Watch out for noctilucent clouds.  This summer they’re the liveliest in years

Watch out for noctilucent clouds. This summer they’re the liveliest in years

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Night clouds over Varbla, Estonia

Now that your local fireworks are over, don’t despair! You may still be in for a visual treat if you look up after sunset for the rest of the month, especially THIS month.

Over the past two weeks, the northern United States, along with Canada and Europe, have been treated to the brightest display in nocturnal (illuminated at night) clouds for at least the past 15 years.

This phenomenon can continue throughout July and even into August, and it is not limited to the northern United States, having been Point as far south as Los Angeles. You don’t need a perfectly dark sky to catch these clouds by any means, as they can easily be seen over cities. These exposures have grown in intensity and global reach in recent years, and it’s not entirely clear why, although climate change can be a factor.

Here’s a time-lapse from a few weeks ago in the UK to give you an idea of ​​what it might look like:

And one Photo of Edmonds, Washington taken on July 1:

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Photographer Sherman Page took this photo showing noctilucent clouds over Edmonds, WA at 3:40 a.m. on Friday, July 1

Noctilescent clouds form at the very upper edge of Earth’s atmosphere, about 50 miles high, mostly over the “border” with outer space. This is much, much higher than conventional “high altitude” clouds like cirrus clouds, which stay in the troposphere up to only 40,000 feet. So unlike conventional clouds, the noctilescent variety can reflect sunlight back to us two or more hours after sunset (or within the hour or two before sunrise), even though the stars are clearly visible.

Here is a photo (not a diagram, a photograph!) taken from the International Space Station which clearly shows the phenomenon:

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Conventional clouds appear in the troposphere, but noctilucemt clouds form much higher up in the mesosphere, where temperatures can drop to -170°F

Here’s a prettier one without all the tags. There are a few wisps up there just before it all goes black:

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Noctilescent clouds are made up of ice crystals that nucleate (start) on small pieces of meteor dust floating in the very cold mesosphere. Crystals need a little bit of something to form to get a few molecules to adhere and line up to start the process, especially in the very thin mesosphere, but once that happens they can really tear it up. To illustrate this, below, a droplet of nucleant is squeezed through a capillary into a highly concentrated saline solution, and off you go racing!

The colder it is in the mesosphere, the easier it is for ice crystals to form, and it turns out rather counterintuitive that it is the coldest up there during the summer, whether you are in the northern or southern hemisphere. Here’s the typical temperature up there as it goes through the year, and you can see we’re in the coldest part right now:

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Seasonal changes in Northern Hemisphere mesospheric temperature around 60°N inferred from the hydroxyl component of nightglow. We are in this trough right now!

But why have these clouds become more prevalent recently? More water in the mesosphere? More nucleation materials? It is tempting to say that the recent volcanic eruption in Tonga (one of the more powerful never observed) could have been spitting dust up there and that’s part of the cause, but it’s far from clear. Or maybe it is more rocket launches over the past few years who have placed extra water up there? Mmm, maybe.

Either way, keep looking north an hour or two after sunset, and maybe you’ll get some this show if you persevere.

“Residents of the northern United States and Canada should definitely be on the lookout for night clouds over the long weekend,” Cora Randall, a professor at the University of Colorado at Boulder, said in an e -mail. “We are near the peak of the night cloud season, and even absent extraordinary events, they may appear over the northern continental United States”

[…]

“This season has been pretty extraordinary the last few days,” said Randall, who is also the principal investigator of the Cloud Imaging and Particle Size instrument on NASA’s Aeronomy of Ice in the Mesosphere (AIM) mission, designed to study the nocturnal brilliance. clouds. “The season started out as a pretty average season, but over the last week or so the cloud frequencies have increased significantly.”

She said the frequency of noctilucent clouds over the past few days has been higher than ever seen in at least 15 years of observations by the AIM mission.

And hey, while we’re talking about spectacular summer cloud formations, check out this treat from the Pittsburgh area from mid-June:

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These are big clouds, which are formed by pockets of cold air descending from clouds, often in the vicinity of thunderstorms. You can see a bunch of more amazing photos of these here on EarthSky.org. Here’s a particularly stunning one, even though it wasn’t taken at sunset:

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Mammatus clouds over Lewiston, Idaho in November 2020.

Anyway, I can see from my porch here that we have cirrus clouds high in the night sky over Boston tonight, but the north side of my yard is obscured by trees. Although I love having these trees, I’m going to have to stop at some of the same places I saw comet NEOWISE a few years ago, like the hill near the college my son just graduated from, with a view of the northern horizon, to see if I can catch any noctilucent clouds this month.

I look at the big sky now….

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