by Dennis Pagen
Let me respond to my respected friend and old colleague, Peter Gray and his article in the Dec. 2001 Hang Gliding and Dec. 2001 Paragliding. Peter performs an exhausting study of some of the physics describing thermals and their behavior, then concludes that our popular concept of “thermal triggers” is a myth. He references my book Understanding the Sky as well as the articles by a number of other writers to point out where in some part we go astray. I cannot be responsible for the exuberance of other writers, so I’ll focus on what I wrote and the research I have done.
To begin, I totally agree with the mathematical analysis Peter has provided. I went through much the same process over the years in writing the various forms of my book (the first edition was written in 1975). The calculation of the weight of a thermal appears on page 213. However, most of the mathematical analysis Peter presents is left out of my book because it is intended for the average reader who wants to know conclusions, not every step along the way. When writing about the real world there is always a fine line between great detail for exactitude and brevity for clarity. We’ll revisit this point later.
I also agree with Peter that surface tension has no role to play in the birth, release and rise of a thermal. However, I disagree with his statement that there is no such thing as thermal triggers. And since I believe that thermals linger near the ground as they build, I think it is a very good model for the average reader to use their observed experience with surface tension to form a working picture in their mind. Dixon White has used the concept of surface tension as an analogy to explain the dynamic of heat being trapped by bushes, trees, rocks, etc. Heat will simply “bleed” off of areas where it isn’t trapped, which is certainly a valid point. Scientific literature is chock full of similes such as this to help us imagine the behavior of nature. For example, in electronics we describe a diode as being like a door that only lets electrons pass in one direction and not the other. No one seriously thinks there is a door on hinges with a little man in uniform regulating passage. The important thing for pilots is to form models in their mind that are easily accessed, track reality as nearly as possible and are not overly complicated in order to avoid using too much mental process while the world of lift passes us by.
If I read Peter’s write-up correctly, he believes that thermals form at the surface when the sun heats the ground which in turn heats the overlying air. All right so far. Then he thinks the thermal automatically begins lifting off as soon as it has a bit of buoyancy (less density than the surrounding cooler air). This lift-off, according to what he wrote, occurs over the entire heated surface in a more or less uniform fashion. As this heated mass rises it coalesces to become the somewhat discrete column that we know and love as a thermal.
I think Peter’s “Realistic Model” is faulty for a couple of reasons. Firstly, note that convection currents form very readily in a fluid due to density differences caused by heating. These currents serve to distribute the heat more uniformly throughout the fluid. They are more or less steady state and continuous. However, when the heat is applied at the bottom of a fluid at such a rate that that convection currents cannot keep up with the process, then more chaotic penetrative convection occurs. In this case the warmer air “comes away from the heated (surface) in lumps, which we call thermals.” The quote is from R.S. Scorer’s book Natural Aerodynamics. [This book was the first comprehensive text written on micrometeorology and is a wonderful source of in-depth information for anyone wanting to really understand how the smaller scale atmospheric effects work. Unfortunately, it may be only available through the libraries of large universities. Scorer himself was perhaps the world’s foremost meteorologist in his day (I believe he died in the 70s)].
When the heating from the sun’s insolation reaches a certain rate, the volume of air being heated over good absorbing surface areas expands. The expansion rate is faster than the convection currents within it can distribute the heat beyond the boundaries of the growing thermal. This process can continue for half an hour or more as you can readily see by sitting in a thermal generating field. With a little patience waiting in the quiet field you suddenly feel a rush of wind as the grass waves and the leaves toss. A thermal has just lifted off. The buoyant thermal air does not bleed off gradually. It sits on the ground until a discontinuity starts it penetrating upwards then it releases in a whoosh. It has been triggered.
There are several mechanisms that account for the tendency of a thermal to rest on the ground despite its lower density compared to the air above it. The first is inertia. Peter correctly calculates the weight of a thermal as being from tens to thousands of tons. In order for this great weight to begin rising as a cohesive mass it must overcome a good deal of inertia. Secondly, in order for a thermal to lift off it must be replaced with air from the side (remember, nature abhors a vacuum). If the air is being heated more or less uniformly over a wide expanse, there is no great horizontal differential that will induce an inflow from the side. As a heated blob expands it exerts a pressure outward that helps oppose the inward pressure from cooler, denser air at the sides.
An important point to note in is that air of different densities does not readily mix, in a manner similar to oil and water. That’s why squall lines or gust fronts maintain their integrity for hours over many miles of travel. That’s why you can watch a cumulus cloud take a half-hour or more to lose its cohesive structure. That’s why fronts persist for days and the jet stream exists. That’s why a thermal maintains its integrity through thousands of feet of rise, despite the mixing from mechanical turbulence along its edges. That’s why a thermal isn’t penetrated by countless incursions of cooler air as it sits on the ground waiting for a trigger.
A thermal growing on the ground is conditionally unstable (or more appropriately, meta-stable). That means it remains in a tenuous equilibrium as long as a certain process continues. That process is the continued growth of the thermal. When that growth slows down (which it always does since the heat comes from the surface and it is getting distributed over a larger and larger volume), the expansion forces are lessened and the thermal is prime for a disturbance to start its upward motion. There is an analogy we can use here to illustrate our model. This is the supercooling of water. Pure water can be cooled several degrees below freezing and will remain a liquid until a discontinuity or disturbance occurs. Then it will ice up in a sudden rush. In the same way a thermal lies on the ground until it is disturbed.
The second point that disagrees with Peter’s analysis is that a thermal lifting off as a large flat pancake blob would have an exorbitant amount of drag. Drag goes up linearly with the surface area, so such a thermal would have to quickly consolidate to the ideal shape (a sphere or column) or be soon torn apart. I don’t believe (and the models don’t show) that thermals lift off even initially like a pancake. They penetrate with arms upreaching in several places until they coalesce, sometimes several thousand feet up (that’s why there are often several cores). What often starts this penetration is a trigger point.
The final thing to observe regarding Peter’s model is that if thermals didn’t sit on the ground for a spell there would be no excess of heat built up on the ground, so there would never be a superadiabatic lapse rate. Any excess heat would be continuously carried aloft by little convection currents that would not be large enough to lift us. We’d be limited to boring soaring. Since our observations indicate that thermals do sit on the ground for a spell as they build and they often release suddenly, we can conclude that the mechanism that releases them is something that disturbs them from their quasi-equilibrium state. We call this mechanism a trigger.
What are possible triggering mechanisms? One of the most ubiquitous triggers is the presence of downdrafts caused by the thermals themselves, especially in wind. Downdrafts are a necessity in thermal conditions, since “what goes up must come down.” Downdrafts are usually more spread out than thermals, but when they reach the ground their gustiness disturbs the tranquil thermal and releases it. Wally Wallington, in his wonderful book Meteorology for Glider Pilots has a good illustration of this matter on page128 and Scorer shows it on page 177 of Natural Aerodynamics.
Another common trigger is a blocking of the flow when wind is moving the heated ground air along. A hill, buildings or a tree line can produce this blocking effect. Elevated ground such as hills and mountains are good thermal sources both because they are often more readily heated than the lower ground, but also because they provide automatic triggers. They do this by virtue of their upslope (anabatic) flows that assist a thermal in its initial upward flow.
Now let’s address the controversial idea of moving objects disturbing the festering hot air to release it. Peter argues that a relatively small tractor, car or glider could have no effect on a multi-multi-ton thermal. I disagree, because, for all the above reasons, I believe that only a very small portion of the thermal needs to be disturbed to start the chain reaction that results in the thermal rushing upwards. Starting a small portion of it swirling or roiling can initiate the process that unbalances the quasi-equilibrium, allows air on the side to undercut the thermal and achieves lift-off. As Dave Broyles pointed out, “chaos theory describes many systems that show massive changes resulting from very small causes. Thus, in all likelihood, chaos theory would strongly support the trigger paradigm.” Small perturbations are often responsible for large effects in nature.
Now we come to the less analytical part. This is where pilots get up and give testimony like at a revival meeting. Peter’s reference to the perpetuation of myths is well founded (I’m glad he reads the Skeptical Inquirer—if more people did we’d have fewer astrologers and Jerry Falwells). However, in this case I don’t believe we are in the throes of a myth. Let me give you some examples. Australian pilot, Rohan Holtkamp, who took second at this summer’s hang gliding World Meet, and who is recognized as being one of the world’s top flatland pilots stated in an interview that he goes to tractors very frequently as a primary source of thermal triggering. He described a number of times in the world meet when top pilots did the same for a low save. The well-known Helmut Reichmann (former world sailplane champion) discusses triggers in his book, Cross-Country Soaring. He specifically mentions ground vehicles (among many other things) on page 6 of the revised edition. On the same page he describes experiences where he went to triggering areas to get a thermal.
Of course, all these top pilots could be subscribers to the myth as well as us mortals (no facetiousness intended). But here’s one last argument: If there were no such thing as thermal triggers or trigger points, it would make sense for all cross-country pilots to simply fly in as straight a line as possible over a flat desert area such as in Australia near Hay, or Chelan (perhaps altering course a bit to avoid stronger sink) and rely on chance to find a thermal. Since we all are rational and don’t believe any one pilot is guided by the “force,” the result would be a mix of pilots going the furthest or winning on successive days. But this is not the case. In fact, certain pilots with superior decision making abilities are consistent winners. In my interviews with them they declare that everyone climbs about the same, and it’s the decision making of where to go that makes a difference. They all believe in trigger points. Perhaps it can still be argued that most of us are deluded, but flying involves hundreds of subtle decisions on the wing and if the model we have helps us excel, then it is a valid model even though it may not be absolutely accurate. All the top pilots know that XC flying is a game of percentages and they try to load the dice by seeking thermal sources and thermal triggers. No one believes that potential trigger points are 100% reliable, but they up the percentages of finding a thermal.
One final point I wish to dispute is Peter’s statement that since I mention thermal bubbles in my book that I “implicitly accept the surface tension idea…” Nothing could be further from the truth, as I’ve already mentioned. I also mention thermal columns in the book. I believe Peter misinterpreted what I’m modeling. When we describe thermals as bubbles or columns, myself and other authors are not talking about how they act or operate on the ground in this case. We are describing their shape in the air after they have consolidated and are rising. This is somewhat a historical matter, which arose as a question when sailplane pilots in the 20s first discovered thermals. As I point out in my book, thermals can be either bubbles or columns, according to how much air is available to supply them from underneath. Here in the Northeast we frequently have bubble thermals since the heated layer is often not as thick as it is out West and the heated areas tend to be determined by field size. It is not uncommon to see a gaggle of gliders or birds define the top and bottom of the lift in a couple hundred feet.
For a really good picture of thermal shape and progress, I recommend the Scorer book. He presents photos of a dense, white solution (barium sulfate) being released as a blob in water. The downward flow, vortex ring, erosion and cohesion of the solution is a very good model (upside down) of a thermal. I would also direct readers’ attention to the OSTIV publications on technical soaring, especially publication nr.11.
Much of the above discussion is a rehash of an article I wrote for my Wingtips column in the 80s for what was Glider Rider magazine at the time. I’m sure Peter and many readers have missed it. As I have been pointing out in the talks and seminars I present, there is a new emphasis on small-scale meteorology due to the military importance of drones. There are many new studies of the effects and characteristics of thermals. For these reasons I have been planning an updated article on thermals for some time. Look for it soon. In the meantime, keep your finger on the trigger.