Context :

Recently, rare iridescent clouds, known as nacreous clouds, were observed in parts of Scotland, northern England, and the West Midlands.

About :

• These clouds, reminiscent of mother of pearl, are notable for their soft, pastel reflections during sunrise and sunset.
• This atmospheric phenomenon occurs when sunlight diffracts around minute ice crystals within the clouds, producing an effect akin to the play of colors seen on a thin layer of oil atop water. In the UK, nacreous clouds are infrequently seen and typically make an appearance when cold air from the polar vortex temporarily lingers over the region.

Mechanism:

The mechanism responsible for the iridescence in these clouds is diffraction, a phenomenon that arises when small water droplets or tiny ice crystals scatter sunlight. It's worth noting that larger ice crystals, while capable of causing halos, do not generate iridescence.