iridescent cloud (photo by Jan Freeman)
Iridescent clouds commonly appear at 5-15° from the sun. They are extremely bright, showing
nacreous colours in irregular structures. This happens frequently in lenticularis clouds; dark
sunglasses are necessary to aid observation. As with the diffraction corona, colours are
formed by diffraction of sunlight by tiny particles of the clouds, the variety depending on the
particle size and the angular distance from the sun. At 5-15° the colour is extremely sensitive
to the particle size, so that small variations result in another colour being produced.
Polarization, however, is absent in these clouds.
