BAN #375: RSS feed back up, Föhn wall clouds

15 November 2021   Issue #375

November 15, 2021

[The planetary nebula M 2-9, winds from a dying star. Credit: NASA / ESA / Hubble Legacy Archive / Judy Schmidt]

Apropos of nothing

Not everything needs to be themed

The website for SYFYWire recently underwent a pretty big behind-the-scenes upgrade, which is great, but it also messed a few things up, including the RSS feed for my blog.

The good news is that’s now been fixed, so if you read it via a feed reader, the new URL is

https://www.syfy.com/syfy-wire/topic/bad-astronomy/feed

Please update your readers, and all should be well again. Thanks for your patience!

Blog Jam

What I’ve recently written on the blog, ICYMI

[Artwork depicting the merger of a neutron star (right) with a black hole (left). Such events make gravitational waves, as described in Tuesday’s article. Credit: Carl Knox (OzGrav)]

A Bit o’ Science

The entirety of science is too much for one sitting. Here’s a morsel for you.

Many years ago on the blog I wrote about a weird atmospheric phenomenon I saw a lot after moving to Colorado where a cloud bank forms on the downwind side of a mountain range and just hangs there, apparently unmoving, sometimes for hours. It wasn’t until years later that I accidentally found out what it’s called: a Chinook arch. In the case where I see them this forms to the east of the mountains, since the wind typically blows from the west.

A similar thing just happened to me again, and once more it involved mountains and airflow. In this case, I’ve seen clouds hanging to the west of the mountains, a stationary line that looks much like a breaking wave or sometimes a waterfall, but it never seems to move east. It always lingers over the mountains. It strongly reminds me of the scene from “The Ten Commandments” when Moses parts the Red Sea and there’s a wall of water just sitting there.

As I write this there was one this morning (indeed all day) that I’ve been admiring. The weather here was pretty calm, but I knew that wouldn’t last. Sure enough, not long after this wall of clouds formed, a wind picked up here on the plains like a switch was thrown. For several hours a warmish gale blew.

I’ve seen this formation literally many dozens of times, maybe over a hundred, but I didn’t know the name for it.

Then this happened:

(I saw this when it was retweeted by the Boulder National Weather Service account).

AHA! It’s called a Föhn or Foehn wall.

In a sense it’s the opposite of a Chinook arch, since it has clouds to the west and clear skies to the east, but also in how it forms. Chinook arches form when moisture-laden air rises over the mountains due to the wind lifting it upslope (this is called orographic lift) and then when it gets high enough, a bit east of and above the ridge, it cools sufficiently to form a cloud.

Föhn walls also form when that moist air lifts up a mountain. It expands due to the decreased air pressure around it, and also cools (squeezing a gas it heats up, and expanding it cools it off). Cooler air can’t hold the water vapor, so it condenses, forming a cloud, or even rain or snow. That gives you the Föhn wall. Similar to a Chinook arch but it forms earlier, to the west of the mountains.

[A glorious Chinook arch over the Rockies on Dec. 15, 2018. Credit: Phil Plait]

But converting vapor to liquid (or solid) adds heat to the air around it. Now you have drier air (the rain or snow has precipitated out) that’s warmer blowing over the mountaintop. It then falls down the eastern slope, getting compressed by higher air pressure as it does. That warms it even more, and what you get is a dry warm wind that blows across the plains: a Föhn wind.

That’s why I expected the wind after I saw the wall today. That wind can be… strong. The first week after we moved to Colorado we had 100 km/hr horizontal winds shaking our house so hard I was worried we were going to have a “Wizard of Oz” situation. You get used to it, but it’s still a bit weird and quite dramatic when it happens.

In the tweet above the poster mentions Kelvin-Helmholtz clouds, now officially called fluctus clouds, which are the breaking-wave features at the top of the bank. People who have followed me for a while know how much I love those; I never saw any until I moved here and now I see them several times a year, and they’re incredibly cool. If a layer of air blows over a lower cloudy layer, it can lift up the cloud in that wave pattern. I’ve seen it many times and I never get tired of it.

The tweeter says they’ve never seen that above a Föhn wall before. I posted a pic of fluctus clouds  here on the BAN once, and looking at it now I can see a Föhn wall beneath it, so it might be rare but not unheard of. Also, one of my favorite photos I’ve ever taken shows them casting shadows on the sky at sunset:

[Fluctus clouds at sunset. Credit: Phil Plait]

I have a lengthier explanation at that link, too.

It was a complicated wind day today, with never-repeating patterns of amazing clouds forming and disappearing. Gravity waves can happen when air blows over the mountains, creating long ripples in the air that can form beautiful wavy clouds; I saw this one for example:

[A gravity wave ripple in a long narrow cloud as air rises and drops along it. Credit: Phil Plait]

I’m not sure how to classify this cloud; it’s long and narrow like some types of cirrus, but not wispy. I think there’s more going on there, too, like maybe gravity waves and a slight Kelvin-Helmholtz effect as well. Air is a fluid — it flows — and fluid dynamics is hard. Really hard; the physics is ridiculously complex.

But my oh my, is it ever beautiful.

Et alia

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!

Reply

or to participate.