Atmospheric Metal Layers Appear With Surprising Regularity

Twice a day, at dusk and just before dawn, a faint layer of sodium and other metals begins sinking down through the atmosphere, about 90 miles above Boulder, Colorado.

Twice a day, at dusk and just before dawn, a faint layer of sodium and other metals begins sinking down through the atmosphere, about 90 miles above Boulder, Colorado. (Image: via Pixabay)