Serengeti Cap Cloud ~ Iridescent cap cloud or pileus imaged in Tanzania by Marc Herpers just before a night of thunderstorms.

"[The cap cloud] caught my eye as it was there for only a few minutes before the sun set completely, showing this beautiful green"
©Marc Herpers, shown with permission.

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The warm air of thunder clouds boils upwards.  It pushes layers of moist air above it to even higher altitudes. As that air rises, it expands and cools.   If it cools enough its moisture condenses out to form a pileus or 'cap' cloud.

Pileus often shines with colour because its droplets are all formed at the same time. They thus have identical histories and similar sizes.   Furthermore the cloud is thin.   These are ideal conditions for bright iridescence - diffraction by individual droplets.