Thermal Behavior

Understanding thermal characteristics and free flight

By Dennis Pagen
Originally published in USHPA Pilot, January/February 2021

I nearly fell out of my chair laughing last July 16 when I read a headline from Fox Views that read, complete with awe and naïveté: Andean condors can fly more than 100 miles without flapping their wings once, researchers reveal! (exclamation added). It went on to say that, “Instead of the simile ‘fly like an eagle,’ perhaps it should be ‘fly like a condor.’” The article was also astounded to note that “once they reach the desired height, flying is relatively stress free” and “they don’t have to flap” (italics added). Okay, spinning reality is par for this news outlet, but still I feel sorry for their ignorance and also for the poor eagles that are apparently limited to 99.9 miles of XC soaring. For the rest of us, we can take heart knowing that all we have to do to be better soaring pilots is to shift our focus to emulate condors and their amazing feats (if only we could find one).

The truth is the vast majority of the John Q. Public have no clue that there are vertical air movements, except when smoke goes up. And they are mostly unaware of the existence of thermals. In fact, it wasn’t until I was in college and a friend took me on a sailplane ride that I heard of them. I still didn’t understand the concept clearly even though we were circling and staying up over the flatlands of Michigan; but gradually I learned more and made sense of the bubbling atmospheric process.

Now, with all the small aircraft exploration we have done throughout the lower sky, we know a ton of thermal lore. In fact, nearly every time we as free flight pilots fly on a good day, we add to our knowledge. In this piece I want to explore aspects of thermal behavior we may not think about regularly. It is my view that the more we know about thermals and all their different forms, the better we can exploit their gift of lift and avoid their curse of sink.

Thermal Nativity

Most thermals are born on the ground. Initially they are amorphous lumps of warmed air, expanding as the sun’s heat continues to cook the surface, which in turn heats the air above it. These lumps of warm air may be all joined together to cover large areas, but they are not usually uniform. Some blobs may be warmer and extend higher than others as shown in Figure 1. As these blobs grow due to continued heat input, they do not necessarily rise as thermals even though they are lighter (less dense) than the cooler air above them because their expansion does not let the cooler air tunnel under them to fill in the empty space that would be left. If a large area is being heated, cool air can only come in at the sides as shown. In fact, the cool air may not move under the heated air at ground level until a large mass of the warm air begins rising as a thermal. Most of us have been on the ground on a hot summer day and felt no air moving until suddenly a thermal lifts and a breeze of cooler air comes in as a gusty wind that blows until all the warm air evacuates. At this point, the wind dies and the warming cycle begins again.

When a thermal does liberate itself from the ground, it is typically not made up of the entire heated surface air mass. As noted, there may be hot spots in this mass, but also an area more prone to lifting (such as a hilltop or a point near a vertical surface) will begin rising first; then things get complicated. An area of lifting air has to move the air above it out of the way in order to rise; this air does not move upward—that takes too much energy—instead it sidles to the side. That’s still some work that has to be done to upset the equilibrium, so perhaps you can see why heated air at the surface takes some time to swell and gather strength before it can release as a thermal.

The cool air moving out of the way of the growing thermal will slide along the top of the warm layer until it finds a “weak” point where it can push downward to seek its equilibrium as shown. When this action happens, the closest, warmest blob or mass will suddenly release because it has the cool air to replace it underneath. The total weight of air in a medium sized-thermal is tons or scores of tons (as we’ve shown in previous articles), so we can see that it takes time to overcome the inertia of all that air and get the thermal process going. We call the birth of a thermal the “trigger” or “release.”

Thermal Types

As most experienced pilots know, thermals can be quite varied in strength, size, duration, and singularity.

Strength is dependent on the excess of heating of the air layer just above the ground and the lapse rate (temperature profile—the drop in temperature with altitude). The amount of heating determines whether the thermal dribbles or leaps off the ground. A surface area that can be superheated compared to its surrounding will normally exhibit the strongest thermals. I have experienced a few of the most powerful thermals of my career over burnt ground. And, of course, we should be aware that desert areas like the Owens Valley and other super-dry terrain can send up boomers.

Size is generally dependent on how much of the warm surface air is lifted all at once, but also on the stability of the air. A high pressure system usually causes smaller, weaker thermals because the air mass is slowly sinking and becomes more stable, so a thermal has to push up through this sinking air, which can erode its edges more readily. The generally sinking air suppresses thermals and tends to limit them to small hot spots with turbulent edges.

We should be aware that east of the Mississippi (hereafter simply called the East) the best thermals occur just after a cold front passes when a good lapse rate is still present before the high in the heart of the cold mass arrives. This rule doesn’t apply to Florida, which makes its own unstable air in a manner similar to the West (west of the Mississippi). The main distinction is that eastern areas are characterized by high humidity and widespread greenery which tend to slow the production of thermals and reduce their power.

In the West, we see the formation of “heat lows,” whereby a vast area of surface heating expands the air aloft, which flows to the side, just as it does close to the surface when a thermal forms. On this larger scale, the outflow of air aloft creates a relative low pressure at the surface and a slow but meaningful rise in the general air mass. Because the air is so dry, it does not form widespread clouds (as it would in the East), and only exhibits cumis where the individual thermals push up in the generally rising air to reach the condensation level. Such thermals are usually large and easy to stay in with a modicum of thermalling skill.

The duration of a thermal mainly depends on how much warm air exists at the surface to feed the individual thermal. In the East with smaller open areas surrounded by trees and hills, thermals tend to be of shorter duration. It seems almost impossible to catch one below 500 feet from the ground and carry it up to cloudbase. It almost always requires two or more to elevator up, like a lift in a department store. On the other hand, in the West with typical huge expanses of semi-arid ground, all the surrounding warm air can feed a single thermal for many minutes and the resulting thermal can stand tall and proud. I have often been in thermals with pilots strung out vertically for at least 3,000 feet.

Singularity is my term for a thermal that consists of one core, rather than multiple areas of good lift. We most often encounter single core thermals in the West and other arid areas with large areas of heated surface air that can feed one point for a relatively long time (Australia and South Africa are other places I have experienced them). This piece is not intended to cover dust devils which are caused by lifting thermals in strong conditions. Most commonly we see dust devils in the West as single towering swirls, denoting a single core thermal. However, I have encountered a large dust devil with little devils racing around its outside in the Chelan, Washington area. I flew into it and did find multiple cores (see cautions below).

As we described above, a thermal that is born of a “lumpy” heated layer will often rise with multiple little areas of “thermalettes,” to coin a term. Multiple core thermals are a common feature of the East with its varied field sizes and close groundcover. I have flown in large meets in Florida, Europe, and Brazil where a gaggle of 20 to 50 pilots would be in three or more cores (tight areas of lift) in close proximity. Usually these cores eventually come together, or the weaker ones die out, but it takes 1,000 to 3,000 feet of rise above the ground before it all gets sorted out. I have illustrated an example multicore thermal in Figure 2. As a side note, one of the valuable things about flying in meets is to learn the nature of the local conditions and thermals in general. Certainly after years of practice, we can predict some thermal behavior and quickly learn to find the best cores. But if we always fly alone, we may never learn that certain thermals have multiple cores and never reap the rewards of finding the strongest and longest cores.

To help you visualize the release and progress of a thermal with multiple cores, go to Google Maps and look at Montezuma, Iowa in the satellite image format. At the northwest corner of the town you will see a grass fire with lots of smoke that perfectly illustrates how a thermal behaves. There is a wind of perhaps 10 mph that tips the smoke over so we can see it from our bird’s eye view. Observe the three larger plumes all coming together to produce one cohesive “thermal.” Also note the smaller plumes that are drawn into the main column by the inflowing air replacing the rising warm air. A fire such as this is almost identical to a thermal except that the heated air is at a higher temperature, at least for a portion of its rise. Note how the smoke seems to come together, then expand into a wider puff, then come together again and repeat the process. I can see at least seven of these cycles. This shows us that thermals can go through cycles as they rise. These cycles may be due to the feedback system where cool air comes in surges to replace the rising warm air or cooler air rolling down along the main “head” of the thermal pinching it off at some point above the ground. We show this action in Figure 3.

It should be apparent that when we are climbing in a thermal of this type there may be surges and periods where we barely climb at all. In my experience, many thermals, especially in greener areas, exhibit this pulsing behavior. The question is always: “Do I wait for it to get better, or move on to the next thermal?” Of course, if we are recreationally flying waiting is a fine choice—we can even learn a bit about thermal behavior, at least on that day at that site. In competition the choice is tougher and depends a bit on what part of the flight you are experiencing. Early on in a task, you probably do not know the full nature of the day’s thermals as well as you will later. Of course, if most of a gaggle moves on or other pilots on course are showing thermal circles, perhaps you should move on. But most of us in competition have seen a few days when the more conservative pilots get further because they hung on to every scrap of lift. Perhaps the main takeaway here is that thermals are often unpredictable so paying attention and being prepared for all possibilities will enhance our performance.

Thermals in Wind

As we can see in the image of the Montezuma fire/smoke, a thermal can maintain its integrity and cohesiveness very readily in wind. This behavior holds true until the wind starts rolling over the ground to mix things up. There will still be thermal globs and masses rising, but they may be very turbulent, short-lived, and hard to work. But many of us have experienced massive thermals blocking the wind somewhat as they rise into faster moving air.

In general, a thermal will behave in moderate wind as it does in light or zero wind. It simply drifts along as it rises and if you keep your circles regular within it you will drift along somewhat in harmony. However, if the whole thermal column is tilted, most often we tend to drift out the back of the thermal because our upward trajectory is flatter than that of the thermal column since our climb rate is less than any part of the thermal. The best procedure in this case is to pay attention to climb rate around the entire circle and expect to have to adjust your circle a bit upwind to stay in the juiciest lift.

When a thermal is next to a mountain or moving up a slope, it can tilt so the lift is not uniform all the way around the thermal circumference. Figure 4 illustrates this point. The friction with the terrain slows the upflow close to the slope so stronger lift occurs at the point in the thermal furthest away from the ground. As soon as the thermal breaks away from the slope (either by rising above it or becoming strong enough to track more vertically) it begins to get more uniform around an entire circle within the thermal. It is important to pay attention to positioning and bank angle when thermalling close to the terrain (for safety reasons), and also to be ready for the thermal to rise more vertically as it breaks away or accelerates downwind a bit as it clears the top and enters stronger winds.

It should be noted that even strong thermals tend to hug a slope because the inflow to replace the rising air can only come from the non-slope side. Thus, the thermal gets pushed towards the slope as shown in the figure. In a column thermal the same thing happens because there is always erosion and cool air pushing inwards on the thermal as it rises. These different pressures and winds acting on a thermal near a slope can cause turbulence and variations in the lift, so it’s always wise to carry a bit of extra control safety near a mountainside. That means speed on a hang glider and safe brake pressure on a paraglider.

The final matter we’ll mention is how thermals tend to often be strung out parallel to the wind they rise into. That means we can often find more lift and multiple cores by flying directly upwind from the point where we left the last lift. From what we discussed before, it should be clear that thermal lift can rise from the ground in pulses because all the warm air reaches the triggering area at different times with different amounts of excess heat. So blobs of warm air can push up one after another, and the stronger ones will rise in front of the preceding ones that have drifted downwind. This situation happens most often in the East where stronger general winds allow us to stay up on ridge lift until we find thermals to climb higher. In the West or when going cross-country away from the mountains in the East, this stringing out of the thermal lift is not so common. But there are always exceptions, and it is worthwhile to explore the possibility on many flights. Of course, if the air is streeting (as it does more often in the East than the West), flying the direct upwind and downwind paths from lift greatly increases the chances of success.

Thermal Spin

In an earlier paragraph I mentioned dust devils. Dust devils are created when a strong thermal lifts off and accelerates upward. Typically the air rushing in to fill the void below it will have some curvature to its flow, so when it comes together it spins rapidly and picks up dust. This is not an article about dust devils, so all we’ll mention here is, in most areas, it is unsafe to fly above them to use their thermals below about 1,000 feet in a hang glider, and a bit higher in a paraglider. Of course, in the Chelan, Washington area, we enter them lower, but the particulates are what we affectionately call moon dust, and it gets picked up by the slightest disturbance (the only other place I have encountered such fine dust is certain sites in Mexico). The main point to know is that in the lower reaches, the thermals based on dust devils can be spinning as it rises above the visible dust. Eventually this rotation stops as it continues to climb, but for safety and performance reasons it is important to enter such a spinning column by circling in a direction opposite the air/dust’s spin.

It should be clear that strong thermals in areas where no dust exists can also be spinning (we frequently see leaf devils in the East). But if you can’t see it, it is just a guess. I have often watched birds to see if they have a preferred turning direction, and if they do I turn in that direction when down low, assuming: 1) they know what they are doing, and 2) the overall air flow has some vorticity that induces the in-rushing air to spin the same way.

Another cause of spinning thermals we usually can’t see is when the wind is somewhat cross to a hill, ridge, or mountain. In this case the air will roll along the slope and tends to twist a thermal in the same manner as a dust devil. Succinctly put, if the wind is cross from the left, the air will twist clockwise when viewed from above, so thermal turns in this vicinity should be to the left (counterclockwise). If the wind crosses from the right, the opposite turn direction is called for. I have experimented with this effect enough to feel its significance in scratching situations near the terrain.

These varied thoughts on thermal evolution and behavior are just a glimpse of the complex world of thermals. After 45 years spent exploring them (it took me a year and a half of flying before I even had a concept of a thermal), I know them as friends, but I also think there is a lot more to discover. I truly wish I could fly like a condor so I could use them more effortlessly, but alas, I find I can only fly like an eagle so far. And my advice to Fox is to stick to something they are less confused about and leave the flight of birds to the aviators and ornithologists.