John Metcalfe is CityLab’s Bay Area bureau chief, based in Oakland. His coverage focuses on climate change and the science of cities.
These immense sky divots are the result of human activity.
This weekend, the people of La Crosse, Wisconsin, were treated to an awesome sight: great, gaping holes in the cloud cover, some as perfectly round as a god's frisbee and others glowing with a spectral iridescence.
Those who ran outside for the Rapture were in for disappointment. These humongous holes are a lesser known, though well-documented meteorological phenomenon known as fallstreak or "hole-punch" clouds. They owe their gut-shot appearance to humans: When an airplane pierces a cloud containing supercooled vapor, it can cause a chain reaction of water crystallizing into ice and drifting downward. (As neat as it would be to see vast, circular snowfalls on the ground, the crystals typically disperse and sublimate before they hit land.)
The weather agency has assembled a wonderful photo array of the recent sky divots. You can see them all here, and below are some of the grander holes that drifted over the region: