What's that smell? Smoke plume on the move in Ontario
Thursday, July 12, 2018, 4:55 PM - One of my Twitter followers asked me an interesting question recently, "can we smell the fires that are ongoing in the Temagami area today?"
I thought this was a pretty simple question to answer, but as I looked into it, turns out that things are a lot more complicated and interesting than I first thought.
Currently, the fires in Ontario are not on the scale of the infamous Fort McMurray blazes, but they are still significant. The smoke is now travelling southward due to prevailing winds from the north.
How far south the smoke travels depends on a few things including, the amount of smoke produced, the stability of the atmosphere, wind direction, and capping inversions.
Let's look at each one in turn.
The amount of smoke a given fire produces is dependent on how big an area is on fire. The Fort McMurray wildfire consumed a final area of 590,000 hectares, whereas the Ontario fires are far smaller. However, there is a lot of them. 90 per cent of the mass emitted by a fire is carbon dioxide and water vapour. The last 10 per cent, hydrocarbons and particulate matter, are what we can see and smell. So, the larger the fire, the more smoke produced, and the more likely we will be able to see or smell it.
However, the more intense the fire, the more the fuel is converted into CO2 and water. So, in order to get a fire that can be smelled far away, we need a big, low intensity fire.
Where those hydrocarbons and particulates go is dependent on the weather systems that are swirling around it. Big, intense fires can modify the weather around them and even produce "pyrocumulus" storms -- thunderstorms that are induced by the heat of the fire itself. The fires in Ontario do not have the intensity to do that, but there is enough heat to drive the smoke high into the atmosphere.
Instability is also key to getting smoke high enough to be moved by the fast winds of the upper atmosphere. Warm air at the surface with cool air above is unstable, while cool air at the surface with warm above is stable, or "capped." The greater the instability, the higher the smoke will be lifted. At the same time, the higher it's lifted, the more it will be dispersed and less able to be detected.
The heat from the fire will push the smoke upwards in a stable atmosphere, but the inversion will quickly force the particulate matter and hydrocarbons back down to the surface, making it detectable by sight and smell well away from its origin point.
Finally, winds at various levels in the atmosphere determine where the smoke is going. In the case of the Ontario fires, the winds are generally from the north at all levels. That means that the smoke is being driven towards southern Ontario and the GTA.
But can we smell it here in Toronto? Smoke has been reported in Muskoka, which means that it is getting at least that far south. The most likely way we would detect smoke in the city, given the dispersal and the distance from the origin point, is a haze in the upper atmosphere. Actually, smelling it would be difficult, but not impossible, especially as the fires increase in size.
So, if you are headed up to the cottage this weekend that wood smoke you are smelling might not be a campfire and marshmallows. It may be something far more dangerous.