Noctilucent clouds: what they are and how to see them

The science behind the summer astronomical spectacle of noctilucent clouds and how to see them.

A view of noctilucent clouds captured over Tallinn, Estonia. Credit: Arsty/iStock/Getty images

Noctilucent clouds – literally ‘night-shining clouds’ in Latin – can be seen in the northern hemisphere in the summer months, roughly from the end of May to the beginning of August.

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They occur about 82km above sea level in Earth’s upper mesosphere, which means they form about 10 times higher than the clouds in our lower atmosphere.

Noctilucent clouds only occur under certain conditions when the warm summer air in the atmosphere expands upwards, allowing the mesosphere to cool down.

Water molecules accrete onto dust particles in the mesosphere, and the colder temperatures form tiny ice crystals.

When the Sun drops below the horizon during the summer evenings and these ice crystals are illuminated from below, they appear as noctilucent clouds.

The science of NLCs. Credit: Steve Marsh / BBC Sky at Night Magazine
The science of NLCs. Credit: Steve Marsh / BBC Sky at Night Magazine

The effect is an almost ethereal phenomenon of silvery blue clouds that appear as though they are glowing.

Because they are so high up, the Sun illuminates them from below the horizon, either after it has set or before it rises.

As a result, these shining clouds are best seen 90-120 minutes after sunset low above the northwest horizon, or about 90-120 minutes before sunrise low above the northeast horizon.

Observing them is relatively easy too, and you don’t need a telescope or any specialist equipment to do so, although a pair of binoculars would not be a bad idea if you want to get a closer look.

The main thing is make sure the skies are relatively clear and free of summer haziness.

You will also need a clear view of the horizon, so go somewhere where you won’t have trees or buildings getting in the way of spotting them.

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As with all aspects of astronomy, practice is key, and you will eventually get better recognising noctilucent clouds whenever they appear.