Direttamente dagli States:

IRIDESCENT THUNDERHEAD: "On June 18th, more than 100 people attending the
Heart of America Star Party near Butler, Missouri, watched a beautiful display of iridescent clouds,
" reports photographer Tom J. Martinez. Iridescent clouds are common enough,
but these were strange because "they were riding on top of a thunderhead."

"This is an iridescent pileus cloud," explains atmospheric optics expert Les Cowley.
"On sunny afternoons, cumulus clouds boil upwards, pushing layers of moist air above
them even higher where they cool and condense to form cloud caps or 'pileus' (Latin for cap).
Pileus clouds formed very quickly have their water droplets all the same size,
the perfect condition for iridescent colors."

"The colors came and went as the cloud changed shape," notes Martinez.