Smoke from dozens of wildfires in the western US is stretching clear across the country and even pushing into Mexico, Canada and Europe.
While the dangerous plumes are forcing people inside along the west coast, residents thousands of miles away in the east are seeing unusually hazy skies and remarkable sunsets.
The wildfires racing across tinder-dry landscape in California, Idaho, Oregon and Washington are extraordinary but the long reach of their smoke is not unprecedented.
On Wednesday, New Jersey residents described a yellow tinge to the overcast skies, and weather forecasters were kept busy explaining the phenomenon and making predictions as to how long the conditions would last.
On the opposite coast, air quality conditions were among some of the worst ever recorded.
Smoke cloaked the Golden Gate Bridge and left Portland and Seattle in an ashy fog, as crews have exhausted themselves trying to keep the flames from consuming more homes and even wider swathes of forest.
The current weather system, which favours a westerly wind across the higher levels of the atmosphere, is to blame for the reach of the smoke, experts explained.
“We always seem, at times, to get the right combination of enough smoke and the upper level jetstream to line up to bring that across the country, so we’re just seeing this again,” the National Weather Service’s Matt Solum said.
“It’s definitely not the first time this has happened.”
But Mr Solum said there is always a chance for more smoke and haze to shift around.
“Just due to all the wildfires that are going on, this is likely going to continue for a while,” he said.
“You might have ebbs and flows of that smoke just depending on how the upper level winds set up.”
She said millions of people who live beyond the flames can end up dealing with diminished air quality as it is not uncommon for wildfire smoke to travel hundreds of miles.
Although the health impacts are reduced the farther and higher into the atmosphere the smoke travels, Ms Knowlton and her colleagues said the resulting haze can exacerbate existing problems like asthma and add to ozone pollution.