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Smoke’s impact on weather

Try not to get too upset with your weather forecasters during smoky spells

The sun rises through a cover of wildfire smoke above downtown Toronto on July 20.

In my last article we took an early look at this summer’s heat. One saving grace (if you want to call it that) with the heat this year is that humidity levels have been low — not surprising, given the drought conditions. This goes back to the discussion we had earlier this year about whether wet or dry conditions exacerbate more of the same. As we discovered, it is not a simple yes-or-no answer — otherwise, we would either stay wet or dry all the time. However, our current dry conditions are not helping to generate any rainfall, and this has been leading to a rash of forest fires and their accompanying smoke. These two topics, smoke and drought, go together. 

Our current long-term drought stretches well beyond the boundaries of southern and central Manitoba. Along with our region and most of Western Canada, most of the western U.S. is also in severe to exceptional drought. Draw a line roughly from Lake Superior to New Mexico and everything west of that line is in drought. Weather systems that would normally trigger thunderstorms that bring most of our summer rainfall just don’t have the moisture they would usually have. Thunderstorms that do form bring only quick shots of rain, with amounts typically less than 25 mm, instead of the usual big dumps of 40 to 60 mm. While any rainfall regions get from these moisture-starved storms is welcome, it will take a major shift in our weather pattern to bring an end to our current drought. 

Take the long-term drought, add heat to it, and you get the recipe for forest fires. While most agricultural regions don’t have to worry directly about forest fires, we all have to deal with the impacts of smoke. Now, luckily for us, the local boreal forest lies to our north and east — directions we do not often see our winds come from. Last week, and in particular, July 19-21, we saw northeasterly winds which, for some regions, brought in thick heavy smoke. Besides the obvious health hazards associated with smoke, it can also have significant impacts on the weather. 

Radiation

Smoke does a really good job of blocking incoming shortwave solar radiation but has very little impact on outgoing longwave radiation. In essence, it is like the opposite of greenhouse gases. For those of you old enough to remember the discussions about nuclear winter, we saw a short-term example of just what that could be like. In the nuclear winter scenario, smoke from all the fires created by a nuclear exchange would blanket the Earth and quickly drop temperatures, creating winter-like conditions for several years before the smoke finally cleared out. 

Last week, not only did we have low-level smoke from local fires, but we had upper-level smoke from fires out west, something we have been dealing with on- and-off this summer. Bring these two together and we get a big hit on temperatures. The smoke and the impact on temperatures drove weather models and forecasters a little crazy last week. Forecasts were calling for daytime highs around the 30 C mark, but in areas that had thick dense smoke, temperatures struggled to make it to around 20 C for highs. The most dramatic example I saw of this was in Edmonton, where on July 18, the overnight low fell to around 13 C, and thick smoke during the day kept temperatures to within one degree of the low. 

One interesting impact that smoke can also have is suppression of cloud development. Studies have shown that the development of fair-weather cumulus clouds (those little puffy clouds we often see develop during a summer afternoon), can be significantly reduced in smoky conditions. This in turn means we lose not only the cooling effect of these clouds but the potential for these clouds to continue growing into late-day thunderstorms. So now, under certain conditions, smoke can sometimes offset the cooling it causes but reducing cloud cover. If you haven’t discovered it yet, cli- mate and weather are not as easy to understand as you might think. You will know when you are finally getting a good grasp of a topic or area of study when you go from “I pretty much know and under- stand everything about this” to “I never thought of that, this is way more complicated than I thought.” 

Put it another way, learning goes from 1) I know nothing about this topic and 2) I know every- thing about this topic to 3) There is so much I don’t know about this topic. It is when you get to the third stage that learning has really begun. 

OK, enough of my teacher rant- ing, school is still a month or so away. As the dry weather looks to continue, forest fires will undoubtedly continue to burn, and we will have to deal with the impacts of these fires for the remainder of the summer. Be sure you keep in mind the impacts these fires and their associated smoke can have on our weather, remembering that it is not always clear-cut, and try not to get too upset with the weather forecasters if the forecast is really off due to smoke. 

For our next issue, it may be hard to believe, but July will have come and gone, so it is time to look back and then peer ahead to see if the much-hoped-for shift in our weather pattern is on the horizon.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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