"Calm winds prevent warmer air above the surface from mixing down to the ground, and clear skies increase the rate of cooling at the Earth's surface," the NWS continued. "Long nights allow for the cooling of the ground to continue over a longer period of time, resulting in a greater temperature decrease at the surface."
If moisture is trapped in this layer, it can form fog, as happened in this case. So far the inversion has taken place on two of the past three days in the Grand Canyon, which rarely happens, especially on days with blue, clear skies, the NPS wrote.
The Grand Canyon, which wends 277 miles (446 kilometers) along a sinuous path, became a national park in 1919; at that time some 44,173 visitors enjoyed the billions of years of history tucked into its colorful rocks. That's compared with the nearly 5 million visitors to the park today, according to NPS.
Article originally on LiveScience.
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