Dark Thunderheads

It might not be obvious but this is an example of supercellular convection. “Dark Thunderheads” was probably as close a description of supercells as one can expect from those unfamiliar with meteorology. Tom was motivated to record this weather event as it was certainly marked by nearly continuous thunder and dramatic wind shifts. Tom had a northerly view as the clouds are clearly front lit and there was some colour to be seen in those distant ridges even under the shadows of the towering thunderstorms. 

Storm Clouds, Algonquin Park or Dark Thunderheads
or Storm Cloud Spring 1916 or Lake & Lowering Sky.
Oil on composite wood-pulp board 8 7/16 x 10 9/16 in. (21.5 x 26.8 cm).
R.A. Laidlaw Thomson Collection at the Art Gallery of Ontario

Tom was quite close to this supercellular cumulonimbus cloud and was unable to fit it on to his 8.5 by 10.5  inch panel – even with the horizon placed so very low on his panel. As a result he painted the portion of the storm that interested him most. 

An Excellent Conceptual Model of the Supercell Thunderstorm

For a supercell, this happens to be the flanking line with the main updraft to the right side of the image.  The underside of the upstream anvil was barely hinted at but to a meteorologist, the large, back-sheared anvil is key to the successful application of Creative Scene Investigation, CSI.  An anvil that spreads upstream reveals that the updraft was very strong and associated with significant  wind shear. Wind shear is essential to generate a rotating supercell but that is another course in meteorology.

Graphic from my PowerPoint Showing the Portion of the
(Dark Outlined Box) Supercell Included in Tom's Weatherscape

The updraft was painted like a “barber pole” indicative of the rotating updraft. This is especially evident when one examines the original painting. Long and bold strokes of the brush alternate upward following the updraft just like the alternating colours of a barber pole. It is certain that Tom witnessed the rotation of the updraft which in itself is the fundamental definition of a supercell.

There could be a series of these thunderstorms toward the north suggesting possible organization along a squall line. However it is challenging to be certain from Tom’s painting – I suspect there is only the one supercell but can’t be sure. A lone, rogue supercell is actually more likely to be accompanied by severe conditions. This severity results from the fact that a single cell is able to access all of the heat and moisture energy in the air mass and convert that into motion. 

Top Down View of Supercell Thunderstorm
Conceptual Model 

The scud (Scattered Cumulus Under Deck) cloud in Tom’s painting is problematic. Remember that scud is formed when rain moistened air is lifted to form cloud at a level below the lifted condensation level for the air mass - the deck. This scud is certainly along the gust front from the rear flank downdraft which moved in after Tom started to paint. Thus the scud cloud appearing in the painting was purely a result of timing. Notice the scud cloud has no uniform base and is formed in the chaotic wind around the supercell and in the heavy rainfall. Tom had no choice but to paint the reality of what he witnessed, leaving it up to us to interpret those observations as best as we can. The scud cloud did hide the details of the flanking line that we know must be there feeding into the updraft. 

Notice that there was a considerable amount of wave action on the lake. There is not a clear enough signal to deduce a wind direction from those water surface gravity waves. However from the location of the updraft and knowledge of the wind profile that occurs around supercell thunderstorms, Tom would have been enjoying a warm if not hot and blustery southwesterly wind on his back when he started to paint. This wind would have shifted dramatically to the west and then northwest with the arrival of the afore-mentioned cooler, gust front winds. The gust front would have also brought the turbulent scud cloud. 

These intense and dynamic supercells are typical of spring and early summer. They are dominated by strong convective energy and wind shear. Supercells tend to be large beasts and thus Tom was able to stay south of the rain while still recording the weather observation. 

In some performance measurements studies I conducted while with Environment Canada, I detected a strong signal that La Niña years of the El Niño–Southern Oscillation (ENSO) climate pattern, were very conducive for supercell convection. The La Niña event that developed in the spring of 1916 and lasted until the spring of 1918 was the 13th strongest of the ‘classic’ La Niña events. It should not be surprising that Tom observed and painted a lot of severe convection during this period when he was at the peak of his artistic expression and tragically, the end of his career.

Singleton Lake Supercell

 Note that there was only one obvious “thunderhead” in Tom’s painting and not more as suggested by the title. The scud cloud was not a cumulonimbus cloud.  The accompanying image depicts the portion of the supercell that Tom recorded on his rather small panel using a photo of a supercell thunderstorm that I observed at Singleton Lake.  

The Singleton supercell was almost an identical twin to Tom's supercell. There was even some scud cloud that drifted into my view-finder at just the right moment but it was not nearly large enough to obscure the flanking line of the storm. All of the observations that I made of Tom’s painting, apply equally well to my photograph. This supercell had already knocked down several swaths of very large trees and cut power to the area. Our laneway was blocked by some large trees that had been felled by the damaging, downburst winds. Supercell thunderstorms can be lethal and are best viewed from a distance - preferably on radar or satellite imagery. 

This painting has been given several different names including Storm Clouds, Algonquin Park; Autumn Woods and Lake; Storm Cloud Spring 1916; Lake & Lowering Sky; and the name I preferred “Dark Thunderheads”. All of these titles miss the mark.  I would suggest that the name of this painting should be "Supercell with SCUD". This explains why I am very careful in naming my art and telling the story behind each individual painting. I also typically discus the nature and science revealed in the scene. I do all of this to eliminate the need for someone like me, making it all up years later. 

Tom painted what he saw… This painting may be viewed as part of the R.A. Laidlaw Thomson Collection in the Art Gallery of Ontario. Pay close attention to the long, bold and overlapping "rotating updraft strokes".

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”

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”

Lightning, Canoe Lake: Summer 1915

Tom Thomson and Lightning, Canoe Lake, 1915

There is no question what motivated Tom to complete this sketch. He wanted to record something that was elusive and pure weather – the flash of a lightning strike. The horizon is very near the bottom of the painting leaving no doubt that this is yet another skyscape - weatherscape. 

Lightning occurs with every thunderstorm and is responsible for more deaths than the very exciting, supercellular convection. Science and technology is still learning about the three dimensional shape in space and the four dimensional structure in time of the lightning flash. It is far more complex than meets the eye and I suspect that Tom appreciated that fact. 

Lightning, Canoe Lake, Summer 1915
(21.5x26.7 cm) 8.5x10.5 inches
oils on plywood 
Dr. J.M. MacCallum, Toronto, 1944 bequest
to the National Gallery of Canada

This painting could be looking any direction but the colour in the landscape reveals that Tom had some light on his back. Weather typically  moves from west to east with the jet stream and one can guess that Tom was looking at an approaching thunderstorm passing to his north.  It is less likely that Tom would be painting plein air just after the storm had passed. The lightning and cloud is unobstructed in his painting and this relatively clear view would only be available with the approach of the thunderstorm. After a thunderstorm there is typically an abundance of precipitation fog and mist as well as stratocumulus and SCUD (scattered cumulus under deck). The darker mass that fills the top third of Tom’s panel could be the shelf cloud of the approaching thunderstorm. The shelf cloud immediately precedes the updraft region of most thunderstorms and it is the updraft region generates most lightning. Tom's view is thus likely to be northwesterly.  

The slide from my PowerPoint along with an image of actual
lightning closely resembling what Tom saw and painted. 

The updraft of the convective cell carries small ice particles and their associated, typically positive charge into the higher reaches of the cloud. The cloud top temperatures required to create lightning processes is about minus 25 Celsius. This leaves the cloud base as a negatively charged area. Under the thunderstorm a positively charged “shadow” follows the updraft area trying to connect with the negatively charged cloud base. Short leaders of ionized air channels attempt to make that connection in jabbing strokes both from the cloud down and the land upward. When the connection is completed somewhere between the cloud and the ground, a tremendous amount of energy is exchanged and you do not want to be in its way! If the hairs on your body stand up, crouch quickly on the tips of your toes, become instantly shorter than everything around you and discard everything conductive like gold clubs and carbon fishing rods... 

Kinds of Lightning 

• There are many words to describe the different kinds of lightning. 
• In-Cloud Lightning: The most common type, it travels between positive and negative charge centers within the cumulonimbus cloud. 
• Cloud-to-Ground Lightning: This is lightning that reaches from a thunderstorm cloud to the ground. About one third of lightning strikes hit the ground. 
• Sheet Lightning: This is lightning within a cloud that light up the inside of the cloud like a sheet. 
• Ball Lightning: Rarely seen, this is lightning in the form of a grapefruit-sized, ionized ball, which lasts only a few seconds. 

What can one say about lightning? Lots! Quick Flash Facts: 

• An estimated two thousand thunderstorms are going on in the world right now... 
• There are about 100 lightning flashes around the world every second. 
• The diameter of a lightning bolt is about a half-inch to an inch wide, but can be up to five inches wide.
• The average length of a lightning bolt from a cloud to the ground is three to four miles long. 
• When lightning strikes a beach, the intense heat fuses the sand into silica glass beneath the ground. 
• A flash of lightning appears to flicker because there are usually several bolts of lightning striking at almost the same time. Together they comprise the lightning flash. 
• Flash is comprised of 6 to 8 strokes occurring in less than a second which is too fast for the eye to resolve. This is the reason for the bass sound. 
• Lightning can occur not only in thunderstorms, but also in snowstorms, sand storms, above erupting volcanoes and from nuclear explosions. 
• Electrical current of 10,000 to 100,000 amps far exceed the 15 amps it takes to toast a household fuse. 
• There are 500,000 tonnes (1,000 kilograms) of water in typical thunderstorm. 
• CN Tower gets about 70 lightning strikes each year. 
• 15 Canadians (150 Americans) die each year from lightning. (American are 10 times “better” than Canadians in almost all severe weather statistics.) 
• 40 Canadians (400 -800 Americans) injured each year from lightning 
• Lightning provides half of the world’s fertilizer. The current creates nitrogen based compounds in the air which are delivered to the ground in the precipitation. 
•  Bolt from the blue: A lightning bolt from a distant thunderstorm, seeming to come out of the clear blue sky, but really from the top or edge of a thunderstorm a few miles away.

Thunder is the sound generated by lightning. Lightning bolts are extremely hot, with equivalent temperature of 30,000 Celsius or 5 times hotter than the surface of the sun! When the bolt suddenly heats the air around it to such an extreme, the air instantly expands faster than the speed of sound itself. The shock wave we hear is an explosion of sound. There are 6 to 8 strikes within a lightning flash that lasts a fraction of a second. The frequency of these shock waves is in the lower bass frequency which explains why thunder sounds the way it does. If you are near the stroke of lightning, you’ll hear thunder as one sharp crack since the sound waves will all arrive at your ear at the same instant. When lightning is far away, thunder sounds more like a low rumble as the sound waves reflect and echo off hillsides, buildings and trees. 

This rumbling effect is enhanced if the lightning is oriented along (parallel to) your line of sight. Sound originating along the length will take different lengths of time to arrive depending upon where along that length it originates. Depending on wind direction and temperature, you may hear thunder for up to twenty miles away. Thunder is only a noise and is nothing to be afraid of but lightning is deadly dangerous.

The lightning flash that Tom painted was a negative flash from cloud to ground. The flash was oriented perpendicular to Tom’s line of sight. This means that the sound generated all along the channel arrived at Tom’s ear at about the same time making this particular flash sound like a thunder crack as opposed to a rumble.

There is a lot of wave action so Tom was probably in the gust front portion ahead of the forward flank downdraft of the thunderstorm. Tom also recorded a lot of turbulence and wind in the atmosphere under the base of the thunderstorm. This was quite an active thunderstorm.

The terrain of Canoe Lake would solve exactly where Tom was positioned to record this lightning flash as Tom was very faithful to the scene in front of him. A good guess for Tom’s vantage point would be Hayhurst Point which was one of Tom’s favourite camping places. It is also the location of Tom’s memorial cairn that was constructed by J.E.H. MacDonald and William Beatty.

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

Frankly we could write another book just about lightning. Although this would be an interesting project as well, maybe it is time to move on.

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”

Thunderhead: Pink Cloud over a Lake - Summer 1916

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. 

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. 

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. 

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.

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.

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

Tom painted what he saw… Tom's patron Dr. J.M. MacCallum felt the truth and power of the thunderstorm and kept this painting until giving it to the people of Canada 

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”