DENVER — Acrid yellow smoke clogs the skies of major Western U.S. cities, a human-caused fire in the Columbia River Gorge rains ash on Portland, Oregon, and a century-old backcountry chalet burns to the ground in Montana's Glacier National Park.

Wildfires are chewing across dried-out Western forests and grassland, putting 2017 on track to be among the worst fire seasons in a decade.

A snowy winter across much of the West raised hopes that 2017 wouldn't be a dried-out, fire-prone year, but a hot, dry summer spoiled that.

Here's what happened, and how bad things are:

HOW DID WE GET HERE?

Heavy snows last winter brought relief from a long, brutal drought across much of the West and produced a lush growth of natural grasses — thicker and taller than many vegetation experts had ever seen. But the weather turned very hot very fast in the spring, and the snow melted much faster than expected.

All the grass that grew high dried out, and so did forests at higher elevations, leaving plenty of fuel for wildfires, said Bryan Henry, a manager at the National Interagency Fire Center, which coordinates wildfire-fighting.

Summer lightning storms then dumped less rain than usual and weather conditions kept the humidity low, creating a natural tinderbox in many states.

"It was kind of a bad combination of things," Henry said.

HOW BIG ARE THE FIRES?

By Thursday, more than 76 large fires were burning in nine Western states — including 21 in Montana and 18 in Oregon, according to the interagency fire center.

So far this year, wildfires have burned more than 12,500 square miles (32,000 square kilometers) nationwide. In the past decade, only two years were worse at this point in the wildfire season: 2015 and 2012.

For all of 2015, a record 15,800 square miles (41,000 square kilometers) burned. In 2012, 14,600 square miles (38,000 square kilometers) were scorched.

WHAT ABOUT CLIMATE CHANGE?

It's making things worse for fires, said Jonathan Overpeck, dean of the School for Environment and Sustainability at the University of Michigan.

Hotter and drier weather is a symptom of human-caused climate change, and that's making fires worse by leaving forests and other vegetation more flammable.

"It's not of course playing the only role," he said. "There's natural variability at work."

"Humans are contributing to an ever-increasing degree to wildfires in the West as they emit greenhouse gases and warm the planet and warm the West," Overpeck said.

WHO'S FIGHTING THE FIRES?

More than 26,000 people are fighting the fires, backed by more than 200 helicopters, 1,800 trucks and 28 air tankers dropping water and fire-retardant slurry. Three of those tankers are military C-130 planes.

The military has also assigned surveillance aircraft and at least 200 active-duty soldiers to fight fires and the National Guard has been called out in at least four states — California Montana, Oregon and Washington.

"We're stretched thin," said Jennifer Jones, a spokeswoman for the interagency fire center.

Sometimes the center gets requests for more crews and equipment than it has, so "fire managers on the ground are adjusting their tactics and strategies to accommodate the resources they can get," Jones said.

"We don't pack up our tents and go home."