Smoky Skies: September 13-15, 2020

solar radiation chart
Chart of Solar Radiation for September 1-15 at the U of M St. Paul Campus
Courtesy: Dave Ruschy

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 drifted in on the moderately strong winds aloft, and unlike other events in 2015 and 2018, this time remained at higher altitudes, without much effect on visibility or air quality near the ground. The visibility at the Twin Cities International Airport was ten miles for the whole period.

The more noticeable effect of the smoke was its apparent "bleaching" of the sky, often giving the impression that it was cloudy, when in fact skies were predominantly cloud-free most of the time. The veil of smoke rendered the sun a pale disk at times, with some spectacular orange and red hues closer to sunrise and sunset.

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.

Modified September 17, 2020

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