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What caused the recent reddish color to the moon?

Dear Tom,
The moon appeared reddish on the evening of Sept. 4, 2017. Is there an explanation?
— William Kessler, Cassandra Dale, George Cutright

Dear William, Cassandra and George,
The reddish color of the moon that evening was the result of smoke in the atmosphere from fires in southern Canada and the Pacific Northwest of the United States. Wildfires burning across much of that huge area were causing smoke that mid- and upper-level winds were carrying across much of the Midwest, and elsewhere. The area of the wildfires also extended south to California and Arizona.
The western U.S. and southwestern Canada have been experiencing a drought in recent weeks (longer than that in some areas), and in the last several days temperatures have risen far above normal (8 to 20 degrees in many places) in combination with periods of high winds and very low humidity.
The reddish color of the moon Sept. 4 was the result of smoke in the atmosphere from fires in southern Canada and the Pacific Northwest of the United States.