Now that your local fireworks show is over, don’t despair! You may still be in for a visual treat if you look upwards after sunset over the remainder of the month, especially THIS month.
Over the last couple of weeks, the northern US, along with Canada and Europe, has been treated to the brightest display of noctilucent (aglow at night) clouds in at least the past 15 years.
This phenomenon can continue throughout July and even into August, and it’s not restricted to the northern US, having been spotted as far south as Los Angeles. You don’t need pristinely dark skies to catch these clouds by any means, as they can easily be seen over cities. These displays have been building in intensity and global reach over the past several years, and it is not entirely clear why, though climate change may be a factor.
Here is a time-lapse from just a couple weeks ago in the UK to give you an idea of what this can look like:
Et un Photo from Edmonds, Washington taken July 1:
Noctilucent clouds form at the very upper edge of the Earth’s atmosphere, about 50 miles up, basically on the “border” with outer space. That’s much, much higher than conventional “high-altitude” clouds like cirrus clouds, which stay in the troposphere at up to only 40,000 feet. So unlike conventional clouds, the noctilucent variety can reflect bright sunlight down to us two or more hours after sunset (or in the hour or two before sunrise), even as stars are clearly visible.
Here is a photograph (not a diagram, a photograph!) taken from the International Space Station that shows the phenomenon clearly:
Here’s a prettier one without all the labels. There are a few wisps up there just before everything goes to black:
Noctilucent clouds are made up of ice crystals that nucleate (get started) on little pieces of meteor dust floating around in the very cold mesosphere. In order to form, crystals need a little piece of something for a few molecules to stick to and line up to get the process started, especially in the very thin mesosphere, but once that event occurs, they can really rip it. To illustrate this, below a droplet of nucleant is squeezed through a capillary into a very concentrated salt solution, and we’re off to the races!
The colder it is up in the mesosphere, the easier it is for ice crystals to form, and as it turns out, rather counterintuitively, it’s coldest up there during summer, no matter whether you’re in the Northern or Southern Hemisphere. Here’s the typical temperature up there as it unfolds over the year, and you can see we’re in the coldest part right now:
But why have these clouds become more prevalent recently? More water in the mesosphere? More nucleating materials? It’s tempting to say the recent Tonga volcanic eruption (one of the most powerful ever observed) could have spewed dust up there and that’s part of the cause, but that’s far from clear. Or maybe it’s more rocket launches in the past few years that have placed extra water up there? Mmm, could be.
Regardless, keep looking northward an hour or two after sunset, and maybe you’ll be treated to this show if you keep at it.
“Folks in the northern US and Canada should absolutely be on the lookout for noctilucent clouds over the long weekend,” Cora Randall, a professor at the University of Colorado Boulder, said in an email. “We are near the peak of the noctilucent cloud season, and even in the absence of extraordinary events, they can appear over the northern continental US”
“This season has been quite extraordinary in recent days,” said Randall, who is also the principal investigator for the Cloud Imaging and Particle Size instrument on NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission that was designed to study the night-shining clouds. “The season began as a rather average season, but in about the last week the cloud frequencies have increased dramatically.”
She said the frequency of noctilucent clouds in the past few days has been higher than ever observed in at least 15 years of observations by the AIM mission.
And hey, while we’re on the topic of spectacular summer cloud formations, check out this tidbit from the Pittsburgh area from mid-June:
These are mammatus clouds, which form by pockets of cold air sinking from clouds, often in the vicinity of thunderstorms. You can see a bunch more amazing photos of these here at EarthSky.org. Here’s a particularly stunning one, even though it wasn’t taken around sunset:
Anyway, I see from my porch here that we do have some high cirrus in the night sky over Boston tonight, but the north side of my yard is obscured by trees. Though I do love to have those trees, I’m going to need to stop by some of the same places I saw comet NEOWISE a couple of years ago, like the hill near the middle school my son just graduated from, with a view to the northern horizon, to see if I can catch some noctilucent clouds this month.
I’m looking at the big sky now….