Tens of millions of Americans and Canadians have been greeted by gray skies, eerily fiery sunrises and sunsets, and a dingy veil of haze obscuring the sky overhead. The culprit? Dozens of raging wildfires burning across the West, emitting smoke some 40,000 feet into the atmosphere and signaling what may be a long, devastating fire season.

The smoke is also compromising air quality in areas near the blazes.

According to the National Interagency Fire Center, 67 large fires were burning across 12 states Tuesday. Seven have reached scales of 50,000 acres or larger, burning in areas from the Four Corners region and the Rockies to the Pacific Northwest. Scores of fires are also burning across Canada.

The National Oceanic and Atmospheric Administration wrote that “light density smoke” covered most of the western half of the United States late Monday, while “thick density smoke” blanketed Southern California, Nevada, Idaho, Montana, Wyoming, Utah, Colorado, Arizona, most of New Mexico and parts of West Texas. It added that much of central and south-central Canada was blanketed in “moderate to thick density smoke.”

As the fires burn and the smoke spreads, nearly 17 million people are enduring temperatures topping 100 degrees amid the conclusion of an exceptional heat wave that has toppled numerous records. Heat advisories and excessive-heat warnings continued to blanket the weather map Tuesday from interior California to the Canadian border in eastern Washington state.

Red-flag warnings, for high fire danger, are in effect in south-central Oregon in the vicinity of the Bootleg Fire, in southeastern Idaho and in large parts of Wyoming on Tuesday due to hot, dry and windy conditions, along with the possibility of new fire ignition from dry lightning.

As of Tuesday morning, the Bootleg Fire, the largest nationwide, had consumed more than 200,000 acres north of the Sprague River. The fire rapidly expanded over the weekend, burning in the Fremont-Winema National Forest. The “extremely active” fire, which was first ignited last Tuesday, is being tackled by more than 1,000 personnel.

On Monday night, a sudden surge in plume height shortly before sunset lofted smoke to nearly eight miles high.

A number of recreation sites, like the Head of the River Campground and Sycan Ford Picnic Area, are closed to the public. Outside the immediate fire zone, however, copious smoke isn’t just affecting visibility and sky conditions — it’s taking a health toll, too.

The Oregon Department of Environmental Quality issued an air quality advisory for south-central and northeastern Oregon through Friday, warning that smoke “can irritate the eyes and lungs and worsen some medical conditions.”

Air quality alerts have also been hoisted within the Columbia River Basin, where numerous blazes are active, including parts of northern Idaho, adjacent western Montana and sections of eastern Washington and Oregon.

In northwestern Montana, an unwelcome situation greets summer visitors at Glacier National Park. According to its Facebook page, smoke from wildfires burning in Idaho is blanketing much of the park, and the air quality reached “unhealthy” levels over the weekend. “You should expect to encounter smoke on a visit to Glacier this summer, as much of the west is experiencing record-breaking heat and historic drought conditions,” the Facebook update said.

Even Denver is under an air quality alert, but that’s for ozone, which can pool in the area amid hot, stagnant air — not the fine particulate matter spewed by wildfires to the west. Still, weather forecasters in the Mile High City are dealing with smoke impacts. The Weather Service in Boulder, Colo., is expecting the shade cast by the smoke to trim a degree or two off afternoon high temperatures. Denver will probably climb into the lower 90s Tuesday.

Satellite imagery late Monday revealed the massive expanse of smoke occupying considerable real estate and becoming entangled in the jet stream. It covers most areas west of the Rockies away from the immediate coastline, swirling east as it is dragged clockwise around high pressure over Southern California. Thunderstorms can be seen in Arizona and New Mexico, courtesy of an active monsoon pattern that even brought dust storms to the desert near Tucson.

The smoke is wafting toward the northern Great Lakes and Canada on the jet stream, a strip of it yanked into eastern Manitoba and Ontario, Canada, as far north as Hudson Bay. A razor-thin back edge to the smoke can be seen there, probably the result of a sharp cold front encroaching.

Wildfires produce carbon monoxide, carbon dioxide and particulate matter, the latter a fine aerosol that brings adverse health effects; this is primarily a concern close to the source of a fire, where smoke is nearer to the ground.

In areas farther away from an active blaze, the smoke is lifted high enough as to not affect health, but it has a noticeable effect on the sky. It dims the deep azure that ordinarily soars overhead, replacing it with a milky-white color. Around dawn or dusk, the skies stand gray. Sunrises or sunsets may appear a vibrant crimson or orange.

Wildfire smoke will thin some in Denver, but is likely to continue to drift over the High Plains on Tuesday. Meanwhile, some could be visible in northernmost New England later this week.

The wildfires, which have already produced fire-induced thunderstorms and fire tornadoes, are made more severe by the exceptionally dry conditions desiccating the West — conditions that have been worsened by human-caused climate change. Continued hot and anomalously dry weather is likely there for much of the foreseeable future.

That will favor a more severe fire season.

Fires are intensified by the effects of climate change and contribute to climate change themselves as their carbon dioxide emissions increase the atmospheric concentration of this heat-trapping gas.

July is relatively early to see wildfire activity of this magnitude. It’s more common in the fall, when the lingering summertime heat combines with periodic bursts of offshore winds that brew extreme fire danger. Nearly 34,000 fires have erupted so far in 2021 in the United States, the most since 2011.

Jason Samenow contributed to this report.