Temperatures below -85 Celsius are needed to produce these clouds of tiny ice crystals 15 - 25 km up in the stratosphere way above ordinary tropospheric clouds.
The crystals scatter, diffract, high altitude sunlight long after it is dark on the ground. The diffraction makes the clouds glow bright and colourful in the darkling sky.
Wigan, Lancashire.
The dark line is the contrail of an aircraft far below the clouds.
While they are brightest after sunset and before dawn, they can - as here - be visible in daytime.
Another daytime sighting.
Nacreous clouds are one type (II) of polar stratospheric clouds (PSCs). Nacreous are the most iridescent. Here there is evidence that they were embedded in other more virulent PSCs that show up faintly.
