Earth & Sky

Cloud Holes

DB: This is Earth and Sky. Bob Teague in Nashville, Tennessee wrote, “I observed two perfectly round, dark blue ‘holes in the sky’ directly overhead one morning . . . “

JB: Bob said he’d never seen this atmospheric phenomenon before. He wants to know what it was and what caused it. Bob, we passed your question along to a few scientists – and they think that what you probably saw were holes in high altitude clouds. These clouds can form a bluish-white veil of ice crystals and water droplets covering much of the sky. When you see this cloud veil, rain is likely – as was the case on the rainy day last March, Bob, when you saw the cloud “holes.”

DB: These “holes” in clouds are somewhat mysterious. One possibility is that sometimes, a stream of ice crystals can fall into a cloud. These falling ice crystals cause some of the water droplets in the cloud to freeze and fall through the cloud. The freezing spreads uniformly from where the stream of ice crystals punched its way through the cloud – and that’s what creates the cloud hole.

JB: Another way that cloud holes form is from the exhaust of jet aircraft, which seeds the water droplets in the cloud and helps them freeze and fall out. By the way, we have pictures of cloud holes at earthsky.org. Thanks today to the National Oceanic and Atmospheric Administration and to the National Fish and Wildlife Foundation. We’re Block and Byrd for Earth and Sky.