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AstroBob: Wildfire smoke smudges our starry skies

Bill Kauffman of Santa Rosa, Calif., took this smoky photo of the near-setting sun on Sept. 3. Credit: Bill Kauffman1 / 4
Encroaching smoke (from right) begins to cover a night that began perfectly clear earlier this month. The light at lower right is from moonlight. Bob King / Forum News Service2 / 4
The jet stream, which is like a “conveyor belt” for air traveling from west to east in North America, can bring wildfire smoke from its origins on the West Coast all the way to the east in a matter of days. Credit: NASA image courtesy Jeff Schmaltz, MODIS Rapid Response Team. Caption: NASA/Goddard, Lynn Jenner3 / 4
Just look at this stuff. The central U.S. is enveloped in a film of wildfire smoke in this photo taken by the GOES-8 satellite on Sept.13. The brighter, white textures are clouds. You can even see shadows, cast by higher clouds, on the haze layer below, especially at left. Credit: NOAA 4 / 4

Red suns? Anemic night skies? Whether you live on the East Coast or the West Coast or somewhere in between, you've been affected by the smoke from Canadian and western wildfires. Those of us in the Midwest, stuck in the middle, have had smoky skies since July with fewer than 10 days and nights in the clear. Forest fire smoke thousands of miles have hitchhiked for the ride west on the jet stream, which knows no borders. The amount can vary day to day and hour to hour. At worst, even the noon sun looks sapped of strength, a glaring orange ball in a white-blue sky, and the night sky lacks its customary starriness.

At times, you can even smell it in the air, though not always as much of the haze circulates several miles overhead. The smoke particles from the fires allow sunlight's longer wavelength colors like red and orange to get through while blocking the shorter wavelengths of yellow, blue and green. We're used to this right at sunset and sunrise, when the sun shines through the greatest thickness of the atmosphere. But this summer, we've seen extreme reddening with the sun a pallid, red ball with light so weak, it barely casts shadows.

If you haven't yet been touched by fire haze, the fireball sun and a general haziness in the air are your tip-offs. A 2017 Georgia Tech study showed the smoke from wildfires spew methanol, benzene, ozone and other noxious chemicals into the atmosphere. If the smoke stays in the jet stream and doesn't descend, the health risks are minimal, but if the winds bring the smoke down low, it can cause unhealthy air quality.

Since you and I are interested also in how the smoke affects the night sky, I can tell you, I've never seen a longer fire haze season than this one. Depending on how thick the smoke is where you live, I've seen nights when only stars of magnitude 3 or brighter are visible against a smoggy, gray — but "clear" — sky. Most nights, stars of 4th or even 5th magnitude are visible in the upper third of the sky, but as you lower your gaze, the smoke piles up. In the bottom third, the limit is closer to magnitude 3.

The moon, bright double stars and planets aren't affected as much as deep sky objects like star clusters, galaxies and nebulae, but I've gone out on a few fire-haze nights hoping to look for comets and simply had to pack it in. One evening, the first after weeks of smoke, began crystal clear, but within an hour, the haze oozed in from the west and blanketed the most of the sky within an hour.

Normally, in fall, temperatures drop, rain returns and the fire danger drops. Let's hope that happens soon. While it's no fun for skywatchers, we all know it's far worse for homeowners who live in the affected areas and those at risk for respiratory problems.

To keep track of active wildfires across the U.S., visit inciweb.nwcg.gov and NASA's Fire and Smoke site at https://go.nasa.gov/2wHUqGX

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