Noctilucently Yours

June 18, 2021 5:22 PM

As it turns out, 2021 may be a good year for Noctilucent Clouds after all.

While early reports had them trapped above the Arctic Circle, here in Seattle we had a sighting yesterday morning.

As I only found out this afternoon gripe gripe mumble mumble.

I saw some about a decade ago while on a pre-dawn bike ride and -- after seeing the southern nighttime sky turn daylight blue from an evening A bomb test in Nevada when I was a wee tad in Idaho -- it was about the coolest dark sky event I ever witnessed. So, get thee out a half hour before sunrise or after sunset and keep looking east or west respectively.

Previously

posted by y2karl (13 comments total)

Who said it wouldn’t be? I’d like to fight that man
posted by subaruwrx at 6:27 PM on June 18, 2021 [2 favorites]

If you start looking only 30min before sunrise you'll miss them! The timestamp on that video in the link shows the best visibility was up to 2 hours before sunrise.

You can also look after sunset, in which case yes, 30min after sunset is an ok time to start looking -- with the expectation that the optimal viewing could be more like an hour or two after sunset, depending on your latitude.

Noctilucent clouds are best observed when the sun is 6° to 16° below the horizon, with optimal conditions at about 10° below the horizon. The times for these angles are highly dependent on your latitude and longitude, so I recommend using a planetarium app to see when the sun is at its optimum positions for any given location and date. On a computer Stellarium is great, cross-platform, and free. There are also lots of mobile apps that can provide this info.
posted by theory at 6:53 PM on June 18, 2021 [6 favorites]

I've looked at clouds from both sides now
From up and down and still somehow
It's cloud illusions I recall
I really don't know clouds at all
posted by kirkaracha at 7:52 PM on June 18, 2021 [7 favorites]

Noctilucent S-H-I-N-E

(not one of my best, I'll admit, but I had to purge the incessant Biggie/Futurama reference out of my system)
posted by Greg_Ace at 7:53 PM on June 18, 2021

If you start looking only 30min before sunrise you'll miss them!

Thank you so much for that. Post in haste, repent at leisure: I was composing on the phone and erased things twice by accident, got frustrated then barreled ahead. I hope I've learned my lesson.

The sad thing is I live in a tree filled neighborhood here. Not much horizon in view.
posted by y2karl at 8:27 PM on June 18, 2021 [2 favorites]

Related: previously—The Cloud Appreciation Society
posted by not_on_display at 9:46 PM on June 18, 2021 [2 favorites]

Noctilucent Clouds are complex:

As the mesosphere contains very little moisture, approximately one hundred millionth that of air from the Sahara,[13] and is extremely thin, the ice crystals can form only at temperatures below about −120 °C (−184 °F).[9] This means that noctilucent clouds form predominantly during summer when, counterintuitively, the mesosphere is coldest as a result of seasonally varying vertical winds, leading to cold summertime conditions in the upper mesosphere (upwelling and adiabatic cooling) and wintertime heating (downwelling and adiabatic heating). Therefore, they cannot be observed (even if they are present) inside the Polar circles because the Sun is never low enough under the horizon at this season at these latitudes.[14] Noctilucent clouds form mostly near the polar regions,[7] because the mesosphere is coldest there.[14] Clouds in the southern hemisphere are about 1 km (3,300 ft) higher than those in the northern hemisphere.[7]

Ultraviolet radiation from the Sun breaks water molecules apart, reducing the amount of water available to form noctilucent clouds. The radiation is known to vary cyclically with the solar cycle and satellites have been tracking the decrease in brightness of the clouds with the increase of ultraviolet radiation for the last two solar cycles. It has been found that changes in the clouds follow changes in the intensity of ultraviolet rays by about a year, but the reason for this long lag is not yet known.[15]

Noctilucent clouds are known to exhibit high radar reflectivity,[16] in a frequency range of 50 MHz to 1.3 GHz.[17] This behaviour is not well understood but a possible explanation is that the ice grains become coated with a thin metal film composed of sodium and iron, which makes the cloud far more reflective to radar,[16] although this explanation remains controversial.[18] Sodium and iron atoms are stripped from incoming micrometeors and settle into a layer just above the altitude of noctilucent clouds, and measurements have shown that these elements are severely depleted when the clouds are present. Other experiments have demonstrated that, at the extremely cold temperatures of a noctilucent cloud, sodium vapour can rapidly be deposited onto an ice surface.[19]

And seeing them in the lower 48 has a harbinger-like feel to it.
posted by jamjam at 11:48 PM on June 18, 2021 [4 favorites]

In the Southern Hemisphere are noctilucent clouds visible around December instead?
posted by Joe in Australia at 2:01 AM on June 19, 2021 [1 favorite]

Noctilucently Yours

This should have been a location in the Harry Potter books, Noctilucent Alley, to go along with Diagon Alley (diagonally) and Nocturn Alley (nocturnally.)
posted by Bee'sWing at 6:03 AM on June 19, 2021 [2 favorites]

Woah, that is beautiful. As a cloud lover (and member of the Cloud Appreciation Society and an owner of their Cloud Spotters Handbook) I would love to see that. Thanks for the post!
posted by bluesky43 at 8:33 AM on June 19, 2021

> In the Southern Hemisphere are noctilucent clouds visible around December instead?

According to https://www.antarctica.gov.au/about-antarctica/ice-and-atmosphere/atmosphere/clouds-and-radiation/noctilucent-clouds/ yes, Summer here too. But also less frequently; 55-65°S is the best range down here.

> Noctilucent clouds have been observed thousands of times in the northern hemisphere, but less than 100 observations have been reported from the southern hemisphere. This could be due to inter-hemispheric differences (temperature and/or water vapour) in the atmosphere at these altitudes. Or, the difference could be due to the lack of observers and poorer observing conditions in southern latitudes.

(Lack of observers seems a strong contender to me, at that latitude?)
posted by xurizaemon at 11:45 AM on June 19, 2021 [1 favorite]

Woah, that is beautiful. As a cloud lover (and member of the Cloud Appreciation Society and an owner of their Cloud Spotters Handbook) I would love to see that. Thanks for the post!
posted by bluesky43 5 ½ hours ago [+] [!]

Noctilucently eponysterical
posted by cynical pinnacle at 2:10 PM on June 19, 2021 [1 favorite]

Lack of observers seems a strong contender to me, at that latitude?

Seems very likely. Tens of millions of people live between 55° and 65°N, and there are some reasonably busy shipping routes. Almost no-one lives past the southern tip of Tierra del Fuego, at 55°S, and there's not been much shipping that way for about a century. Potential observers in the northern hemisphere probably outnumber those in the southern by thousands to one.
posted by howfar at 3:47 PM on June 19, 2021