Friday, October 2, 2020

Answer: Digging deeper into the story behind a photo?


... is one of those personality traits that some people find endearing while annoying others.  Personally, I'm a curious person, so I find a curious person to be engaging and fun.  

This week's Challenges are drawn from things I saw that made me say why is THAT the way it is?   

Let's unpack these Challenges and see how curiosity helps us find some understanding:  

1. Here's a pic I took the other day in my back yard.  I live not too far from San Francisco International airport, so it's common to see contrails in the sky.  But this one seems unusual to me.  For lack of a better term, it's unusually poofy with lots of blobs along its length.  What's going on with these poofs on the contrail?  Does this happen often?  Is there a name for this phenomenon? 

This one of those fairly common moments in SearchResearch:  what do you search for here?  That is, what search terms are most likely to tell you what you need to know?  

In cases like this, you have to count on learning as you search--picking up terminology that you can then use for your actual search.  

I started with: 

     [ contrail puffy ] 

led me to an article by the Royal Meterological Society (UK) called Contrail lobes or mamma? The importance of correct terminology, which seems singularly appropriate as I'm casting about for better search terms.  This article talks specifically about the lobes that sometimes form on contrails. 

As this article points out, the terminology is varied: 

The lobular cloud regions in contrails have been variously called ‘drop‐like formations’ and ‘pendulous lumps’ (Ludlam and Scorer, 1953), ‘blobs’ (Scorer and Davenport, 1970), ‘pendant swellings like inverted mushrooms’ (World Meteorological Organization, 1975, p. 66), ‘pendules or fingers’ (Schaefer and Day, 1981, p. 138), ‘puffs’ (Lewellen and Lewellen, 2001), ‘clumps of condensate’ (Rossow and Brown, 2010), ‘smoke rings’ (Unterstrasser et al., 2014), and ‘tear‐drop structures’ (Paoli and Shariff, 2016). They have also been called ‘mammatus’ (Ludlam and Scorer, 1953; Schultz et al., 2006; Unterstrasser et al., 2014), ‘akin to mammato‐cumulus’ (Day and Schaefer, 1998), and ‘mamma structures’ (Paoli and Shariff, 2016).

This discrepancy in terminology in the literature (as well as public‐facing websites discussing contrails and meteorology) raises an important question as to what should be the appropriate scientific name for these features... 

What should I use for my next search?  Well, many of these terms are fairly common ("drop-like formations" can describe icicles as well as clouds).  The big question for meteorologists is Why does a smooth cloud develop these lobes?  In meterologist terms, What causes a mammatus formation in a contrail?

But since the term mammatus is fairly rare I tried: 

     [ mammatus contrail cause ] 

And that led to a bunch of articles, all of which are fairly technical.  Seeing so many tech-focused articles makes me think that we should look in Google Scholar (which gives us 52 hits for this query).  But they're pretty good hits.  

One paper in particular seems right up our alley:  The Mysteries of Mammatus Clouds: Observations and Formation Mechanisms, by Schultz, D. M., Kanak, K. M., Straka, J. M., Trapp, R. J., Gordon, B. A., Zrnić, D. S., ... & Lilly, D. K. Journal of the Atmospheric Sciences, 63(10), 2409-2435. (2006).  

Fortunately, this is an open access paper, so we can read it carefully. 

As the authors write, sadly for us: 

Because they are not directly related to significant weather events on the ground and they do not apparently hold insights into forecasting severe convective storms, mammatus generally have been viewed as no more than a curiosity in the atmosphere. Consequently, published research on mammatus is rather limited, and what literature exists is either highly speculative or severely constrained by the limited nature of the observations.

Well... rats!  But I'm curious enough to read through the rest of the paper.  

Pressing on, though, we read that in the "Mechanisms" section of the paper there are several different thoughts about why a cloud (such as a contrail) might form mammatus  shapes.  If we ignore the proposals for mammatus formation in anvil-shaped clouds (thunderheads and the like that don't apply to long, linear contrails), we see there are 3 different suggestions: 

1. local inhomogenities: that is, slight differences in the way the air is moving, causing a linear cloud (contrail) to be pushed into lobes. There's some evidence from Doppler radar studies for this, showing that the air really is moving at different speeds in the lobe vs. outside the lobe. This could be just due to local differences in winds from the sides, spreading out the contrail a bit.  

2. "gravity waves" in the atmosphere: This sounds more sophisticated than the reality--"gravity waves" are really just pressure waves. The idea is that as the plane moves through the air, it generates a series of waves (think of little shock waves) that persist for some time after it passes.  As the homogeneous contrail cloud is formed, it encounters pressure waves of air left in the wake of the plane, forming regular pulses in the contrail.  

3. Kelvin–Helmholtz instability: This happens when a stratified fluid (the air) has strong vertical wind shear (air currents at an angle to the direction of the contrail formation).  In at least one case, wave-induced vertical motions inside clouds seem to be associated with mammatus clouds at the cloud base, perhaps the most convincing evidence published to date that K-H instability causes mammatus clouds.  Would this work on contrails?  A quick search for

     [ Kelvin-Helmholtz contrail ] 

leads us to the Earth Science Picture of the Day and this image: 

A Kelvin-Helmholtz contrail. P/C John Adams. From Earth-Science Picture of the Day

Overall, there are several different mechanisms that could create the pulses (or lobes) that we see on contrails.  They're all somehow related to local "inhomogeneous" conditions in the contrail, each causing a volume of air to disperse differently than the one next to it... and that leads to a regular pattern of lobes along the old contrail.   I'm willing to bet that the pressure waves are caused by the plane itself.  (But I admit that this is a guess based on the papers I read.)  

Note that older planes can also create contrails, although they're usually at lower altitudes and are mostly vapor, rather than ice crystals as we see in jet contrails.  Vapor droplets happen when the air is humid and there's a sudden pressure drop, as sometimes happens over winds and at the tips of propellor blades. 

Contrails created by the propellors of B17 bombers during World War 2. Note the mammatus structure of the earlier contrails in the image. It's not just a jet-age phenomenon. 

As you might expect, there are many different kinds of contrails, different conditions and probably different explanations for the lobes in the contrails.  As I was reading the above papers, in the reference list I noticed a reference to time-lapse study of contrails... so I naturally look for videos:  

      [ time lapse contrail ] 

and find all kinds of videos, including this one from Encyclopedia Britannica: 

which has this image at 0:15: 

It's just at this point you can see the puffy / blobby / mammatus structure starting to form. What clearly starts as a fairly smooth and undifferentiated cloud fairly quickly organizes itself into lobes. 

Regular Reader Jon (the Unknown) points us to Crow Vortex instability (or the Wikipedia article) as another mechanism for making pulses in contrails.  It happens when the wingtip vortices interact with the engine contrails, producing visible distortions in the shape of the engine contrails. 

As an example, check out this compelling YouTube video

At 2:26 in this video you can see a striking image of two helically rotating vortices: 

While Crow vortex effects are beautiful, they probably didn't cause my original picture, which seems very one-sided.  

I'm not sure I was able to quite answer the Challenge question ("what causes.."), but I think we got to the limit of scientific knowledge.  

2. Here's another photo I took while hiking on a trail next to a channel in the greater Los Angeles area. I'll spare you from having to extract the lat/long from the photo (it's 34.1628333,-117.9922528). It's not the most exciting trail in the world--it follows along a fairly barren path next to this concrete channel for quite a ways before getting to Monrovia Canyon Park (which is quite nice).  

As you can see, for most of its length, the concrete channel has plain square walls.  Here, though, there's a kind of angled buttress on one side of one corner of the place where the ramp enters the channel.  Why is it there?  Why would someone feel the need to build this special buttress?  

To get an idea of what I was looking at here, I first checked Google Maps. There, I saw that this channel was connected to the "Sawpit debris basin" via a concrete channel in "Sawpit  Canyon."    

Click on image to see at full size.
This is taken from Google Maps at  34.1628333,-117.9922528

First question that occurs to me: What's a debris basin?  If you do that as a query, you'll quickly find LA Public Works answer to that query

Debris basins are facilities designed to capture sediment, gravel, boulders, and vegetative debris that are washed out of the canyons during storms but allow water to flow into the downstream storm drain system, thereby reducing flood risk for communities downstream of the facility. They are typically located at the mouths of canyons and are key components of the Los Angeles County Flood Control District's flood risk management system.

In unburned watersheds, debris basins are cleaned out once they are 25 percent full. The number of years it takes to reach that level varies. In burned watersheds, where the potential for mudflows is higher, debris basins are cleaned out once they are 5 percent full. A watershed that has had more than 20 percent of its area burned within the previous 5 years is considered a burned watershed. For some debris basins in burned watersheds, this may lead to multiple cleanouts within a year.

If you look just a bit upstream from our spot on the channel, you'll see Sawpit debris basin, behind what looks like a dam.  But if you look for: 

     [ list of dams in Los Angeles county ] 

 you'll find the official LA county list of dams and reserviors, and Sawpit isn't listed there.  Hmm.  

When I see something like this, I get curious--so I will try different kinds of maps.  If you check out Open Street Maps, you'll see this: 

You can see what looks a LOT like a dam right where the Google Map shows "Sawpit debris basin." Compare this to the Bing map of the same area: 

Look at that!  Right where the OpenStreetMap indicates a dam, there's a lake.. and even farther north, there's another lake!  

After searching for variations on my queries: 

     [ sawpit debris basin ] 

     [ sawpit dam Los Angeles county ] 

I found this map at the LA County Flood Control district

This shows the Sawpit Sediment Placement Site (SPS), the Sawpit debris basin, and the Sawpit dam way up at the north.  The northern Sawpit dam is no longer in use (which is why it's not on the list of LA dams), but what looks like a dam (and is marked as a dam in this map), is actually a debris basin.   

Basically, a debris basin is a kind of  dam often used in areas where there is extensive flash flooding. This mountain range (the Santa Gabriels mountains) are notorious for this. (See John McPhees' book, Control of Nature for more details on this.) 

 When the rains come and the ground gets soaked, any more rain instantly runs off, bringing a massive amount of boulders, rocks, trees and brush. A debris basin catches most of this junk and has to be cleaned out every once in a while.

A sediment placement site is where the excess water runs (below the debris basin) to let sediment slowly settle out, rather than filling the channel.  

Why am I telling you all this? 

Because the channel in the first photo shows a kind of ramp leading up from the channel to the surface streets.  That exposed gap in the channel wall means that anything coming down the channel will preferentially strike that corner.  A corner like that is pretty delicate--it will need a bumper or some kind of protection to keep it from rocky harm.  

And why does that matter? 

Because this channel buttress is right after the debris basin and before the sediment placement site.  When this channel fills up, that particular corner is going to be in for a lot of abuse by rocks and logs and everything that's coming down the spillway from the debris basin.  

So while I wasn't able to find out anything definitive about this structure (none of the obvious queries worked!), it seems pretty clear that it's built to handle the stresses of massive water + debris flows coming down from above.  

Interestingly enough, the current fire (still in flames as I write this!) is the Bobcat fire, which has burned all of the upstream brush and woodlands.  This winter will be a real test for the channel and the buttress. 

SearchResearch Lessons

As I said, there are all kinds of ways to think about these SRS Challenges.  You could extrapolate the questions:  Why aren't all contrails poofy like this?  Or, Why does Los Angeles have all of these strange channels that obviously don't have water in them?  Assuming that this channel sometimes does carry water, where does that water go?  Or you could expand your range of searches to include items that are nearby or potentially relevant.  Count on your curiosity to lead you to those new topics.  

In these two SRS cases we're left with not-quite-complete answers to the Challenge questions.  We have some hypotheses about why some contrails grow lobes, but not a definitive answer.  

In the case of the channel buttress, we're left with a good guess based on what we found nearby... but I was hoping for an engineering report or a blueprint to say what it was, how it was designed, and why it's only at that location. 

But in both cases my curiosity drove a lot of discovery, and I learned enough about contrails, mammatus clouds, and low non-homogenous clouds to be fairly sure that I understand the lobes.  Likewise, I learned about debris basins and the way large amounts of debris flow down channels to be pretty sure that this is what's going on--it's there to protect the relatively weak and exposed corner.  Not conclusive... yet... but pretty high probability.  

What did we learn from these Challenges? 

1.  Not all Challenges have neat answers!  We've seen this before, but it's worth remembering.  I spent about 12 hours trying to answer these Challenges (especially the channel-buttress and debris basin question), but never really got to a conclusive answer.  At this point, if I was getting paid for this, I'd call up the LA County Flood Control district and talk to someone. (But I'm out of time for the week.)  

2. Look at different maps to get different information You'd think that all of the maps would have pretty much the same data on them, but that's not quite right.  The Bing maps shows exactly where the debris basin is, while the Google map shows different details more clearly.  In general, look at more than one map to get a broad perspective on your research.  This point generalizes to the contrail challenge as well... 

3. Let your curiosity guide you, especially down adjacent topics, but learn as you go.  Curiosity is a delicate thing: it can be incredibly productive IF you keep track of the amount of time you're spending following a topic.  But you have to be willing to cut the curiosity tangent off when it starts to be unproductive.  (That's why I always leave a visible window (or sticky note) with the original research goal on it.)  Every 5 minutes or so I see that goal and ask myself, "Is this getting me closer to answering the goal?"  If not, I cut off that tangent and go back to my primary task.  (Note that it's perfectly okay to change your goal, but if you do that, then change the note!)  

4. Take discovery notes along the way. In particular, as you pursue these side tracks in your search, take notes as you go.  Write down the special names and terminology, maybe even dates and organizations that might prove valuable in the future.  I find that this is a great way to get back to something I ran across once, but then found useful many minutes later.  My memory is good, but it needs the metacognitive assistance of notes that capture the good stuff along the way.  This skill (recognize the good information) is something that will improve with time.  Be aware of it as you do your own search challenges.  

Hope you enjoyed this Challenge.  I certainly did!  (And now I've got a few spare minutes, so I'll keep checking other notes that I didn't have time to follow-up before.  I might post an update if I get really great, clear answers!  

Search on!  

P.S.  These topics  itch my curiosity something fierce.  As I learn more, especially about the effects of the Bobcat fire on the channel, I'll be sure to keep you updated!  


  1. This was a fun one. It reminded me of that feeling Los Angeles gives me that there is always way more going on than what you see when you drive around.

  2. Thank you for sharing this. Your learnings are very useful and I will heed to them. What was even more exciting was to find a kindred spirit in curiosity and using search strategies and tools to find answers. I should’ve found this blog years ago. What an inept searcher I am! Thanks again.