Secrets of Thermal Soaring

If model sailplane pilots ran the Discovery Channel, they'd make videos like Paul Naton's "Secrets of Thermal Soaring."

Article Thumbnail

Features An informative and in-depth look at the physics of thermals and thermal flying.  Features 135 detailed color illustrations, and a library of thermals in action.
Formats DVD, VHS (NTSC or PAL)
Available from Radio Carbon Art


If model sailplane pilots ran the Discovery Channel, they’d make videos like Paul Naton’s "Secrets of Thermal Soaring." This instructional video should appeal to both the beginner and expert sailplane pilot alike. It presents the environment our hobby takes place in (the air!) in a detailed, and novel way.

A bit of background on your reviewer might be in order for some perspective: I’m primarily a slope pilot, and for a long time in my early flying career I’d never really paid much attention to thermals, except perhaps when I blundered into big boomer while zipping through ridge lift.

A few years ago, I started dabbling in hand launch gliders, so my appetite for thermal knowledge increased quite a bit. I read Dave Thornburg’s classic "Old Buzzard’s Soaring Book" and tried to apply as much as I could to my new flat field gliding activities. My skills have continually improved, but I still don’t consider myself to be very good at it, which is where "Secrets of Thermal Soaring" comes in. I’ll take every extra bit of knowledge I can get!

This video won’t make you a great sailplane pilot: you need to actually get out there and practice for that. But for the beginner pilot especially, it does a lot to explain away much of the mystery of how and why thermals form, and where, when, and under what conditions you can find them.

A look at the DVD menu.

I reviewed the DVD version of "Secrets of Thermal Soaring", which has its advantages of over the VHS version. For example, there are lots of graphs and charts in the video (over 100), and the ability to freeze frame on a graphic to get a better look at it, yet have it remain crystal clear, is really nice. Of course, the quality of the sound and video is excellent too. There are also some extras on the DVD, which I’ll discuss a bit later. My one gripe is the same as with the previous Radio Carbon Art video I reviewed, "Endless Lift 3." That is, the main content of the DVD is only broken down into two chapters, despite the fact that there are many, many more segments. This makes it hard to quickly jump to a segment in the middle of a chapter that you might want to repeat, or perhaps review later.


The first part of the video focuses on what makes thermals happen. The goal here is to increase the viewer’s ability to read the air, and to forecast (or at least guess at) good or poor soaring conditions. Some of this can be a bit dry, as it’s presented with lots of charts and numbers, but it’s all information that is illustrated later when the video gives us a look at some real thermals.

"Secrets" explains some of the basics about the layer of air we R/C pilots fly in (called the convective boundary layer, or C.B.L.), which extends about 1000 meters above ground level. Our activities more often take place in a lower portion of the C.B.L., called the micro convective boundary layer, which extends from ground level up to about 300 meters.

Next we’re introduced to the lapse rate. This is an important concept that describes the rate at which the air cools as altitude increases. Lapse rate, along with atmospheric density, temperature differential, and the moisture content of the air are among the prime determiners of how good thermal conditions will be.

Ah, the sun, source of all thermals. The sun powers our atmosphere, of course, so the video next gives some insights into how the sun will effect thermal production. This includes a helpful chart showing the reflective properties of various ground materials such as soil, asphalt, sand, grass, etc. The preceding is helpful in determining where "hot spots" on the ground are more or less likely to occur and create thermals. It also covers some of the more obvious things, such as the time of day and year (more direct sunlight from a steep angle is good), and some less obvious things, such as how ground covered in gravel or rocks can be a better thermal producer (because there’s a greater surface area to absorb the sun’s radiation). There are graphics accompanying this part showing examples of good thermal terrain as well.

With the preceeding out of the way, Paul gets into explaining actual thermal production.The concept of lapse rate is reintroduced, and we see why it’s important. A "steep" lapse rate (one in which the air cools fairly quickly as you go higher) is needed to produce good thermals. A thermal, as you probably know, is air that rises because it’s warmer, and therefore less dense, then the surrounding air. A "parcel" of warm air rising from the ground will cool more slowly then the surrounding air that the parcel is rising through. A steep lapse rate means that the rising parcel will stay relatively warmer then the air around it for a longer period of time, meaning it can rise higher through the atmosphere and produce a better thermal. When the parcel reaches air that is at the same temperature as it is, the parcel/thermal stops rising: end of the elevator ride.

So with this in mind, why are some days better then others for thermals? The video explains a key factor in this…the inversion layer. The inversion is simply an inversion of the lapse rate: that is, the air starts warming with increasing altitude rather then cooling. This means that a thermal rising from cooler low-level air will stop rising when it reaches the warmer air of the inversion. If the layer is down low, it means the thermal elevator ride will be short indeed.

The video goes into detail explaining when inversions are likely to occur (nights and mornings, for example), and how to identify them. A bit that I had never really given a lot of thought to is exactly why thermals are typically weak earlier in the morning. Obviously, the sun hasn’t had time to fully heat the ground that early in the day, but from an atmospheric perspective, it’s because the thermals cannot punch through the morning inversion layer. Eventually, these morning thermals will mix up and dilute the inversion layer to the point where bigger late morning and early afternoon thermals can form and rise higher. Neat stuff. The video goes on to cover some other indicators to watch for to identify both good and poor thermal flying days.

The inversion layer: a thermal killer.

Most of us have probably wondered what a thermal would look like if we could actually see it. There has been much speculation about exactly what shape a thermal takes: Are they rotating vortexes, a series of bubbles, or big donut shaped things rising through the air? The later, also known as the vortex ring model, has been a popular theory, but "Secrets of Thermal Soaring" seems to dispel that concept in favor of a different shaped model.

"Secrets" presents the thermal as a "plume," something like a skinny mushroom cloud. It backs up this model with some laser radar/thermal images of actual thermals (and some photographic evidence a bit later). There’s a graphic showing the various parts of a thermal plume, and we also see how the plume’s shape can be distorted by the wind or from combining with other thermals.

The video illustrates what thermals look like, and how they're affected by the wind.

Part 2 of "Secrets" takes a more dynamic look at thermals. First we get a look at that all important factor, the wind. Thermals, of course, are affected by the wind, and Paul explains how they are stretched and tilted by this force.

In addition to being affected by the wind, thermals also affect the local wind themselves, which is key to identifying where they are. Distortion of the local wind can give away the thermal’s "feeders," the warm air supplies that thermals need to suck in order to keep going. These feeders can then be used to estimate the thermal’s position in the field.

"Secrets" illustrates how to practice spotting thermals using thermal poles: tall polls with streamers on top, which can be used to give away the tell tale sign of a thermal feeder.

Graphics are used to show the effects of a passing thermal on the poles’ streamers, and there is also some video showing what this looks like in real life. This is isn’t exactly new stuff…Dave Thornburg covered this in his book as well, but it’s still a useful tool for illustrating what effect a thermal has at ground level.

Streamers reveal a thermal's feeders.

In the next segment, entitled "Invisible Revealed," things get really interesting. Here we get to see actual video of thermals in action. This unique opportunity is provided by the fine powder soil that covers the farmland of Oregon. When stirred up by tractors and plows, this lightweight stuff does a great job of showing the form of rising thermals.

Dust reveals rising thermals.

The action starts off closer to the surface, though, with a look at energized air rising from ground level, and water vapor pulling into mini thermal plumes. Some color-enhanced video illustrates this more clearly.

Quickly, though, the video gets to shots of full-blown thermals lifting off from fields, forming rising plumes in the Oregon sky. The great thing about this segment, aside from some really great images, is that it illustrates many of the concepts discussed in part one of the video. We see thermals of various shapes and sizes, thermals tilted by the wind and drifting with it, and thermals joining with other thermals. In one nice clip, we see a thermal drift over a stand of trees, which promptly cuts off it’s supply of surface air, converting the thermal from a ground hugging plume to a separated, rising bubble.

"Flight School" is the final part of this video, and as the name implies, it provides some flying tips for finding and making the best of thermals with your sailplane. This covers how to identify when the airplane has encountered lift or sink and suggestions for thermal search patterns. It goes on to cover what to do once your plane is actually in a thermal, for example, how to best find and circle the core (where the best lift is), which may vary in shape and size depending on the wind’s distorting affects.


With the main part of the "Secrets" over, the DVD has some extras to explore. This includes a catalog of the many diagrams and charts shown during the video. You can easily flip through these in a slideshow type presentation. Also included is a "thermal gallery," a collection of even more videos of visible thermals in action over the Oregon fields.


"Secrets of Thermal Soaring" offers a detailed look at the air we spend so much time flying through. I must admit that on first viewing, the shear number of charts and graphs was a bit daunting. However, on watching it again, everything fell into place (okay, so I’m a little slow). I think I now have a much better understanding of what to look for to indicate that thermal conditions might be good or poor.

I was at first a bit surprised that there wasn’t more flying instruction in "Secrets of Thermal Soaring." Realistically, as noted before, you aren’t going to learn how to fly your plane from a video anyway. The main challenge is learning what signs to watch out for to find the thermals in the first place. Once you can do that the rest is easy (well, easier, at least).

I think this video should be really helpful to the beginner thermal pilots who might not be sure exactly what he is supposed to be looking for when tracking these sometimes-elusive beasties. It should also appeal to more experienced pilots, who may have a good gut-level ability to find thermals, but may not know some of the science behind what makes them tick.

Thread Tools

Quick Reply

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Wanted Secrets of Thermal Soaring rcflyinff Aircraft - Sailplanes (FS/W) 3 Jun 16, 2016 08:02 PM
Found DVD for High Performance Thermal Soaring and Secrets of Thermal Soaring hepavirus Aircraft - Sailplanes (FS/W) 0 Jun 28, 2012 03:20 PM
Question about Paul Naton's Secrets of Thermal Soaring Miami Mike Thermal 16 Jan 22, 2004 01:21 AM
FS - Secrets of Thermal Soaring DVD tsheehy Aircraft - Sailplanes (FS/W) 2 Oct 20, 2003 02:58 PM