Our national parks have fascinating weather, from the hottest spot in the United States to massive glacial snow to a veritable rain forest.

Thursday, as the National Park Service celebrates its 100th birthday, it’s an opportune time to reflect on the amazing weather of our parks and how it is changing.

Death Valley N.P. is the hottest, driest and lowest-elevation park in the system. Located in the Mojave Desert in California, the highest temperature ever recorded on Earth occurred here, a scorching 134 degrees (although some question if the reading is credible). The average high temperature in July is a stifling 115 degrees with an average nighttime low of 88.

Its lowest point is 279 feet below sea level, and annual rainfall is a meager 1.5 inches. There have been years when not a single drop fell.

It snows a ton in Glacier N.P. Straddling the Continental Divide in northwest Montana, moisture coming off the Pacific is turned into over 400 inches a year of snow. At the peak of winter, settled snowpack can pile up to 16 feet. “Early summer snow removal on Going-to-the-Sun Road, which crosses the Continental Divide, is so spectacular it has become a spectator sport,” the park service says.

The Great Smoky Mountains N.P. is part of the Appalachian Temperate Rainforest and is located half in North Carolina and half in Tennessee. Around 50 inches of rain falls per year in the lowest elevations but up on Clingmans Dome, it can receive in excess of 85 inches.

Because of the abundant rainfall, southern latitude and elevations ranging from 850 to 6,643 feet, Great Smoky Mountains N.P. has the highest biodiversity of any park in the system. There are over 100 species of trees, more than in all of Europe. They range from warmth-loving species at the base of the park to cold-hardy species found high up, like Red Spruce, that are more typically found growing farther north, in New England and Canada.

In its next 100 years, the national parks will face many environmental challenges but none bigger than global warming. By 2100, the glaciers in Glacier N.P. will have melted away completely; in fact they are expected to disappear in less than two decades.

A study published Wednesday by Climate Central calculated how much warmer certain parks will be by 2100, and the results are staggering. For example, Big Bend N.P. in Texas currently sees, on average, only 17 days over 100 degrees. By 2100, this is expected to increase almost 700 percent to 113 days.

Many of the Rocky Mountain parks that are cool refuges in the summer “could be as hot as the Plains,” the report said. “Parks in the Southeast, already a pretty hot place, will face even more extreme temperatures with a climate more like southern Texas,” including parks in our backyard, like Shenandoah N.P.

The report concluded: “There’s no denying that national parks will look a lot different by the end of the century, but that won’t make them any less a part of the fabric of American identity”.

In Ken Burns’s poetic documentary “America’s Best Idea,” he makes a strong case for the national park system being a uniquely American concept — the first national park in the world, Yellowstone, was founded in 1872, in a wild corner of the Wyoming and Montana territories.

In honor of the National Park Service centennial, admission will be free at all parks from Aug. 25 to 28. So get out this weekend and visit “America’s Best Idea.”