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?
'A sea of luminous silvery white'
Noctilucent clouds were discovered relatively recently. The first recorded sighting was made in 1885 by British man Robert Leslie, who wrote to the journal Nature, saying "a sea of luminous silvery white cloud lay above a belt of ordinary clear twilight sky."
He compared their oddly bright glow to phosphorescent paint, and said "though they must have received their light from the sun, it was not easy to think so, as upon the dark sky they looked brighter and paler than clouds under a full moon."
Krakatoa discovery
Noctilucent clouds owe their initial sightings to a volcanic eruption at the other side of the world - the 1883 Krakatoa eruption.
The volcano erupted with the force of a thousand nuclear bombs, firing a dust plume 50 miles (80.5km) into the sky. This led to years of unusual atmospheric activity, including fantastically vibrant sunsets and making themoon look blue. Noctilucent clouds almost certainly did exist before 1883, though their unique nature was not realised.
Tiny seeds of ice
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.
Noctilucent clouds (blue) observed in front of the moon, taken from the International Space Station
Noctilucent clouds were observed and studied throughout the 20th Century, and have been sighted with increasing frequency in the 21st.
So why do they appear to be getting more common? Tom McEwan, who has run the Noctilucent Cloud Observers' homepage since 1995, says "Sightings are certainly more frequent but it remains to be seen if this is due to stronger noctilucent cloud activity or is the result of increased awareness of the phenomenon amongst the general public and amateur astronomers."
Ordinary observers can now record the clouds with ease using digital cameras.
"Even basic, compact digital cameras are capable of capturing the fine detail of noctilucent cloud," McEwan explains. "There is now a network of such cameras, operated automatically by computer software, spread across the northern hemisphere (the UK station is located in Port Glasgow) which have been returning very interesting results."
AIM to understand
The greatest insights into noctilucent clouds, however, have come from space itself.
Noctilucent clouds imaged by NASA's AIM satellite
In 2007, NASA launched the Aeronomy of Ice in the Mesosphere (AIM) satellite for the specific purpose of gathering more data on noctilucent clouds. The satellite was equipped with cameras and detectors to observe the fine details within the clouds.
Using the Solar Occultation For Ice Experiment (SOFIE) onboard the satellite, Hervig's team saw the ice crystals each contained a small bit of dust.
This dust is not volcanic, however - it is extraterrestrial.
In 2012, Mark Hervig's team confirmed that the dust comes from the thousands of tiny meteors that burn up in the atmosphere every day, leaving vaporised trails as they burn up.
The particles are the same size as those that Krakatoa would have thrown into the upper atmosphere in 1883.
The dust provides the vital surface for water to form tiny ice crystals.
These crystals are a hundred times smaller than those which would form in lower clouds. Their tiny size explains the cloud's blue colour, as the wavelength for blue light is most effectively scattered by crystals at this scale.
"Previous instruments were not sensitive enough to detect the optically thin ice layers which AIM has no trouble measuring," says Hervig.
"SOFIE was the first satellite instrument to measure meteoric smoke in the stratosphere and mesosphere, and provided the first continuous smoke measurements over periods of years," explains Hervig.
This meant they were able to see patterns emerge and also found that the clouds exist in the Arctic Circle nearly all year round.
Cloudy future
Despite the power of the AIM mission and the stacks of data it has generated, noctilucent clouds still harbour mysteries and contradictions between predicted and observed behaviour.
"Past experiments have shown a reduction in noctilucent clouds during periods of increased solar activity, due to increased heating and destruction of water vapour," says Hervig.
"However, during the current solar maximum, AIM measurements show increasing noctilucent cloud activity."
Clouds caused by the launch of Space Shuttle Atlantis in 2007
The team is now researching whether this is due to increasing greenhouse gases.
"Increasing methane leads to more water vapour in the mesosphere, and increasing carbon dioxide is thought to cool the mesosphere," says Hervig. "Both of these effects lead to more noctilucent clouds."
The team also found that the Space Shuttle's exhaust was enhancing noctilucent clouds. "With the Shuttle program over we are very interested to observe what changes occur and understand the effects of the next generation of launch vehicles," says Mark Hervig.
Noctilucent clouds can regularly be seen in the UK. For a detailed guide on how to spot them for yourself, visit the BBC's guide on how to spot noctilucent clouds
