Naturally Curious: Tom Thomson’s Lake Shore and Sky

Tom’s motivation to “record” this particular observation –the cloud structure.

This sky pattern that intrigued Tom is also a frequent flyer. I have many examples. The clues to determine the viewing angle are subtle and require being familiar with the patterns. An educated, plein air guess would always be toward the north. In Tom’s painting this is where we see the hints of a westerly cold flow with the clouds growing in size from left to right. The sun was getting low on the western horizon and this would explain the illumination on the west facing tops of the stratocumulus. There is also some yellow hue in the distant sky that could be associated with a setting sun and Rayleigh scattering removing the blue colour from the sky. The flow that Tom painted was cold, unstable and increasingly moist with low based turbulent stratocumulus. This would require that the painting was done late in the afternoon in which case Tom was certainly looking northerly.

The orientation of the deformation zone is the other important clue. The thick altostratus on the moist side of the deformation zone is associated with a low pressure area that is retreating eastward. In the northern hemisphere systems generally move from west to east. The deformation zone is oriented as though the system is approaching from the left in which case “west” must be to the left. Note how dark the altostratus is. This is a result of the thick layered cloud that much be associated with the retreating low pressure area.

With this diagnosis the winds at Tom’s vantage point would shift from the west to the northwest in the next several hours and these winds would be distinctly chillier as they usher in colder and drier air in the wake of the low. The overcast skies would abruptly clear to be replaced by northwesterly streets of stratocumulus which would also gradually dissipate with the weakening winds and increasingly drier air mass.

Initially with this painting, I investigated several other alternative interpretations but one by one they all became less probable than the one presented above. From a painter’s perspective, it is likely that Tom was confined to his tent all day with the rain. He would have been eager to get out to capture the skyscape when the rain tapered off – even though the western flange of the deformation zone cloud still hung overhead.

Tom’s location would be just west of the low pressure centre with his viewing angle within that depicted by the yellow arrows. The surface flow would be westerly while the flow aloft would be turning toward the northwest as the low continued to pull away.