Rough estimate: There are 1.4 × 10^17 liters of water in atmosphere (https://whyfiles.org/2010/how-much-water-is-in-the-atmosphere/index.html https://whyfiles.o

Rough estimate: There are 1.4 × 10^17 liters of water in atmosphere (https://whyfiles.org/2010/how-much-water-is-in-the-atmosphere/index.html https://whyfiles.org/2010/how-much-water-is-in-the-atmospher...). Your number "about a half a billion metric tons" is 0.5 × 10^12 liters of water, which is 5 orders of magnitude lower number. Or, less than 1/10 000 of atmospheric water. To say in your words, "If I was quoting this number as a significant contribution, I would be embarassed."