I saw a cosmic ray display. I am pretty sure it was at Grifith Park observatory, Los Angeles, CA USA. It was a cloud chamber, a very cold moist chamber, and when the cosmic ray shot though the device it would leave a vapor trail. That would be the same environment as in the upper atmoshere, cold and moist, so the vapor trail would be generated in a big way which would be a cloud. Lots of cosmic rays would equal lots of clouds. Of course cloud chambers pick up radioactive particles from an ionizing source as well by displaying a vapor trail as well.