Nacreous clouds

These rare clouds, sometimes called mother-of-pearl clouds, are 15 - 25km (9 -16 miles) high in the stratosphere and well above tropospheric clouds.

Nacreous clouds shine brightly in high altitude sunlight up to two hours after ground level sunset or before dawn। Their unbelievably bright iridescent colours and slow movement relative to any lower clouds make them an unmistakable and unforgettable sight।

http://www.collthings.co.uk/2008/06/10-very-rare-clouds.html