13, BBC Scotland)

Noctilucent clouds are the world's highest types of cloud. Although uncommon, they can regularly be seen from the UK in the summer, with the annual peak in activity about 20 days after midsummer.

The rarefied clouds appear 50 miles (89.5km) above the Earth in the mesosphere, right on the edge of space, and glow with a white-blue light.

Fluffy cumulus clouds tend to sit at just one mile from the ground. The highest common cloud, the thin and wispy cirrus, still only reach around eight miles (12.8km) high.

So what makes noctilucent clouds so different to ordinary clouds, and why do they form? [...]

Noctilucent comes from Latin for "night shining," and these ethereal clouds can only be seen in summer after the sun has set.

"This is because the sun still illuminates the noctilucent clouds while the observer is in darkness, thus providing the necessary contrast to see these optically thin phenomena," explains Mark Hervig, research scientist at GATS inc, an atmospheric science research company.

His work has helped to explain many of the mysteries of the formation and composition of noctilucent clouds.

"Noctilucent clouds consist of tiny ice particles," says Hervig, which only form below -120C. This happens in summer as the upper atmosphere is at its coldest when hot air from below rapidly cools when it expands upwards, allowing noctilucent clouds to form.