Too faint to be seen in daylight, they are visible only when the observer and the lower layers of the atmosphere are in Earth's shadow, but while these very high clouds are still in sunlight.

[7][8] Data from the Aeronomy of Ice in the Mesosphere satellite suggests that noctilucent clouds require water vapour, dust, and very cold temperatures to form.

[10] The exhaust from Space Shuttles, in use between 1981 and 2011, which was almost entirely water vapour after the detachment of the Solid Rocket Booster at a height of about 46 km (151,000 ft), was found to generate minuscule individual clouds.

[10] As the mesosphere contains very little moisture, approximately one hundred millionth that of air from the Sahara,[14] and is extremely thin, the ice crystals can form only at temperatures below about −120 °C (−184 °F).

Other experiments have demonstrated that, at the extremely low temperatures of a noctilucent cloud, sodium vapour can rapidly be deposited onto an ice surface.

Studies have shown that noctilucent clouds are not caused solely by volcanic activity, although dust and water vapour could be injected into the upper atmosphere by eruptions and contribute to their formation.

He had been doing detailed observations of the unusual sunsets caused by the Krakatoa eruption the previous year and firmly believed that, if the clouds had been visible then, he would undoubtedly have noticed them.

[28] The first physical confirmation that water ice is indeed the primary component of noctilucent clouds came from the HALOE instrument on the Upper Atmosphere Research Satellite in 2001.

[29] In 2001, the Swedish Odin satellite performed spectral analyses on the clouds, and produced daily global maps that revealed large patterns in their distribution.

[35][36] The United States Naval Research Laboratory (NRL) and the United States Department of Defense Space Test Program (STP) conducted the Charged Aerosol Release Experiment (CARE) on September 19, 2009, using exhaust particles from a Black Brant XII suborbital sounding rocket launched from NASA's Wallops Flight Facility to create an artificial noctilucent cloud.

Since the naturally-occurring clouds only appear in summer, this experiment was conducted in mid-winter to assure that its results would not be mixed with a natural event.

[41] Type III billows are arrangements of closely spaced, roughly parallel short streaks that mostly resemble cirrus.

In the early 1970s, visible airglow photometers first scanned the atmospheric horizon throughout the summer polar mesospause region.

The very bright scattering layer was seen in full daylight conditions, and was identified as the poleward extension of noctilucent clouds.

[46] On 8 July 2018, NASA launched a giant balloon from Esrange, Sweden which traveled through the stratosphere across the Arctic to Western Nunavut, Canada in five days.

The giant balloon was loaded with cameras, which captured six million high-resolution images filling up 120 terabytes of data storage, aiming to study the PMCs which are affected by the atmospheric gravity waves, resulted from air being pushed up by mountain ranges all the way up to the mesosphere.

[49] Noctilucent clouds are generally colourless or pale blue,[50] although occasionally other colours including red and green have been observed.

[52] They can appear as featureless bands,[50] but frequently show distinctive patterns such as streaks, wave-like undulations, and whirls.

[56] They seldom occur at lower latitudes (although there have been sightings as far south as Paris, Utah, Italy, Turkey and Spain),[50][57][58][59] and closer to the poles it does not get dark enough for the clouds to become visible.

Also, some noctilucent clouds are made of smaller crystals, 30 nm or less, which are invisible to observers on the ground because they do not scatter enough light.