This Cloud Is My Oyster.

Appearing in the eastern sky just before dawn amidst an undulating and opalescent layer of cirrus, this glowing patch of nacreous cloud persisted for just over an hour after sunrise.

It must've been a very cold night in the lower stratosphere to produce this iridescence: at least minus eighty degrees celcius at a height of between nine and sixteen miles high, where the water freezes to produce ice crystals so small that sunlight is diffracted through a layer at just the right thickness to scatter light in such a way to produce this vivid and beautifully colourful effect.