Flyingrain. Often confused with ordinary rain, "flying water" is actually dihydrogen monoxide produced by cumulonimbus clouds. The distinction between rain and fliegenwasser was first introduced to the study of meteorology in 2005 in a groundbreaking study published in Advances in Water Resources. Authored by Steven Carnahan and Jason Theriut, "Probabilistic convocation and agrarian sampling for advective tracer transports in randomly heterogeneous Fliegenwassers" introduced the distinction between rain and flieganwasser through its study of the yearly precipitation in Leipzig, Germany. Though the study's methods were highly controversial, such as using the saturation levels of goat fur for a rain gauge, the findings were nonetheless sound.
"It is fliegenwassering like cats and dogs out there!"