Look up in the sky: It's a bird, it's a plane, it's a … volutus, or is that an asperitas? If you're not sure what kind of cloud you're seeing above your head, consult the World Meteorological Organization's International Cloud Atlas, newly updated just in time for World Meteorological Day on Thursday. The Weather Channel reports that a bunch of new classifications have been added to the atlas (the first fully web-based version) for the first time in three decades, including the volutus "roll cloud" and aforementioned asperitas, as well as human-created cloud types like contrails (the condensation trails left in the wake of airplanes). There's even a new "accessory" cloud known as the flumen (aka "beaver's tail"), which sometimes follows along with a convective storm, a WMO press release notes.
The science behind the atlas's cloud classifications, which Live Science calls "not for the faint of heart," is a sophisticated one. The 10 cornerstone clouds, referred to as "genera," are the ones you've likely heard of—stratus and cumulus, among others. Then the classification gets more granular with "species," which break down the genera clouds further based on shapes, and from there to "varieties," which indicate transparency and arrangement. The recent additions fall under the species category; there's also a new "supplementary features" section of "unusual parts"—such as the aforementioned asperitas, a wavy look to a cloud's underside—and five "special" clouds, including the contrail and clouds formed by waterfall mist. "Few natural phenomena" are as inspiring as clouds, WMO Secretary-General Petteri Taalas notes. (Shifting cloud patterns are worrisome.)