To see more clearly, we must start at the beginning: at the beginning of this fire episode.
A week ago, temperatures in eastern Canada in particular (Southern Ontario, Quebec and the Maritimes) exceeded 30°C, while May was drought due to significantly insufficient rainfall.
So all the ingredients were there for a simple spark to ignite the powder, as they say.
The Maritimes, particularly Nova Scotia, were the first to be affected by these fires, but the fires also began to spread further west, in Quebec.
However, a low-pressure system helped contain the blaze in Nova Scotia thanks to a very large amount of rainfall. But in the meantime, an atmospheric blockage — a weather phenomenon in which conditions persist for days or even weeks — has kept this system nearly frozen over the Maritimes, increasing the smoke stream driven by winds blowing into the Northeastern United States flow is carried counterclockwise. This explains why winds mostly from the north drove the smoke towards the United States.
In addition, another low-pressure area that occurred in the middle of the Atlantic Ocean slowed the progress of the system over the Maritimes, allowing the latter to dump significant amounts of rain on the Maritime provinces but also on the north shore. However, a low-pressure area affects not only precipitation but also winds.
In the northern hemisphere, a low-pressure area’s winds move counterclockwise around its center, while a high-pressure area’s winds, usually responsible for good weather, move clockwise. Note that the directions are reversed for the southern hemisphere.
The low-pressure system responsible for the rain has so far been blocked over the Maritimes, sweeping all of Quebec with rain that has relieved the north coast, which has been ravaged by major fires in recent days. But the fact that this system in eastern Quebec is almost frozen meant that the north winds did not change direction for several days.
Outcome: A corridor emerged that drew smoke generated by the Quebec fires into the United States. The atmospheric blockade kept the winds toward Canada’s southern border.
It is important to note that atmospheric blockage is not a rare phenomenon but has become increasingly common in recent years.
For example, the extreme heat that suffocated British Columbia in June 2021 was again the result of an atmospheric blockage. Remember that this year the town of Lytton broke a dismal absolute heat record of 49.6°C, the highest temperature ever recorded in Canada.