One of the great pleasures of late June and July, as the summer nights reach their zenith and when the skies never really get dark, is chasing those delightful summer nocturnal displays: the noctilucent clouds.
These whispy tendrils of silver, set against the deep blues of dusk, are a beautiful sight to behold. However, the last couple of years have only seen some very understated shows. So, it was with great delight that I saw in the start of July 2017, staying up until the wee small hours, enjoying the best show I've seen since 2014. It wasn't the highest I've seen, but it was bright and broad, and full of wonderful structure. These clouds are truly things of beauty to see.
But, for a science geek like me, the science behind these clouds is at least as beautiful. They form in the mesosphere, at the furthest edge of our atmosphere, around 80-100km high. Too high for weather balloons, too low for satellites, this is a strange and mysterious place. But our best theories suggest that the clouds form as follows.
Clouds need particles for the water vapour to condense around (condensation nuclei, as they're called in the business). Scientists believe that these tiny specs of dust that act as seeds for the clouds have been deposited there by meteors as they burn up on entry to our atmosphere. Bits of cosmic rock that have been travelling around the solar system for billions of years, burning out in one last glorious fiery display, have one final trick up their sleeves, so it would seem.
Then, in June and July, when the mesosphere is at its coolest, water condenses around these nuclei to create the ice crystal clouds (when the temperatures are warmer, the extremely low pressure there does not allow the ice crystals to form). The clouds that form create the most wonderful of patterns: tendrils here, herringbone ripples there - quite unlike any other cloud you will see in the sky.
But they are also incredibly thin and ephemeral. So translucent are they that they are invisible during the day, washed out by the overwhelming brightness of the sun (it is estimated that they only reflect 1000th of the light they receive). We can only see them at night, when the skies are dark. How then do they shine?
By the sun - shining from below them! The sun, long since set below the horizon, is now shining upwards from the other side of the world. And these clouds are so high in the sky that the sun lights them from below. It's as if the sun itself is so impressed by this last hurrah of ancient meteor dust that it just has to shine its spotlight on this wonderful display as if to say: "Here - look at this!"
Aesthetically compelling - scientifically delightful. Noctilucent clouds - what's not to love?!
For a much more detailed discussion of NLCs, their formation, history and structure, check out Martin McKenna's blog post on them here.