I remember reading in the Almanac years ago about a phenomenon involving clouds — that they sometimes appear to have wispy threads hanging down, suggesting rain, but there is no rain. The threads don’t extend all the way down, and they appear to converge as they descend. I noticed it tonight in dozens of clouds just at sunset. What causes this, and what is it called?
The phenomenon is known as virga, a mysterious rain that never reaches the ground. (A feature on the subject appeared in The 1989 Old Farmer’s Almanac.) Specifically, water or ice particles fall from a cloud, usually in wisps or streaks, but evaporate before reaching the ground. This occurs because the raindrops are too small to sustain themselves all the way down. They are moved back up into the cloud by the air around them, and the process begins again. Scientists estimate that a raindrop with a diameter of 0.004 inch or less will not make it to the ground. Even if the raindrop is larger, the air beneath a precipitation-bearing cloud is often warmer and drier than the cloud, causing the moisture to evaporate on its way through. The drier the air beneath the mother cloud, the more likely the chances for virga. The name comes from the Latin, meaning a branch, twig, or stick — descriptive of virga’s appearance.