A fantastic display of nacreous (mother of pearl) clouds occured across NE England in February 2016. These photos were taken above Durham Cathedral and Castle at about 7:15am. These clouds are also known as Polar Stratospheric Clouds
Nacreous Clouds are quite rare. They can glow very brightly due to iridescence and are much higher than other tropospheric clouds, a height of 15-30km above the ground is typical. They are caused by wave-like motion of air, normally due to the proximity of mountain ranges. Best viewing is just before dawn and just after sunset.