Incredible cloud display on Nov. 19, 2017 over Colorado, caused by rising air over the mountains. Credit: Phil Plait

Jaw-dropping time-lapse of clouds over the Rocky Mountains at sunset

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Nov 22, 2017

When you look up into the sky and see a cloud, it's easy to think it's a thing unto itself.

By that I mean an object that exists with an obvious border, a self-contained bundle of water droplets suspended in the sky, perhaps influenced by the air moving around it but in a sense separate from it.

However, that's not true. Clouds are dynamic, and sometimes just the visible bit of a much larger system that's mostly invisible. But sometimes nature flips a switch, and makes the invisible visible.

Such was the case on November 19, 2017, over the Colorado Rocky Mountain foothills. That afternoon, I saw clear skies to the west over the mountains, but it was sharply delineated from a cloudbank that stretched all the way to the east. As soon as I saw it I knew I had to take a time-lapse video of it. That, it turns out, was a very, very good decision. Behold!

 

Whooooooaaa. That sunset! The clouds were incredible, and incredibly beautiful. Here's a photo I took showing just some of that amazing structure and color:

So what's going on here? What could cause such beauty?

Air! Seriously. Air, and of course some water. It starts with moisture-laden air flowing from the west toward the Rocky Mountains. The mountains are tall, some towering over 4,300 meters (14,000 feet — which is why we Coloradans call them fourteeners) above sea level. The air moving east flows up and over the mountains… but remember, as you go up, air gets colder. As it rises above a certain altitude the temperature drops enough that the water in the air condenses. It forms a cloud!

But this air isn't just a narrow stream: It's a broad, wide sheet, stretching north and south for hundreds of kilometers. So you don't just get a cloud forming in one spot, you get a huge north/south line in the sky: That line is where the rising air cools enough to reach the dew point, and water condenses. So west of that line the air is slightly lower, warmer, rising, and clear; and just east of it the air is higher, cooler, and the condensing water in it forms a cloud.

Incredible cloud display on Nov. 19, 2017 over Colorado, caused by rising air over the mountains. Credit: Phil Plait

Incredible cloud display on Nov. 19, 2017 over Colorado, caused by rising air over the mountains. Credit: Phil Plait

What's really peculiar about this when you see it is that, while the clouds are clearly moving to the east, that line stays in the same spot in the sky! That's because the line marks the spot where the cloud forms and the water in the air becomes visible. So the cloud starts there, and is continuously generated at that spot.

It's very odd to experience this. Right after I moved to Colorado I saw this same thing, and the line stayed put for three days! It was incredible.

But there's more. The air flows over different mountains at different rates, and causes condensation at different altitudes. If you watch the video carefully, you'll see a pair of mountains taller than the others (Longs Peak and Mount Meeker). Look above them: The air stays clear for longer, creating a deep notch in the cloud line that goes farther east. I'm not sure of the precise details of this, but I expect the air flowing over those tallest mountains has less vertical velocity than the air to the sides, so the air moves farther east before rising high enough to form clouds. This is a guess, but it makes some sense.

You can see this in another video, this one from a webcam located just to the northeast of the town of Lyons, right on the edge of the foothills. The view is facing west, and you can see Longs and Meeker in the center. Watch the clouds above them…

 

Wow. The video was sent to me by Noah Newman, a research coordinator at the Colorado Climate Center, who, like me, loves watching the clouds over Colorado. Technically, the clouds in the video are called lenticular altocumulus. Lenticular means lens-shaped, sculpted by the winds flowing over the mountains (these can sometimes look eerily like flying saucers hovering over the peaks). Altocumulus are puffy clouds arranged in patches or layers. They look smooth and featureless in the videos until the Sun drops down low enough to light them from a shallow angle underneath, then suddenly the incredible flow lines pop into view together with the spectacular colors you get with sunsets. Just gorgeous.

The thin crescent Moon sets just under the cloud bank caused by air rising dramatically over the Rocky Mounatins. Credit: Phil Plait

The thin crescent Moon sets just under the cloud bank caused by air rising dramatically over the Rocky Mounatins. Credit: Phil Plait

Oh, and did you see the bonus near the end of my video? The setting crescent Moon makes an appearance! My phone ran out of power (grumble) before you see it actually set behind the mountains, but it's visible to the left, very small but visible.

We get formations like this over the mountains pretty often, but it's more rare to get such a gorgeous light display as well. I saw a similar one a few years ago when I was in Denver:

 

Yeah. Seeing something like that is unreal.

As a human astronomer, I'm a little conflicted about clouds. They do block the view for optical astronomy (for radio astronomers they aren't a problem since radio passes right through them), but they are so ridiculously cool! And their beauty can be staggering.

So it's the best of both worlds: No clouds means unobstructed views of the stars… but clouds are also worth watching just on their own. They're cool all the time, and sometimes you get a real payoff.

Keep looking up. Day and night!