July 2024

Noctilucent Clouds – Shining the Night Away

We cloudspotters describe formations like Cirrus, Cirrostratus, and Cirrocumulus as ‘high clouds’. But terms can be deceiving, as these are high clouds only relative to the rest of the weather clouds – the ten main types that form in our troposphere, the lower eight miles (13 km) or so of the atmosphere. In fact, the highest-cloud award goes to the noctilucent cloud. This forms much, much further up – so high that it’s on the fringes of Space and can only be spotted at night, once the Sun has dipped beyond the curvature of the Earth.

Spotted here by Ken Kennedy over Broughty Ferry, Dundee, Scotland, noctilucent clouds form way above the lowly troposphere – up in the mesosphere. Their altitudes are at around 50 miles (85 km), which is why we refer to them as extreme-altitude clouds. They are known more formally as polar mesospheric clouds, and they can be seen on summer night skies, during nautical twilight, when the Sun is between 6° and 16° below the horizon. Typically, you need to be at a latitude roughly between 50° and 65° – whether in the Northern or Southern Hemisphere – because at latitudes any higher, the summer sky is too light, and at lower latitudes, the Sun dips too low beyond the horizon to light them.

Sharp-eyed cloudspotters will have noticed that noctilucent clouds can take on patterns found in the tropospheric clouds, such as ripples that we’d refer to as undulatus and which can be seen in Ken’s spotting. Honeycomb textures also appear, known in the lower clouds as lacunosus, as well as streaks akin to the fibratus species of Cirrus. The distinctive blueish appearance of noctilucent clouds is a result of the longer, red-looking, wavelengths of sunlight being absorbed by the ozone of the higher atmosphere, leaving more of the shorter, blue-looking, wavelengths.

The mesosphere is very dry – up to a million times drier than the Sahara Desert – and so for the ice crystals of this extreme-altitude cloud to form, temperatures need to be cold. Very cold. But the mesosphere is also the coldest part of our atmosphere, with temperatures often reaching as low as –125°C (–190°F). Counterintuitively, the mesosphere is at its coldest during summer months, when the troposphere below is warmest. This is why noctilucent clouds tend to form from late spring and into summer, when temperatures reach the requisite –120°C (–185°F) needed for the region’s scant moisture to freeze around tiny dust particles.

These dust particles are likely meteor dust – from the burning up of meteors as they enter the Earth’s atmosphere. The majority of the water vapour likely originates from a chemical reaction of methane that has been pushed up from the troposphere way below via gravity waves. As this methane travels up through the stratosphere and into the mesosphere, it goes through chemical transformations that break it down into water.

To a lesser degree, the tiny particles and water vapour can reach the mesosphere from other sources, such as particularly strong volcanic eruptions. The first recorded sighting of noctilucent clouds was in 1885, two years after the infamous 1883 Krakatoa eruption. Climate change is also thought to play a role in the increase in reported sightings of noctilucent clouds. Warming in the troposphere would affect the mesosphere temperatures, and higher methane emissions would also likely lead to more water vapour in the mesosphere.

July is peak noctilucent cloudspotting season in the Northern Hemisphere. Cloudspotters in higher latitudes should keep an eye out during the hours before sunrise or after sunset when the lower sky is clear to be lucky enough to spot these ghostly formations shine the night away.

Noctilucent clouds spotted over Broughty Ferry, Dundee, Scotland by Ken Kennedy. View this in the gallery.

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