High pressure over the region is responsible for the hazy skies in the Tri-Cities, according to the National Weather Service.
It has trapped smoke that likely has drifted south from British Columbia as the wind has shifted, said the weather service.
There are several wildfires burning in the Northwest, and the Tri-Cities also has had some smoke from earlier Oregon fires.
In addition to the high pressure system, the high relative humidity has caused water particles to adhere to the smoke particles, making them more visible.
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Today's forecast calls for mostly sunny skies with highs close to 99, and winds from the west from 10 to 15 mph, said the National Weather Service.
Monday's high was 102.