Updraft® - Minnesota Weather News

Wildfire smoke aloft over Minnesota cut solar radiation by 20 percent this month

Sunny day solar radiation fell to mid-October levels on some sunny days in September.

Thick elevated smoke layer above the Twin Cities Tuesday
Thick elevated smoke layer bleaches the sky above the Twin Cities on September 15, 2020.
Paul Huttner/MPR News

Here’s a little tidbit that caught my eye today.

The Minnesota DNR Climate Working Group finds our smoky September skies have cut inbound solar radiation up to 20 percent on the smokiest days above Minnesota this month.

The thick smoke aloft actually cut a few degrees off our forecast highs temperatures in the middle of September.

Smoke from the wildfires over California, Oregon and Washington spread over Minnesota and was thickest from the September 13th to 15th, keeping high temperatures a few to several degrees below forecast values on the 15th.

The smoke reduced the sun’s rays by about 20 percent at peak. Solar monitors measure solar radiation units called langleys.

Solar Radiation for September 1-15 at the U of M St. Paul Campus
Chart of Solar Radiation for September 1-15 at the U of M St. Paul Campus Courtesy: Dave Ruschy
Minnesota DNR Climate Working Group

The smoke did cut down a bit on solar radiation, affecting daily high temperatures as well. On a perfectly sunny day in mid-September, a location in southern Minnesota would have peak hourly average solar radiation values around 1.1 langleys per minute. Despite the fair skies, however, values peaked between .87 and .95 langleys per minute at the St. Paul climate observatory (see graph included with this story).

Similarly, during peak daylight on a clear day in mid-September, Minnesota receives incoming solar radiation measuring between 690 watts per square-meter in the far north, to 770 in the far south. On September 15th, however, the DNR's pyranometers (instruments that measure incoming solar radiation) across the saw values depressed by 100-240 watts per square-meter during the early afternoon despite few if any clouds. 

To put these sunlight shortfalls in perspective, the smoke suppressed the sunlight intensity to levels we'd expect during mid-October.

Large wildfires in the western U.S. have more than tripled since 1970. Climate change is the single biggest factor in that increase.