The electric blue color of Noctilucent clouds, or night shining clouds, are only visible in a deep twilight. From near the edge of space, the meteorological phenomenon shines when the Sun's rays hit them from below while the lower atmosphere is in darkness. They are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 75 to 85 kilometers (47 to 53 mi), and are not fully understood as meteorological phenomenon. Clouds generally are not able to reach such high altitudes with such thin air pressures.

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Noctilucent clouds were first described in 1885, two years after the massive eruption of Krakatoa, a volcanic island in Indonesia, sent up a plume of ash and debris up to 80 km into Earth's atmosphere. The eruption affected global climate and weather for years and may have produced the first noctilucent clouds. It was originally believed, with good reason, that the clouds were made up of volcanic dust, or maybe meteoric dust. However, it is known now that they are composed mainly of ice grains.

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Reflective Properties

Twenty-five years ago, researchers at Poker Flat, Alaska, discovered that the clouds were highly reflective to radar. This unusual property has long puzzled scientists. Bellan, reporting in the August issue of the Journal of Geophysical Research-Atmospheres, now has an explanation: the ice grains in noctilucent clouds are coated with a thin film of metal, made of sodium and iron. The metal film causes radar waves to reflect off ripples in the cloud in a manner analogous to how X-rays reflect from a crystal lattice.

Sodium and iron atoms collect in the upper atmosphere after being blasted off incoming micrometeors. These metal atoms settle into a thin layer of vapor that sits just above the altitude at which noctilucent clouds occur. Astronomers recently have been using the sodium layer to create laser-illuminated artificial guide stars for adaptive optics telescopes that remove the distorting affects of atmospheric turbulence to produce clearer celestial images.
- Science Daily

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Atmospheric Indicators

It is said that noctilucent clouds are indicators for atmospheric (mesospheric) changes and variability. Wilfried Schröder has demonstrated the relationship between mesospheric transition periods and frequency of noctilucent clouds in Northern Hemisphere. More recently, scientist have seen an increase in the number of these clouds:

"The incidence of noctilucent clouds seems to be increasing, perhaps because of global warming," says Paul M. Bellan, a professor of applied physics at Caltech.
- Science Daily

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Best Time and Place to see them

Noctilucent Clouds are most often observed during the summer months at latitudes from 50 to 70 degrees north and south--although in recent years they have been seen as far south as Utah and Colorado. Noctilucent clouds are a summertime phenomenon because, curiously, the atmosphere at 85 km altitude is coldest in summer, promoting the formation of the ice grains that make up the clouds.
- Science Daily

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Source: Science Daily, Wikipedia: Noctilucent clouds

Header image from Wikipedia