What happens to cumulus clouds?

[qbynewbie]

I had a flight last night at 7:00 pm and it reminded me of a question that I've yet to find the answer to.

The afternoon sky was full of cumulus clouds. A low over Canada had been sending troughs through for days, so we were seeing occasional rain followed by bright sun for a few minutes, followed by another cycle, all day. By afternoon, the radar was showing what I think of as "pop up" storms all over the region but especially to the south. Some had lightning, again to the south.

By 7:00 pm, most of the cumulus clouds were gone but there were a lot of stratus clouds around. So I found myself wondering if:

1. The cumulus clouds had drifted away and stratus clouds had drifted in or simply formed (not my choice), or

2. The cumulus clouds had collapsed into stratus clouds.

That led me to wonder if stratus clouds were denser than cumulus clouds... it would seem they would have to be if cumulus were collapsing into stratus. If not, where did all the extra moisture go to?

So if anyone can help this fledgling pilot-cum-meteorologist understand this aspect of clouds, I'd be very grateful.

 

[Areeda]

Brian,

I believe the cumulus collapse into cirrus as the ground cools and the atmosphere becomes more stable.

If you have Scott Dennstaedt's discussion of the SkewT-logP diagrams you can see this happening in the diagrams attached. Note the CAPE (Convective Available Potential Energy) measurement drops significantly as the ground cools showing the increase of stability while the LCL (Lifted Condensation Level) stays pretty constant at 850mb (about 5000) which is where I, with my limited experience, would expect the clouds.

I get these diagrams from here. The atachments are from the GFS model which goes out for days and tends to be not as accurate as the RUC model which Scott recommends for near term flight planning.

Joe

 

[sarangan]

I had a flight last night at 7:00 pm and it reminded me of a question that I've yet to find the answer to.

The afternoon sky was full of cumulus clouds. A low over Canada had been sending troughs through for days, so we were seeing occasional rain followed by bright sun for a few minutes, followed by another cycle, all day. By afternoon, the radar was showing what I think of as "pop up" storms all over the region but especially to the south. Some had lightning, again to the south.

By 7:00 pm, most of the cumulus clouds were gone but there were a lot of cirrus clouds around. So I found myself wondering if:

1. The cumulus clouds had drifted away and cirrus clouds had drifted in or simply formed (not my choice), or

2. The cumulus clouds had collapsed into cirrus clouds.

That led me to wonder if cirrus clouds were denser than cumulus clouds... it would seem they would have to be if cumulus were collapsing into cirrus. If not, where did all the extra moisture go to?

So if anyone can help this fledgling pilot-cum-meteorologist understand this aspect of clouds, I'd be very grateful.

Cumulus can become stratus when the vertical flow ceases after the heat source (sun) goes away. Regarding moisture, the white stuff you see is only part of the moisture that exceeds 100% RH. The majority of the moisture is contained in the air beneath the white clouds.

 

[TangoWhiskey]

Did he mean collapse into stratus instead of collapse into cirrus? Joe, what's the magenta line, and why does it disappear in the later sounding?

 

[qbynewbie]

Brian,

I believe the cumulus collapse into cirrus as the ground cools and the atmosphere becomes more stable.

If you have Scott Dennstaedt's discussion of the SkewT-logP diagrams you can see this happening in the diagrams attached. Note the CAPE (Convective Available Potential Energy) measurement drops significantly as the ground cools showing the increase of stability while the LCL (Lifted Condensation Level) stays pretty constant at 850mb (about 5000) which is where I, with my limited experience, would expect the clouds.

I get these diagrams from here. The atachments are from the GFS model which goes out for days and tends to be not as accurate as the RUC model which Scott recommends for near term flight planning.

Joe

Joe,

Thanks! I've never seen the SkewT-logP diagrams before. I read what you wrote several times and realized that it was sort of like a foreign language. So I Googled SkewT-logP and got a bunch of references. After reading three of them, it's still like a foreign language -- just one that sounds sort of like it will make sense sometime soon if you just keep listening to it.

I have to go mow the lawn. I'll come back later and read some more and see if I can make some sense of the diagrams. Thanks for the info -- and the challenge.

PS: If you are home now, I hope you both had a great trip.

 

[qbynewbie]

Did he mean collapse into stratus instead of collapse into cirrus? Joe, what's the magenta line, and why does it disappear in the later sounding?

Troy,

Yes, of course I did! I was rushing this morning to post before heading out to the airport and I was simply being a bonehead. I've corrected the references, above (and added a bonehead reference as the reason for the change!)

Thanks for catching my mistake!

 

[gprellwitz]

Joe,

Thanks! I've never seen the SkewT-logP diagrams before. I read what you wrote several times and realized that it was sort of like a foreign language. So I Googled SkewT-logP and got a bunch of references. After reading three of them, it's still like a foreign language -- just one that sounds sort of like it will make sense sometime soon if you just keep listening to it.

I have to go mow the lawn. I'll come back later and read some more and see if I can make some sense of the diagrams. Thanks for the info -- and the challenge.

PS: If you are home now, I hope you both had a great trip.

The place to go to learn about SkewT-logP diagrams is www.chesavtraining.com, where Scott Dennstaedt has a CD for sale that's well worth it! They pack a LOT of information into a very compact format, and fully understanding them takes time. (I'm only just starting going through my copy of the CD ) I think SkewT-logP diagrams are the sort of things of Edward Tufte's dreams (http://www.edwardtufte.com/tufte/books_vdqi)!

 

[Cap'n Jack]

To put it into simple terms...

The cumulus clouds don't have any forces causing them to expand upwards anymore. The visible moisture needs some time to evaporate, it it doesn't disappear right away. The results are the stratus clouds you observed.

 

[2256Q]

If you just watch them long enough, you can see them evaporating.

 

[dmccormack]

Cumulus don't collapse into stratus clouds. They are produced by two different processes. Cumuliform clouds are vertically-developed clouds produced by significant vertical motion driven by surface heating or lifting through a front or other density discontinuity. When the vertical motion ceases to exist, they typically go through a process of dissipation through entrainment of dryer air and essentially evaporate. How quickly they dissipate depends on just how dry the environmental air is around the cumulus clouds. If there's a particularly moist altitude (look at the dewpoint depression on a Skew-T diagram), they may hang around a bit longer at that altitude before they completely dissipate.

All clouds come in different "densities," but cumulus clouds have much more condensate in them which typically blocks out more sun and makes the bases appear darker than most stratus clouds.

If there's a change of air mass during the time the cumulus clouds are dissipating (such as a frontal passage), you can see a different temperature profile behind the front...one that does not promote vertical development and may be somewhat "capped." Depending on the temperature profile below the cap you can see stratocumulus clouds form near the surface which may seem like the other clouds "collapsed" when in fact, the temperature profile changed instead. Also, when the heating of the day quits, that also stops the vertical motion unless there's some kind of other outside energy contributor such as a front.

Excellent discussion.

When I started thinking about clouds and weather in terms of energy -- lost and gained, it helped me visualize what should occur, over time (after all, anyone can look out a window and determine what is).

So clouds with extensive vertical development (cumuliform) require energy to push the moisture up from the surface.

During a clear summer day, we may begin with some ground fog. Solar energy lifts that and other moisture until it reaches the dewpoint altitude (thus the transition from invisible vapor to visible moisture --clouds).

Clouds with extensive horizontal development (stratus, cirrus) exhibit less vertical energy, thus are less turbulent, and typically don't rise as high.

Thunderstorms have extensive vertical development. Energy is absorbed in the transition from vapor to droplets, causing large changes in energy (temperature) within the same cloud. Downdrafts are caused by air going up has to be filled in by air coming down.

 

[2256Q]

Referring to the energy that drives vertical development of cumulus clouds, I would point out a little piece of information for the newer pilots. When the air is rough and your passenger(s) is/are getting air sick, climb up above the clouds, if your aircraft will do that. Once on top of the clouds, the air smooths out.

 

[dmccormack]

Not quite. Most cumuliform clouds exhibit turbulence, but it is highly dependent on the strength of the prevailing wind and the strength of the updraft. Depending on the vertical temperature profile and wind speed and direction, even stratus and cirrus clouds can exhibit moderate or severe clear air turbulence (only 75% of CAT is in clear air).

Good point -- I was being a bit too vague and rule-of-thumby.

We tend to find smoother rides in stratus (horizontally layered) clouds than in cumuliform (vertically developed) clouds.

Of course -- as you mentioned -- there is no guarantee -- other factors can make even stratus and cirrus rough ride type clouds.

This is a typical topic that will become part of my member workshops in my new web site.

We're all looking forward to that discussion!

Thanks!

 

[etsisk]

Referring to the energy that drives vertical development of cumulus clouds, I would point out a little piece of information for the newer pilots. When the air is rough and your passenger(s) is/are getting air sick, climb up above the clouds, if your aircraft will do that. Once on top of the clouds, the air smooths out.

Yep, the tops of the clouds are the tops of the turbulence! Mostly.

Same for the top of the haze layer, in my experience.

 

[jpflys]

????? ..............My head hurts, I'm gonna get a drink !

 

[etsisk]

How do you tell where that minimal or isothermal lapse rate is, Scott? Look at the skew-T and see where the temp and dewpoint lines separate?

 

[thepetrostate]

????? ..............My head hurts, I'm gonna get a drink !

Not alone you don't, move over.

 

[thepetrostate]

Coming soon...AvWxWorkshops.com

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