Watch out for noctilucent clouds. This summer they are as lively as they have been in years

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Noctilucent clouds over Varbla, Estonia

Now that your local fireworks show is over, don’t despair! If you look up after sunset for the rest of the month, you’re still in for a visual treat, especially THIS month.

In recent weeks, the northern US has been treated to the brightest display of , along with Canada and Europe noctilucent (glowing at night) clouds in at least the last 15 years.

This phenomenon can last throughout July and even into August and is not limited to the northern United States sighted as far south as Los Angeles. You definitely don’t need a pristine dark sky to capture these clouds as they are easy to see over cities. These displays have increased in intensity and global reach in recent years, and it’s not entirely clear why, though 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 this may look like:

And a photo from Edmonds, Washington, recorded on July 1st:

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

Noctilucent clouds form at the very top of Earth’s atmosphere, about 50 miles high, essentially on the “boundary” to space. This is much, much higher than traditional “high altitude” clouds, such as cirrus clouds, which reside only up to in the troposphere 40,000 feet. So, unlike traditional clouds, the noctilucent variety can reflect bright sunlight down to us two or more hours after sunset (or an hour or two before sunrise), even when stars are clearly visible.

Here is a photo (not a diagram, a Photo!) taken by the International Space Station clearly showing the phenomenon:

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

Here’s a nicer one without all the labels. There are some will-o’-the-wisps up there just before everything goes black:

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Noctilucent clouds are made up of ice crystals that nucleate (get started) on tiny bits of meteorite dust floating around in the very cold mesosphere. To form, crystals need a small piece of something that a few molecules can stick to and align to start the process, especially in the very thin mesosphere, but once that event happens they can really rip it apart. To illustrate, below, a droplet of nuclear agent is forced through a capillary into a very concentrated saline solution, and we’re off to the race!

The colder it is up in the mesosphere, the easier it is for ice crystals to form, and it turns out to be rather counterintuitive coldest up there in summer, whether in the northern or southern hemisphere. Here’s the typical temperature up there as it evolves throughout 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, derived from the hydroxyl component of the nightglow. We’re at this low point right now!

But why have these clouds become more common lately? More water in the mesosphere? More nucleating agents? It is tempting to say that the recent eruption of the Tonga volcano (one of the strongest ever observed) might have spewed dust up there and that’s part of the cause, but that’s far from clear. Or maybe yes more rocket launches in recent years that have placed extra water up there? Mmm, could be.

Regardless, look further north an hour or two after sunset and you might be treated this show if you keep at it

“People in the northern US and Canada should definitely keep an eye out for noctilucent clouds over the long weekend,” Cora Randall, a professor at the University of Colorado Boulder, said in an email. “We are nearing the peak of the noctilucent cloud season, and barring exceptional events, they may occur over the northern mainland United States.”

[…]

“This season has been pretty extraordinary over the past few 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, which was designed to study noctilucent clouds. “The season started out as a fairly average season, but around the last week or so cloud counts have increased dramatically.”

She said the frequency of noctilucent clouds in recent days has been higher than at any time in at least 15 years of AIM mission observations.

And hey, while we’re on the subject of spectacular cloud formations in summer, check out this tidbit from the Pittsburgh area from mid-June:

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These are Mammatus Clouds, formed by pockets of cold air descending from clouds, often near thunderstorms. You can see a number of other amazing photos of it here at EarthSky.org. Here’s a particularly stunning image, although it wasn’t taken at sunset:

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Mammatus clouds over Lewiston, Idaho in November 2020.

Anyway, from my porch here, I can see we’re going to have some high cirrus clouds in the night sky over Boston tonight, but the north side of my yard is obscured by trees. While I love having these trees, I have to stop by some of the same places I’ve seen Comet NEOWISE a few years ago, like the hilltop near the middle school my son just graduated from, looking towards the northern horizon to see if I can catch some noctilucent clouds this month.

I’m looking up at the big sky now…

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