A guest blog post today, from John Pahl:
If you were to ask me what my favourite cloud would be, I wouldn’t hesitate: it has to be the noctilucent cloud.
I remember the first time I saw them. It was during a night watch sailing a yacht down the east coast of Greenland, approaching Tasiilaq. I was cold and tired but the night’s sky made up for it, with satellites, the aurora and noctilucent clouds:
What are noctilucent clouds (NLC)?
The word “noctilucent” means “night shining” and is a clue as to what makes them special. Most clouds are dark at night but noctilucent clouds appear to glow a brilliant white or electric blue, for they are so high up they can reflect the light of the sun over the horizon, as in this figure:
Noctilucent clouds form in the mesosphere, between 76 and 85 km high, right at the borders of space, high enough for observers in darkness to see them lit up by the sun. They are a bit of a mystery, as 80 km is too low for satellites but too high for aircraft to reach, and there is no record of their observation prior to 1885.
They are thought to comprise tiny ice crystals seeded by dust, which could come from micrometeors, though volcanoes might contribute. The formation is very sensitive to conditions and so only form when the upper atmosphere is cold enough, and at latitudes from 50° to 65° north or south. To be seen, the sun must be just below the horizon, as in the figure above, so in the northern hemisphere they tend to be visible in summer months such as June to August.
NLCs are unpredictable, but appear on radar, so you can look on social media such as Twitter for updates as to whether they are likely or not. They can also be spotted by satellites, and NASA’s Aeronomy of Ice in the Mesosphere (AIM) spacecraft has already detected them this year, as described here.
NLCs have also been spotted on Mars by the Mars Express mission. Rocket launches also can seed the mesosphere leading to NLCs at lower latitudes, such as after a SpaceX Falcon 9 launch in Florida.
To see them yourself, you need to be in the right latitudes for noctilucent clouds, wait until the sky is truly dark, and then look at a low elevation angle in the direction where the sun would be. So in the northern hemisphere look between north-west and north-east, several hours after sunset and before sunrise.
I’ve typically seen them in the early hours, something like 2 am or 3 am, and they can be seen as far south as London, here low on the northern horizon:
There can be stunning details, such as the vortex seen here, but also streaks, waves and whirls. These features can move rapidly, again with the mechanism not entirely understood.
To be able to capture these details in a photograph, it is best not to have too long an exposure – a bit like when taking photos of the aurora. Indeed, the motion of NLCs is a bit similar to that of the aurora, with fluttering, waving and swaying.
As the sky lightens towards dawn they fade into nothing – unlike a normal cloud which will brighten.
Well worth looking out for during June and the following summer months!
John Pahl is the author of the YA SF novel “Martian Blood”, first in the Noctilucents trilogy.