What's going on with these clouds?

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allPrimes

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allPrimes
I took this time lapse yesterday after noticing that the clouds seemed to be building, but not going anywhere while out with my dog. This is in Bozeman and these mountains are the south end of the Bridger Range. I'm sure I've seen this a thousand times but this is the first time I thought to get a time-lapse of it.

Temperatures yesterday were into the upper 80s and lower 90s. I took this at ~2pm yesterday, hottest part of the day. This mountain range tops out at ~11k'.

Link to timelapse: https://i.imgur.com/lr1N8PR.mp4

My take: clearly, there's unstable air, causing the clouds to roil like this, but there must not be enough vertical development to have really built anything big. This is the western side of the Bridgers (windward, even with as little wind as we had yesterday). Even with little wind on the windward side, the clouds grow to whatever extent they can (not massive), the air cooled and would sink, then warm back up, and do the same thing on the other side of the range and into another range.
 
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Insufficient humidity for cloud growth?
 
Stable air, but wind hitting the peaks pushing moister air upward until it hits the warmer air above. The clouds form as it gets cooler, until it hits the warm layer above.
 
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Salty said:
I'm sure I can learn more, but that could cause it. If there's a cooler pocket of air stuck below the peaks, as the moist air is pushed up it condenses, then hits the warmer air above and stops rising as well as the air being able to hold the moisture again.
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No.
 
dmspilot said:
No.
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Maybe you're right. I guess I'll go read everything available on weather to find out why you think you are right since you aren't even attempting to explain where I'm going wrong. ;)
 
Cloud Development - Topographic Lifting

• Simply, air is forced up and over a topographical barrier - such as a hill or mountain
• The windward side will be cloudy and wet as air ascends
• The leeward side will be warmer and drier as the air descends - often called a rain shadow
rain shadow.jpg
 
Looks like it was a great day to do some cloud surfing with a glider plane. The person who referenced the sheppard air link was probably intending to point you toward pages 42-49, where several factors around cloud formation are involved to produce what you saw on the video.

You have an upslope = rising air, blown by whatever breeze was present and aimed toward the range
You have a downslope (not visible in your video, but on the other side of the range)= adiabatic warming on the backside
(relatively) moist Thermals were being generated and were topping out (expending their energy) at the top of the clouds. Because it was otherwise dry, I suspect that the clouds couldn't generate enough energy to do any more than what you saw. I'm not a meteorologist, and I am sure some of this is wrong. But I am an enthusiatic glider pilot and this explanation fits what little I know.

FWIW, I've seen the same phenomenon in the flatlands of MN. Clouds will repeatedly form and dissipate over the same geographical spot. It's how we can call certain pieces of ground our "house thermals" near the airport.
 
AlphaMike said:
Cloud Development - Topographic Lifting

• Simply, air is forced up and over a topographical barrier - such as a hill or mountain
• The windward side will be cloudy and wet as air ascends
• The leeward side will be warmer and drier as the air descends - often called a rain shadow
View attachment 109885
Click to expand...

Topographic lifting, yes, without a doubt. We learn about that early on in our PPL when you learn to fly in the Rockies!

"Cloudy and wet" are relative, I take it? It was nearly dry as a bone yesterday; relative humidity when I took the time-lapse was 15-20%.

The top image on Page 17 of the link you shared has a great image. I would expect that there was a layer of stable air on top of everything keeping things from building too much.
 
Ozone said:
Looks like it was a great day to do some cloud surfing with a glider plane. The person who referenced the sheppard air link was probably intending to point you toward pages 42-49, where several factors around cloud formation are involved to produce what you saw on the video.

You have an upslope = rising air, blown by whatever breeze was present and aimed toward the range
You have a downslope (not visible in your video, but on the other side of the range)= adiabatic warming on the backside
(relatively) moist Thermals were being generated and were topping out (expending their energy) at the top of the clouds. Because it was otherwise dry, I suspect that the clouds couldn't generate enough energy to do any more than what you saw. I'm not a meteorologist, and I am sure some of this is wrong. But I am an enthusiatic glider pilot and this explanation fits what little I know.

FWIW, I've seen the same phenomenon in the flatlands of MN. Clouds will repeatedly form and dissipate over the same geographical spot. It's how we can call certain pieces of ground our "house thermals" near the airport.
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So it wasn't necessarily a "stable" layer on top but simply lack of energy to grow much higher.
 
allPrimes said:
So it wasn't necessarily a "stable" layer on top but simply lack of energy to grow much higher.
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The air is going to keep rising as long as it has more energy (heat) than the air above it.
 
allPrimes said:
So it wasn't necessarily a "stable" layer on top but simply lack of energy to grow much higher.
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That would be my take, since you said the humidity was very low (at least by MN standards...we are hovering around 50-70% most summer days).

Again: i'm not a meteorologist, so I could be wrong.
 
Salty said:
The air is going to keep rising as long as it has more energy (heat) than the air above it.
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This is true. But in this case the air is being forced up by Topographic Lifting. Then it quickly falls back down the other side of the hill. I'm not a meteorologist at all, but I do like reading about weather.
 
AlphaMike said:
This is true. But in this case the air is being forced up by Topographic Lifting. Then it quickly falls back down the other side of the hill. I'm not a meteorologist at all, but I do like reading about weather.
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My point is that if the air is not warmer above the mountain, the air is going to continue rising rather than falling down the other side.
 
Salty said:
My point is that if the air is not warmer above the mountain, the air is going to continue rising rather than falling down the other side.
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That's why I started with "This is true" :)
 
Salty said:
The air is going to keep rising as long as it has more energy (heat) than the air above it.
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Condensation is exothermic. The rising air loses a lot of energy as the cloud forms. If the air still has a lot of energy when the temp/dew point spread reaches zero, you get a lot of vertical development.
 
Salty said:
I'm sure I can learn more, but that could cause it. If there's a cooler pocket of air stuck below the peaks, as the moist air is pushed up it condenses, then hits the warmer air above and stops rising as well as the air being able to hold the moisture again.
Click to expand...

Your premises are wrong therefore conclusion is also wrong. Cumulus clouds are not formed in stable air. You also speculated a temperature inversion, which did not exist. Finally wind pushing air up the mountain would require winds greater than the 5 knots or so that were reported.

Stationary front over the area with winds coming from opposite directions around 10,000 MSL combined with ordinary convective activity. Doubt the mountains actually played much of a role, especially considering the clouds appear to be much the same on both sides of the peaks, however, a slight topographic lifting effect may be boosting the convective and frontal lifting effects to form the result seen.

The closest radiosonde data I could find is from Great Falls, MT at around 6pm local, and does not support Salty's analysis. Graph | Text
 
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dmspilot said:
Your premises are wrong therefore conclusion is also wrong. Cumulus clouds are not formed in stable air. You also speculated a temperature inversion, which did not exist. Finally wind pushing air up the mountain would require winds greater than the 5 knots or so that were reported.

Stationary front over the area with winds coming from opposite directions around 10,000 MSL combined with ordinary convective activity. Doubt the mountains actually played much of a role, especially considering the clouds appear to be much the same on both sides of the peaks, however, a slight topographic lifting effect may be boosting the convective and frontal lifting effects to form the result seen.

The closest radiosonde data I could find is from Great Falls, MT at around 6pm local, and does not support Salty's analysis. Graph | Text
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I’m not arguing what your saying but, Your using data from over 100 miles away and 4 hours later. Does that typically hold true in MT?
 
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AlphaMike said:
I’m not arguing what your saying but, Your using data from over 100 miles away and 4 hours later. Does that typically hold true in MT?
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OP said wind was light. The clouds aren't moving which also indicates the wind is light. The winds aloft don't change that much over distances less than that, but of course, the explanation I proposed is only a best guess based on the data available, and I'm not a meteorologist.

It's more of a counterargument to what I was replying to. If the air was stable the clouds would be lenticular. If it was an unstable layer capped by a stable layer the cloud tops would be smooth and flat.
 
AlphaMike said:
Cloud Development - Topographic Lifting

• Simply, air is forced up and over a topographical barrier - such as a hill or mountain
• The windward side will be cloudy and wet as air ascends
• The leeward side will be warmer and drier as the air descends - often called a rain shadow
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Yeap, you see this all the time on tropical islands.
 
Holy smokes! Comprehensive answer. Thanks so much for this explanation (and the introduction to a figure I don't understand at all)!

scottd said:
Very cool time lapse. I am a meteorologist, so here's what's going on. This moist convection you see in the video has reached it's equilibrium level (EL) and that limits the growth of the clouds to higher levels. Depending on the updraft strength, some of the clouds may overshoot the EL more than others. You can easily see the height of the EL on a Skew-T log (p) Diagram as I discuss in my new eBook, The Skew-T log (p) and Me. Here's one of the diagrams from that text.

View attachment 109892

A process called entrainment also plays in what you see. Essentially dry air aloft (and there's likely plenty of dry air) gets entrained into the building cu and they tend to collapse within their own updraft and dissipate over time. But if there's still plenty of instability for cumuliform cloud growth, these build ups will continue to develop, thus, keeping the convective process going. When insolation becomes less throughout the late afternoon into early evening, they lose their source of energy and instability disappears.

The mountains do play a role in this. Much of the convection west of the Continental Divide develops along the ridgelines and rims of the mountain ranges. This tendency for cumulus clouds like the ones in the video (and even deeper convection) to form over the mountains like this is due to the air being more strongly heated during the afternoon over the slopes (especially those facing the sun) and higher terrain plateaus/rims than the atmosphere at comparable elevations that are further from the mountain peaks. This is a process called thermally driven upslope flow with rising air and convergence that ultimately allows these cumulus clouds to build over the mountains. As it turns out, this effect is most common when the winds aloft are on the lighter side.
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Mostly Thermal Activity generated mostly by the heating of the Terrain underneath it. The Vertical slopes of the mountain provide lot Surface area per Horizontal Square Meter and thus radiate a lot more heat than the Valley Floor. A gentle wind against the side of the mountain can help but is not required. Usually these will generate their own winds.

Earlier in the day you would have seen less clouds that would have formed and disappeared.

I use the example of a watching a pot of boiling water. As the water heat up you get bubbles of heat (like hot air Balloons) rising to the top. As the surface gets hotter these become little tornados (Dust Devils) that continuously suck air into them and rise.

In the glider often you can catch the top of the bubble, climb several thousand feet until you fall out the bottom and have to wait the the next bubble to cycle or move over to where another one is has already formed. When flying these we can often watch cloud from over us and then disappear as the bubble dissipates into the cooler air above.
Once it transitions to a continuous updraft (Thermal) then you climb, often in relatively smoother air, to the bottom of the Cloud.

Excellent conditions for soaring/ Fair chance my friend Greg from Flying Y was over there soaring under them.

Brian
CFIIG/ASEL
 
That was a really cool time lapse, thanks for sharing it here
 
brcase said:
Mostly Thermal Activity generated mostly by the heating of the Terrain underneath it. The Vertical slopes of the mountain provide lot Surface area per Horizontal Square Meter and thus radiate a lot more heat than the Valley Floor. A gentle wind against the side of the mountain can help but is not required. Usually these will generate their own winds.
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But can get these clouds JUST from the wind blowing again a mountain slope.

Again, look at a tropical island. And in that case, the windy side is in shadow, so not surface heating thermals.
 
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