallery Accession number 4689

View of snowsqualls looking northeast from Watershed Farm
at the crest of the Oak Ridge Moraine. There are three parallel
snowsquall lines in my photo. The convective clouds appear
similar to Tom's even though I was not at the leading edge.
4 pm local time November 8th, 2004 

My photographic comparison with Tom’s painting matches the appearance of the clouds. Tom was painting exactly what he witnessed. 

• 
  There are multiple snow squalls in both Tom’s painting and my photo. 
• The snowy underside of the closest snowsquall was dark meaning that the clouds were optically thick. Mie scattering from large snowflakes distributed the light forward away from the observer. Any backscattered light was blocked by the thick cloud. 
• The snow and cloud had advanced further aloft with stronger guiding winds than at the surface. Friction effectively reduces the wind at the surface causing the cloud and snow to appear as though it was tumbling or falling forward as it advanced. This tumbling appearance was certainly of interest to Tom. 
• Shafts of snow virga descend from the snowsqualls in both the painting and the photo.
• My photographic comparison with Tom’s painting has some important differences which in themselves are instructional. 
• In Tom’s painting the sun was to his back and from the left. The convective tops and distant snowsquall clouds were front-lit from the left. With a west-to-east orientation of the snowsquall bands, this requires that the sun was in the western sky. These optics reveal that Tom was painting in the afternoon. 
• The clouds in my photo are weakly front-lit by the setting sun at 4 pm on November 8th, 2004. The photo was taken a bit later than Tom painted his weather observation. Tom was looking north to northwest while my photo was looking northeasterly.
• The bare ground of Tom’s painting indicates an early season snowsquall event which is quite consistent with the timing of my November snowsqualls. 

  

A classic dendritic snowflake is depicted to the right. The science of snow and ice crystals and how they grow into these unique shapes would occupy several books. Like humans, no two ice crystals share an identical life story from microscopic birth through development as they drift down through infinite variations in temperature, humidity and other atmospheric conditions. The microscopic nature of ice crystals is all different under detailed photographic examination. No two snowflakes may be exactly identical but like humans, still very much the same. There are still a few things we should know about different types of snowflakes and what they might reveal about the weather that created them. 

Snowflakes associated with convective cumulus are typically large, dendritic flakes like the one pictured that can accumulate very quickly. A small amount of water can create quite a depth of dendritic flakes. Typically these large flakes are manufactured in an unstable environment within towering cumulus clouds - ideally at temperatures between minus 12 and minus 15 Celsius. These cumuliform clouds may cover 10 square kilometres of the landscape at a time. A typical towering cumulus crossing a location may drop large snowflakes but only do so for a short period as they move with the winds. If the cumulus is especially vigorous, the turbulence inside the convective updrafts will smash the delicate arms of the dendritic flakes resulting in "snow (ice) pellets type B". The balls of hard snow or ice do not accumulate nearly as much as the larger, intact dendritic flakes. Snow pellets also hurt upon impact if you happen to be outside. You won't want to catch them on your tongue!

Snowsqualls are a bit different. A snowsquall is more like a parade of towering cumulus clouds riding a never-ending conveyor belt. Each convective cloud will drop a load of large, dendritic snowflakes along its path. The large snowflakes can really accumulate with a snowsquall as long as the wind does not change direction and parade the towering cumulus clouds across different regions. 

Dendritic snowflake (left) and synoptic snowstorm snow (right)

My hand in a deep snow accumulation from
a real snowstorm. 

The snowstorm alluded to in the mistakenly applied title is very different still. Large, synoptic-scaled storms associated with low-pressure areas move slowly. The air riding the warm conveyor belt rises gently generating smaller snow crystals (image above right) as opposed to the convectively constructed, large snowsquall flakes and their aggregates.

The very significant difference between towering cumulus snow and snowstorms can be found in the snow crystals and the duration of the event. The small snowflakes typically fall for an extended period of time while the large low-pressure area crosses the landscape. The lows move slowly and the snow has time to really accumulate. The towering cumulus convective clouds produce large snowflakes but they come and go quickly. The following poem summarizes these differences quite nicely: ice crystal description followed by the associated accumulation.

“Big snow, little snow; Little snow, big snow.”

In other words, big convective snowflakes (with transient towering cumuli) result in little snow accumulation. However, little snowflakes (with large, slow moving low pressure areas) can result in big snow accumulations. The exception to this little ditty is of course the snowsquall for the many reasons I tried to explain and why Tom painted what he did. 

A cold frontal passage on November 30th, 2022 resulted in
early-season snowsqualls similar to what Tom would have
recorded in his brush strokes. Tom painted what he lived.

Tom was looking north to northwesterly when he made this observation of snowsqualls. We do not know how much snow accumulated or how long the snowsqualls aligned with that piece of upslope topography on the western slopes of the Algonquin Highlands. The snowsquall might have seemed like a snowstorm to Tom and to Dr. MacCallum. The painting should really be renamed as “Approaching Snowsqualls” We have no way of knowing if the landscape was deep with the first snowfall of the season after the squalls were done. 

Tom would have appreciated the difference between the cumulus congestus and a real snowstorm. This plein air sketch was probably completed by mid-October 1915 after which Tom went to paint on the decorative panels at MacCallum's cottage until deer season opened in early November. Tom stayed in  Huntsville with Winnifred Trainor after that until late November 1915. 

Recall that in “Snow Pillars in the Sky”, Tom was painting westerly snowsqualls in the morning. With “Approaching Snowstorm” Tom was painting westerly snowsqualls in the afternoon. Both observations were completed in 1915 but I wonder whether it might have been on the very same, autumn? The afternoon snowsqualls appear more intense as would be expected with the ongoing arrival of fresh and colder Arctic air. The addition of some daytime heating over the land also strengthens the convection during the afternoon hours. No one knows for certain. I am only guessing. 

The length scale of synoptic scale events is the order of 1000 kilometres. The passage of synoptic weather events takes at least several days depending on the speed of the jet stream. The probable separation in time between storms and cold Arctic outbreaks is perhaps a week, more or less (but likely more). Tom certainly did spend some weeks within the snow belts off Georgian Bay in the autumn of 1915. That amount of time was more than sufficient to witness more than one day of snowsqualls so anything is possible - but it is good to wonder and imagine the possibilities. 

Both of the unsigned paintings pictured above ended up in the collection of the good doctor MacCallum. Like snowflakes, the panels are similar and share nearly identical subjects and histories before ending up in the National Art Gallery of Canada in 1944. We will never know the complete life stories of these sketches and that is OK. Not knowing the path of every crystal of ice through the atmosphere does not stop us from appreciating its beauty. 

Warmest regards and keep your paddle in the water,

Phil Chadwick

PS: Tom painted real weather that gets repeated. Here is a current  radar image of snowsquall bands coming off Georgian Bay. He would have loved to sketch these storms. Dec 24th, 2022. 

5:23 am Christmas Eve, 2022.
Intense parallel snowsqualls in the wake of
a historic low pressure bomb crossing southern Ontario.
The blizzard event was extremely well predicted. 
Congratulations to the teams at the Ontario
Storm Prediction Centre

PSS: Tom Thomson Was A Weatherman - Summary As of Now contains all of the entries to date.