IN DEPTH: How wildfire smoke changes temperatures throughout the atmosphere

Posted on 16 September 2020 at 9:34pm

COLUMBIA – California, Oregon, Washington, and Colorado are the main states that have seen extremely large wildfires in 2020. These fires began in July and August and quickly started to break records. By the first half of September alone, California had burned more than 2 million acres and bested 2018 for the record most acres burned in a fire season. The thing is, fire season doesn’t typically peak until October and November. If fire conditions persist this fall, the outcome will be unimaginable.

These wildfires have, of course, pushed tons and tons of smoke into the sky. This smoke rises high in the troposphere, the weather-producing layer of our atmosphere, and then gets caught up in the upper-level jet stream.

The jet stream is what pushes most of our weather systems along and it also has a big say in our temperature trends. For example, when Rapid City, South Dakota went from 102-degrees on September 5 to measurable snowfall on September 7 breaking the record for the quickest time for that to happen, the jet stream was taking a huge dip into the southern U.S. allowing for cold temperatures to spill in from the north.

So, wildfires occur, smoke fills the air and reaches high enough to get into the jet stream and is abundant enough for the jet stream to move a lot of it. This smoke can be carried thousands of miles, and it has been. This week not only has Missouri been seeing the smoke, but so has the likes of the east coast such as Washington D.C.

If you are wondering how this plays a role in our forecasts and temperatures, you’ve come to the right place. Let me explain.

On a clear sunny day, solar radiation reaches the Earth’s surface and heats it. Some of the heat is absorbed and some is reflected into the atmosphere. This makes the surface air warm and the rest of the atmosphere cooler. That is good because it allows the heat to rise and that mixes the air and keeps everything flowing overhead.

Think of a grill when you can see the heat coming off it. It rises and mixes the air molecules, creating a wind and moving the air. For our atmosphere, that’s a good thing because without that process nothing would move much.

When smoke is introduced into our air it blocks some of the sunlight from reaching the surface. When solar radiation interacts with a layer of smoke: some radiation will get through and reach the Earth’s surface, some will be reflected, and some will be absorbed by the smoke. When this radiation is absorbed it warms the atmosphere around it.

This brings a dilemma for the atmosphere because now the surface air is cooler than the air above it. When the higher air is warmer than the lower air it is called a thermal inversion.

A thermal inversion often leads to stagnant air because nothing is mixing; there is very limited motion in the atmosphere. This leads to smoke and pollution being trapped overhead with nowhere to go. It will take a strong system like a large swing of the jet stream, a tropical cyclone, or heavy rains to kick things into gear and move the smoke out.

While daytime temperatures can be cooler if the smoke is thick enough to block a significant amount of solar radiation, at night everything is back to “normal”.

Smoke does not act like a cloud at night. When clouds are overhead at night, they will insulate the lower atmosphere and keep warmer surface temps from escaping. Smoke will simply let any thermal radiation from the surface escape trough it. Therefore, nighttime temperatures are generally not affected by smoke while daytime temperatures can be.

Due to climate change and a lack of efficient forest fuel management creating increasingly favorable conditions for the spread of wildfires this is a situation we will be seeing more and more in the coming years.