Long after sunset or before sunrise on summer nights, mesospheric clouds can become visible against the darkened sky. Appearing in shades of pale blue and silver as they’re lit by a Sun that’s well over the horizon, these are known as noctilucent clouds, from the Latin for ‘night-shining’.

These are the highest clouds in our atmosphere. They are extreme-altitude formations that develop as high as 50 miles (85 km) up in the atmosphere. That means they’re near the top of the mesosphere, the region above the stratosphere, which is itself above the troposphere – the lowest 8 miles (10 km) or so where normal weather clouds reside.

Since the mesosphere is a particularly dry part of the atmosphere, the ice crystals that make up noctilucent clouds are few and far between. This means the clouds are far too subtle to be seen during the day. Only at night do they appear, when their extreme altitudes mean they catch the light while the atmosphere below is in shadow. Linda Holtby (Member 20,966) spotted these noctilucent clouds over Winchester, England at 3.00 am. That’s cloudspotting dedication right there.