Is it true some clouds are 'seeded' by aliens?
Yes...sort of. The clouds in question are the fairly rare 'noctilucent' clouds...(meaning 'night-shining'). Those are found on the edge of our atmosphere...about 48 miles above the earth. That's the same altitude where meteors leave 'dust'...which attract scarce water vapor...needed to form the ice crystals that make up noctilucent clouds. That meteor dust works as 'seeds' for these clouds. Interestingly, scientists are finding that noctilucent clouds are showing up more often, and farther south than was historically the case. Scientists have found that's due to increased levels of methane in the upper reaches of the atmosphere...a greenhouse gas.