BBC Sky at Night - UK (2021-08)

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octilucent clouds (NLCs) are the

highest clouds in the atmosphere and

we can see them around astronomical

twilight in a summer’s night sky. The

name comes from Latin, meaning

‘night-shining clouds’, and they occur

when the Sun is below the horizon and the clouds

are still in sunlight. Although no one as yet fully

understands why, this appears to be a relatively new

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frequency in recent years.

First sightings

While these clouds can be a spectacular sight,

curiously no one appears to have recorded observing

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volcano Krakatoa erupted, destroying land, creating

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after-effects of the eruption were felt for years and

included vivid red sunsets, which were seen across

the world as the ash from the volcano drifted and

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this volcano to look more closely at atmospheric

phenomena in the night sky, a number of observers

began to notice these curious, shining clouds and

EXPLAINER

The fundamentals of astronomy for beginners

Noctilucent clouds: an ongoing discovery

Emily Winterburn investigates the history of NLC observations

naturally assumed these, like the deep red sunsets,

must be linked to the volcanic eruption.

Thomas Backhouse, an astronomer and

meteorologist based in Sunderland, was interested

in cloud formation. He had studied lots of different

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he considered to be a new type of cloud which he

noted was rather unusual in only being visible after

sunset. He kept watching, even after the effects

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At around the same time over at Armagh

Observatory in Ireland, the observatory’s director,

Thomas Romney Robinson, was possibly observing

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he did note the appearance of “luminous clouds”

in his meteorological record books over several

decades. These observations were not often in the

summer, however, which suggests that they might

relate to some other phenomenon.

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widely supposed that the clouds were a result of the

volcanic ash from Krakatoa, but the clouds persisted

even after the volcanic dust settled and the link was

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continued to observe and study these new clouds.

At the Berlin Observatory in Germany, Otto

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þ Early photos

of noctilucent

clouds, as taken

by astronomer

Otto Jesse in

the late 1880s

72 BBC Sky at Night Magazine August 2021

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A noctilucent cloud display can be one of

summer’s most stunning spectacles