This week’s weather article is coming to you from one of my favourite places to camp, Riding Mountain National Park. As I write this the skies are nice and blue, with a yellow sun rising up through the trees. That’s right, blue skies and a yellow sun! The biggest weather story over the last couple of weeks, besides the heat wave earlier this month, has been the smoke from British Columbia’s forest fires.
With the general atmospheric circulation across our region being from west to east, it was inevitable that the smoke from all those fires in B.C. would eventually make it into our region. Just be thankful that we are not having to live with it every day like in southwestern Alberta and most of southern and central B.C. With the fires still raging, don’t be surprised if we see more of the smoke move back into our region over the next several weeks. The smoke from these western fires didn’t just stop here in Manitoba. Satellite images showed fairly thick smoke making its way to the Atlantic Ocean, with smoky, hazy skies making it all the way to the United Kingdom.
All this smoke does have an impact on the weather. For our region, the smoke resulted in cooler daytime highs as the smoke particles reflected solar radiation back to the atmosphere, preventing it from reaching Earth’s surface. This also impacted rainfall potential, as the lower surface heating prevented strong updrafts from forming, which hindered the development of thundershowers. A number of people I talked to about the smoke thought it would act much like cloud cover does, cooling temperatures down during the day (which it did) and keeping it warmer at night. However, clouds are made up of water droplets, which are very good at reflecting longwave radiation (heat). At night, Earth radiates longwave radiation back toward space, which makes us cool down. If there are clouds, some of this longwave radiation is reflected back down, which keeps us warmer. Smoke, stating it simply, is not made up of water droplets, and therefore, does not do a good job of reflecting back longwave radiation. So, while the smoke reflects incoming shortwave radiation during the day, which helps to keep us cooler, it is not as effective as water at reflecting longwave radiation (heat) back at night. Just think of how warm it might have been over the last couple of weeks if there wasn’t any smoke around!
What other effects can smoke have on weather and climate? At first glance it seems like this is a fairly simple and straightforward weather question. Like most weather questions, they start off simple, but quickly become complex the deeper you dig into them. The reasons are twofold; the specific impacts smoke can have depend not only on the initial atmospheric conditions, but also on feedback loops. For example, smoke during the day reflects sunlight back into space, resulting in an overall cooling effect. Smoke particles landing on a snow- or ice-covered surface will darken that surface, resulting in more solar radiation being absorbed which, in turn, depending on the overall atmospheric conditions, may result in warmer temperatures. What will result is more of the snow and ice melting, which will then lead to more of the surface becoming darker, which means more energy being absorbed, which leads to more heating and that takes us back to more melting. We call this a positive feedback loop.
Another example we saw here in Manitoba, is that the smoke led to a suppression of the development of showers and thundershowers. On several days over the last couple of weeks, conditions were just favourable enough for showers or thundershowers to develop in the afternoon. Basically, if we were able to heat up to a certain temperature, updrafts would have been strong enough to overcome the warm upper atmosphere in place across our region. However, the smoke prevented strong surface heating, which then didn’t allow for the updrafts to break through the warm air, thus preventing the development of thundershowers — or weakening the ones that did develop.
Now, under different atmospheric conditions, all the smoke particles in the air could act as additional condensation nuclei and, in theory, could result in an increase in rainfall. For rain or precipitation to form, water vapour has to condense from the gaseous state to a liquid (or solid). To do this you need some kind of tiny solid material for the water to condense onto — condensation nuclei. The atmosphere typically has lots of these nuclei, but if you add a whole bunch of extra ones (smoke particles), then the whole process of water vapour turning into water droplets and then into rainfall can be greatly increased, resulting in either higher rainfall rates and/or greater amounts of rainfall.
We could go on — heck, we didn’t even begin to look at smoke’s impacts on health — but it seems we’ve already run out of room for this week.