Clouds have substantial mass because they are composed of billions of tons of water droplets and ice crystals suspended in the air, though they appear weightless due to the vast volumes they occupy and the updrafts supporting them.
The claim that clouds have substantial mass despite appearing weightless is a matter of straightforward physical fact and common everyday observation in atmospheric science, well-represented by standard meteorological definitions of liquid water content rather than needing contested empirical studies from the provided list.