After every storm there is always a “silver lining” and in Tom’s case, that tempest had all the colours of the rainbow.
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Rainbow -1916 Oil on wood 8 1/8 x 10 1/4 in. (20.6 x 26 cm)
Inscription verso: Snow-Covered Trees, oil on wood,
Vincent Massey Bequest to the National Gallery of Canada in 1968 |
The optics of the rainbow require that the sun be on the back of the observer and that the sunlight is unobstructed to reach a collection of spherical raindrops. Perfect conditions for the plein air artist! Raindrops are most spherical when they are small. As the size of the raindrop increases, they become distorted into an oblate spheroid – a flattened bun-like drop .. a Big Mac. Large drops are not effective refractors or reflectors of sunlight and create clumsy rainbows if at all.
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I prefer graphics rather than words to explain natural wonders.
Here parallel rays of sunlight encounter a host of small water
droplets. Refraction is when light passes from one medium into another
and the colours are separated - shorter wavelengths bend more than longer
wavelengths. The single internal reflection (within a medium)
from the back of the raindrop returns some of that light at roughly
40 (violet) to 42 (red) degrees from the incident light rays. |
It is unusual to have rainbows in the morning since convection and the associated rain showers are less likely in the morning. More importantly with the morning sun in the east, the cloud producing the raindrops will typically also obscure the necessary sunlight. Rainbows appearing in the western sky in the morning light are rare.
It is much more typical to have a rainbow in the afternoon with the sun in the west. The convection that produced the raindrops would be moving away toward the east and followed by clearing. This clearing is vital since it allows the sunlight to fully illuminate the last drops as the rain tapers off and the clouds move away with the prevailing winds of the westerly jet stream. This gentler precipitation on the back edge of the heavier rain area is typically comprised of smaller, more spherical drops which are perfect for producing rainbows.
Note that there can be no rainbow at noon… midday rainbows would have to be viewed between the observer and the ground. Rainbow effects can happen at noon but it involves a water hose and probably washing your car.
The following text is from the first draft of "Rainbow". That interpretation of "Rainbow" was playing the meteorological odds but in this case, gambling did not pay!
"As a result, it is safe to say that Tom was looking eastward in the late afternoon after a band of showers had moved across his vantage point. This is another excellent example of Tom recording his observation of a weather phenomenon in oils. He was probably anxious to get back to painting again after having his plein air session temporarily rained out."
The real story emerges with boots on the ground! Terrain matching has located the painting location for "Rainbow". Thomson was actually looking toward the west-northwest from the cliff overlooking Tarn Lake in the summer of 1916 (Latitude 45.799988 Longitude -77.697180). That story is told below. Recall that Tom was working as a fire ranger and must have been quite surprised to see a mid-morning rainbow. There was not much cloud in the eastern sky so this was a line of convection along a cold front approaching Tarn Lake. The sunlight refracted in the small raindrops that were about to make Tom wet.
The black arrows on the upwind flanks of the cumulus and cumulus fractus clouds indicate that the wind direction at cloud level had a southerly component. Tom would have been in the southerly winds of the warm conveyor belt with a cold front on the way.
From the orientation of the rainbow as it hits the ground, one can also estimate the sun’s elevation and thus the time of day. A steep arc to the ground occurs right at sunset when the sun is at its lowest elevation but still able to illuminate the raindrops. The rainbow that Tom painted had a shallower slope to the ground so his observation was probably made at least two hours from either sunrise or sunset.
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PowerPoint Slide from my "Tom Was a Weatherman Presentation"
I knew the time and thus the hours until sunset in those rainbow
images I took to calibrate the rainbow angle and time to sunset. |
In addition, the colours in the sky are not those of sunset. Although the sun was low on the horizon, there was still possibly as long as three hours after the sun rose or until it would set. If the sun was indeed lower on the horizon, the colour of the setting sun would have been more orange as illustrated in the accompanying graphic - top left rainbow at sunset. This is especially true given the May 22nd, 1915 eruption of Lassen Peak in north-central California. Volcanic aerosols from that eruption just over a year previous to "
Rainbow" would still be lingering in the stratosphere (
exponential decay time of approximately 1.5–2 years) and thus colouring the sunrise and sunset skies.
The path between the rain and the observer is the
adjacent base used for the 42-degree rainbow. The longer this line, the higher and larger the rainbow must appear in the sky. The tangent (tan) of the 42-degree angle must always be the same as adjacent base (
a) increases, the height (
o) must also increase - simple trigonometry. The density of the small, spherical raindrops and thus the intensity of the rainfall must both increase in order to see a rainbow distinctly as this baseline (
a) distance increases. The rain-producing Tom's large rainbow was quite a distance away!
The physics of internal refraction and reflection within a raindrop surrounded by air is also very interesting. On each internal reflection, some of the light energy is lost due to refraction as it is transmitted into the other media. That is why the secondary rainbow… is secondary… plus it requires two internal reflections. Each of these internal reflections within the raindrop loses a bit of light energy to refraction. A double internal reflection within exceptionally spherical raindrops can produce a secondary rainbow based on a 51-degree reflection. Tom did not see and thus did not paint the faint secondary rainbow on that late convective afternoon in 1916.
Tom carefully and correctly matched the colours of the rainbow. This chromatic feat belies his dexterity in handling his oils and brushes. If you are in doubt of Tom's prowess, try it yourself. I have many times. I always make time to enjoy a rainbow - as Tom obviously did.
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A photo I took of primary and secondary rainbows at sunset on Foley Mountain looking east.
The sky between the primary and secondary bows is noticeably darker and this effect was first described in 200 AD by Alexander of Aphrodisias. “Alexander’s Band” is darker since light rays undergoing a single internal reflection brighten the sky inside the primary rainbow. Light reflected twice deviated to form the secondary bow and brighten the sky outside. Raindrops along the lines of sight between the two rainbows cannot send light to your eye and so the sky is darker there.
There is a considerable amount more of science and optics in this image but I will leave
that detailed information to the textbooks. |
The Tom Thomson blogs are always a work in progress - striving to complete the story behind each painting. These investigations have evolved from being purely about the weather and science to include more historical background behind each work. Since the efforts started in the early 1980s, the circle of Thomson friends who have contributed to these efforts keeps getting bigger and bigger. That is a great thing! The wealth of knowledge from these individuals adds considerably to the history.
One Thomson friend introduced me to a trio of new Thomson enthusiasts in May 2024. Scott Rodgers, Mark Rubino and Mark Scarlett completed an epic trip in May 2015 and at the urging of my Thomson friend were persuaded to visit the Tarn Lake cliff. They were armed with prints of some Tom Thomson paintings that might have been completed in that area. They were very successful and did indeed follow in the footsteps of Tom Thomson.
Maps by Jeff Maps by Jeff V4 showed a historic trail from Upper Pine to Grasspink with a dogleg southwest of ZigZag. As the canoeists recall:
"we turned back at the beaver pond as we had run out of flagging and, sadly, after lunch we had nothing left to use for bread crumbs to finish the job. These are some shots of what we found, with a white line beside the trail we were following."
The following photo looking west-northwest from the cliff reveals a terrific match in the terrain depicted in Thomson's painting. Tom did not make stuff up - clearly, he painted what he saw. Somehow Tom also included what he felt in those brush strokes and therein lies the magic of the artist.
The McElroys had already visited Tarn Lake in 2012 and chronicled that experience in "
The Cliff at Tarn Lake". Their exploits always make for an interesting read. Positive experiences, learning and exploration never get old. It takes a circle of open minds to really learn and discover the truth.
The information from this image helped to reveal the true story of Thomson's "Rainbow". That required a bit of a rewrite as noted above but most text was left unchanged. I was playing the safe weather bet of a departing shower moving toward the east producing a rainbow with the sun in the western sky. The odds were in my favour but it was still a losing gamble.
The following graphic depicts the updated truth behind Tom's weather observation. For mid-summer (arbitrarily picked as July 15th), the painting would have been done just after 9 am about 3.5 hours after sunrise. Tom painted pretty much exactly what he witnessed but this timing does put a lot of emphasis on the azimuth angle to the sun. I believe that Tom was that particular about what he painted! The sun would have been about 22 degrees above the eastern horizon and Rayleigh scattering would not have been sufficient to cast a reddish hue to the illumination. The facts all fit together nicely.
Note that as the date of the painting deviates from the above July 15th guess to an earlier time approaching summer solstice on June 22nd, the time of the observation gets slowly later to around 9:19 am with the solar declination maxing out at 23.45 degrees. The earliest sunrise for Tarn Lake Cliff at summer solstice was 5:21 am.
As the painting date gets later than July 15th, the sunrise of course gets later and the observation time gets gradually earlier. A September 1st painting date would require Tom to be painting around 8 am and the solar declination would be only 8 degrees above the horizon. By the summer equinox on September 22nd, the sun would rise at 6:55 am and Tom would have to be painting by 7:30 am. The solar declination would be barely above the eastern horizon.
Clearly, the plane of the rainbow and the intersection with the earth is determined by the solar declination. The angle to the sun above the horizon then reveals the date Tom painted the rainbow. I am still tinkering with that estimate but expect the date to be around July 1st. It is safe to say that Tom was painting by 9 am on that summer morning. This post will be updated only when I have a convincing estimate that I am satisfied with...
There is some additional meteorology for the weather enthusiasts. The convective clouds producing the raindrops that created Thomson's rainbow were actually ahead of the surface position of the approaching cold front. The split cold front pattern where the cold air aloft gets ahead of the surface front is quite common. Southerly winds were still evident in the clouds to Tom's west but the cold front had already produced weather overhead. Such a cold front is also termed as being active or anabatic. In contrast, the weather with an inactive or katabatic front is found behind the frontal passage. The col in the deformation zone which is the cold front, separates the anabatic portions associated with the cyclonic companion from the katabatic and anticyclonic companion. These facts further place Thomson within the weather pattern of the summer morning at Tarn Lake. The following graphic summarizes these considerations. This material will not be on any exam... See "
Finishing The Dry Conveyor Belt" for more details.
Tom Thomson did not autograph this weather observation - just like he rarely signed any of his artistic records. A very cursory look at the art reveals 1916 written in the upper left corner of the panel - probably in pencil. The Tom Thomson Estate Stamp was pressed in the lower right. After Tom passed in 1917, his friend and future charter member of the Group of Seven, J.E.H. MacDonald designed a stamp that was made in both metal and rubber. The Estate Stamp was pressed into the paintings that Thomson had left behind in his Shack beside the Studio Building in Toronto. Apparently, both stamps have been locked in the National Gallery for decades and I have not yet seen them in person. Tragically, the estate stamp caused some damage to a few of these paintings.
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| Tom Thomson had some very good friends including these two Jims. |
Tom Thomson pushed away from the Mowat Dock at 12:35 pm on Sunday, July 8th. He paddled southward remaining on the east side of Little Wapomeo Island on that hot and muggy summer afternoon. Tom passed out of sight between Little Wapomeo and Big Wapomeo Islands. There his body would surface from the depths of Canoe Lake eight days later on Monday, July 16th, 1917. He was born on Sunday, August 5th, 1877. Tom's brilliant painting career ended mere days shy of his 40th birthday. He was just discovering his wings as an artist.
Tom Thomson has been called the Canadian Vincent van Gogh. Indeed there are similarities of artistic genius passing tragically and inexplicably way too young - 37 years old in the case of Vincent. Some people, including members of his own family, felt that the mystery of Tom's death was essential to his legacy. I respectfully disagree with that opinion. Understanding why Tom painted what he did and listening to what he was saying with his oils and brushes, are the essential ingredients to appreciating his art. Herein lies the motivation for these blogs in an attempt to place his art within the context that they were created and that they deserve. There are many more Blog entries to come. Thank you for reading!
Warmest regards and keep your paddle in the water,
Phil the Forecaster Chadwick
Tom Thomson Post 11
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