At higher latitudes, on some midsummer nights, long after the sun has dipped below the horizon, a ghostly silver-blue lacework appears low over the northern horizon. These ethereal wisps are noctilucent clouds — the highest clouds on Earth – and among the most visually striking phenomena in the night sky. Their name comes from the Latin for “night-shining,” and they appear exactly as that: illuminated clouds light shimmering in the dark where no light should be.
Noctilucent clouds (NLCs) form in the Earth’s mesosphere, the atmospheric layer that sits roughly 76 to 85 kilometres above the planet’s surface. By comparison, the weather-bearing clouds we see every day form in the troposphere, no higher than about 12 kilometres up. Noctilucent clouds exist in a layer of the atmosphere so thin and cold it’s more akin to outer space than the relatively thick air we breathe at the Earth’s surface. When they appear, well after sunset, NLCs appear as delicate, luminous tendrils — often rippled, banded, or arranged in intricate wave-like patterns that appear to fluoresce.
The formation of noctilucent clouds requires a peculiar combination of conditions. At mesospheric altitudes, temperatures can plunge to around −130°C, cold enough to freeze tiny water vapour molecules onto microscopic dust particles. Some of that dust originates from meteoric debris — the fine residue left behind as meteoroids burn up entering the atmosphere. The resulting ice crystals are extraordinarily small, just tens of nanometres across, so much like air molecules they scatter blue light preferentially with remarkable efficiency, an effect called Rayleigh scattering.
That sunlight is key to the whole spectacle. The mesosphere at those heights is too high to be shielded from the sun by the Earth even when it has set for observers on the ground. The sun continues to illuminate the cloud layer from below the horizon, causing it to glow against the darkened lower atmosphere — which is why the clouds are only visible at twilight or during the night. This effect also happens only at higher latitudes, roughly from 50° and 65° north or south, and near midsummer when the sun never gets too far below the horizon. In the northern hemisphere, the viewing window runs roughly from late May through early August, with peak activity in June and July. The equivalent window in the southern hemisphere falls between November and February. Places like Scotland, Scandinavia, Canada, and northern Russia in the north, and Patagonia and the southern tip of New Zealand in the south. To see them, look to the north in the northern hemisphere (or south in the southern) in the hour or two after sunset or before sunrise, when the sky is dark but the upper atmosphere still catches the sun’s rays.
In recent decades, scientists have noted that noctilucent clouds appear to be forming more frequently and at lower latitudes than in the past. Many researchers attribute this to rising levels of methane in the atmosphere, which when broken down in the upper atmosphere produces additional water vapour, providing more raw material for crystal formation. Increased rocket traffic might also be a contributing factor. And climate change, which warms average temperatures near the surface,
So, if you’re out enjoying the long summer nights, don’t forget to look towards the horizon for noctilucent clouds. They don’t appear every night. But with a clear horizon and a little patience, you may see them emerge near midnight, and they are a beautiful sight to behold.
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