Every now and then, I read a list article on the internet describing weird and wonderful cloud formations occurring in some part of the world. (Here’s one by the BBC.) For a long time I’ve been a little envious because I’ve never had a chance to witness these beautiful phenomena.
Thankfully, all that changed during this year’s summer solstice.
While watching a movie at home in the evening, I turned towards my window around 23:30 and saw something intriguing. Living this far north of the equator, in the summer the sun sets very late, and so the sky is still blue around that time. However, there were clouds in the sky that evening and they were glowing.
I knew immediately that these weren’t ordinary clouds, and soon after recalled from one of those list articles about clouds that the glow I was seeing came from sunlight reflecting off the ice crystals making up the clouds. This got me very excited. I paused the film and started making photos of the clouds, at the same time googling for more information about what I was looking at (read this and this to start with).
My goodness, these clouds were even more mesmerising after I read about how they form.
How (and when) noctilucent clouds form
Known as noctilucent clouds, or ‘night shining clouds’, these form only in the warm months of the year and in locations that are far from the equator. (In other words, I would’ve never seen them while growing up in Singapore, which lies only a degree north of the equator.) They can only be seen in moments after sunset or before sunrise – when the sun is still below the horizon. Being the highest clouds that we have, the weather also has to be clear for them to be visible.
Furthermore, noctilucent clouds can only form in very cold temperatures – below 120 degrees Celsius. Somewhat paradoxically, these temperatures are achieved only during the summer when the upper part of the atmosphere, where the clouds form, is at its coldest.
Frigid temperatures alone aren’t enough to guarantee the formation of night shining clouds. Water vapour and dust are needed, and the source of the dust particles is an interesting one. Meteors are believed to be one source of dust needed for the water vapour to condense on. That said, they’re not the sole source of dust particles, and these can also come from volcanoes or even a rocket launch.
A recollection of what I learnt in chemistry class
Going back to the counterintuitive idea that temperatures in the uppermost atmosphere – or mesosphere – are coldest in the warmest months of the year, a principle I learnt while studying chemistry in middle school helped me understand how this happens.
I had a chemistry teacher who was, shall we say, somewhat eccentric. For example he taught us to conduct titration in a way that would, these days, likely result in parents writing angry letters to the education ministry. (His position was that pipette fillers were unnecessary when we had mouths and young, healthy lungs that could do what those rubber things did. You can probably guess what happened during my two years of chemistry lab classes.) That aside, though, he was brilliant at teaching and made (inorganic) chemistry fun and interesting for me. I credit him with giving my interest in science a huge boost.
Anyway, my chemistry teacher taught us once that air cools when it expands.
This is a concept that most of us will be familiar with. As an example, if I were to take a can full of air freshener and release all the pressurised gas within, the can would soon turn cold from the escaping gas, which expands and cools as it leaves the can. While writing this I’ve also just recalled that this is the same principle by which we fractionally distil air, which is a fancy name for separating air into its components for various uses (e.g. liquid nitrogen). But I digress.
Apparently this is what happens in the atmosphere during the summer, as this article nicely explains:
During summer, air close to the ground gets heated and rises. Since atmospheric pressure decreases with altitude, the rising air expands. When the air expands, it also cools down. This, along with other processes in the upper atmosphere, drives the air even higher causing it to cool even more. As a result, temperatures in the mesosphere can plunge to as low as –210 degrees Fahrenheit (–134 degrees Celsius).EarthSky.org
There’s even a name for this process: adiabatic cooling. So now you can use this term and sound highly
pretentious learned during dinner conversations.
Coolest summer solstice ever
I would’ve missed seeing this rare type of cloud had I not turned my head and faced the window. But I did look out the window, and I was able to both learn something new and appreciate something very beautiful that evening.
Luckily for me, noctilucent clouds will continue being visible since it’s only June. If you live at least 45 degrees north or south of the equator and it’s a clear evening, try looking in the direction of the sun some moments after sunset. (Or if you’re an early bird, look in the direction of the sun in the moments before sunrise.) You might be in for a treat!