Tom Thomson's Rainbow

After every storm there is always a “silver lining” and in Tom’s case, that tempest had all the colours of the rainbow.  

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. 

I prefer graphics rather than words to explain natural wonders.
Here parallel rays of sunlight encounter a host of small water
droplets. Refraction 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 rain drop 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 as well. More importantly with the morning sun in the east, the cloud producing the rain drops 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 rain drops 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. 

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.

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 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 that his observation was probably made with at least two hours to go before sunset. 

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 getting low on the horizon, there was still possibly as long as three hours until twilight. 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

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 so 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 base line (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 rain drop looses a bit of light energy to refraction. A double internal reflection within exceptionally spherical rain drops 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 at 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. 

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 are 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. 

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 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. 

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 had been 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

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”