Friday photo

Clouds, Grand Canyon National Park. February 1989.

The Grand Canyon is not a glacial artifact, but if it were, it might have looked like this once upon a time. Maybe not so fluffy, but there would have been ice and snow piled thousands of feet deep high. But there has never been a glacier in the Grand Canyon, either in the deep past or when I lived there. What we're looking at here is a low bank of clouds.

This particular event was caused by a temperature inversion, when the air near the surface - normally warmer than the air above it - cools down until it's colder than the layer above. When that happens, the higher and warmer air acts like a lid, preventing the surface air from rising and remixing as it normally would. Here, that warm layer lay below the rim of the canyon, leaving the clouds nowhere to go. From above, you could easily imagine that the clouds filled the canyon all the way from the bottom, that wasn't the case: it only took a short hike down the trail to come out into clear air again.


Temperature inversions in populated areas can pose a serious health hazard, as they trap air pollutants near the ground. They frequently occur in the winter, when the long nights allow greater cooling, and in the mid-20th Century, when every home ran its own coal furnace, the effects could be horrific. In 1952, an inversion settled over London for five days, trapping their smog at ground level. When it finally lifted, as many as four thousand people had died, most of them probably with preexisting respiratory problems.