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Cooling Some
Thermal Superstitions
by Peter Gray
"I swear by tractor thermals," my friend Scott Rutledge, ace Chelan
XC pilot, said two years ago, "I just fly from tractor to tractor. It
works every time."
"Every time?"
"Well, maybe not every time, but."
A few days later, I had my longest open-distance Chelan flight to that date,
naturally the day after the XC Classic ended.
"Scott, I flew almost to
"Of course not, silly." Scott scoffed, "It was
Sunday and all the tractors were in their barns!"
Silly Rabbit, Wicks are for Candles
Scott has plenty of good company. The idea of "thermal triggers" is
widely accepted, with several recent accounts in the USHPA magazines. My old
friend and respected colleague, Dennis Pagen, wrote about them in his excellent
1992 book, Understanding the Sky. In "Bird Man," an April 2001
Paragliding article about famous instructor Dixon White, Tom Harpole wrote that
thermals can be "released mechanically by something as small as a rabbit
running through them." In "Thermals: Collectors, Wicks and
Triggers" (Dec. 2000 Paragliding, Jan. 2001 Hang Gliding), Will
Gadd describes similar "active triggers," as well as passive ones
such as ditches, hedges, and power lines that supposedly "trip"
thermals loose or "wick" them into the sky. After many lively
discussions and lots of research and calculation, here's my conclusion:
tractors, cars, gliders, animals, power lines, and assorted objects on the
ground are effective at breaking thermals loose. just as effective as rain
dances are at producing thunderstorms.
Physics for Fun and Distance
Why do I say this? For a sense of scale, let's start with a brief tour of the
math and physics. How big is a thermal large enough for us to soar in? In
theory, the minimum practical turning radius for a hang glider is about 40 feet,
at stall speed and a 45-degree bank, and a paraglider could reduce that radius
to about 22 feet. A steeper bank is rarely profitable, as it swaps a heavy sink
rate toll for slight reductions in radius. In reality, a paraglider's inside
wingtip in such a small turn would be far below stall speed. More realistic
minimum thermaling radii are on the order of 13 m (40ft) for a
paraglider, 20 m (60ft) for a hang glider. The best climb rates are
typically found by circling at about half the radius of the entire thermal, and
workable small thermals are usually at least five times taller than their
diameters. I've measured thermaling circles with my GPS tracklog interval set
at one second, map page at highest resolution, and have rarely found myself
circling at smaller than a 70 m (200ft) diameter on a hang glider. The
reader might enjoy trying the same experiment. Walking a 30 m (100ft)
and a 70 m (200ft) circle on the ground before flight can be good for
getting familiar with the display scale.
For illustration, let's define a "small hang glider thermal" as being
70 m (200ft) in diameter and 300 m (1,000ft) tall, while a
small paraglider-workable one is 50 m (140ft) by 230 m (700ft).
Note: I emphatically do not suggest that real thermals are shaped like tennis
ball cans, or that they have sharply defined edges. With that caveat, it can be
enlightening to model them as simple cylinders. Using well-known values for air
density, the paraglider thermal, at sea level on a warm day, will have a mass
of about 400 tons, while the hang glider thermal weighs 1,200 tons (in my
experience flying both aircraft, the discrepancy in workable thermal volume
seems less than 3:1, but that's only a personal impression).
A common large thermal would be 330 m (1,000ft) in diameter, 3300 m (10,000ft)
tall. That one has a mass of 300,000 tons. For comparison, a high-end nuclear
aircraft carrier weighs some 91,000 tons. Toward the extreme but plausible end
is a thermal of 700 m (2,000ft) by 6,000 m (17,000ft),
weighing two million tons, equivalent to more than five Empire State Buildings
(due to the net effects of pressure drop and cooling, such a thermal would
expand to about a 800 m (2,300ft) diameter at its top). We can expect to
encounter thermals in a mass range of several thousand to more than 100,000
tons.
If these numbers sound, well, inflated, it's because we're accustomed to
thinking of air as wispy, insubstantial stuff, partly because it is invisible.
For another angle, compare a glider/pilot combo weighing 113 kg (250 pounds),
which I'll refer to as a Standard Glider (a
medium-sized pilot on a high-performance hang glider, or a heavy pilot on a
paraglider). Under typical conditions, one SG weighs the same as a sphere
of air 6-7 m (18 -20ft) in diameter.
We all know that thermals, and weather in general, are solar-powered, but what
does this mean in practical terms? Air under atmospheric conditions obeys the
Ideal Gas Laws of physics to a very close approximation, so estimating the
energy requirements for lift production is a matter of looking up standard
values, converting units, and applying simple math, most of which I will spare
the reader (for background and discussion, see here).
Let's refer back to our Standard Glider. To produce buoyancy to balance one SG,
we must heat a volume of air by adding 8.2 million calories. Within a
reasonable range, the volume is irrelevant. With the same energy input, we can
get 113 kg (250 pounds) of buoyancy by raising the temperature of 8909
cubic meter (314,000ft3) of air (one percent of a small HG thermal)
from 26.7°C (80°F) to 27.3°C (81.2°F), or by heating 92 m3 (3,245ft3)
(the 3 meter-diameter sea level SG-equivalent) to 326°C (620°F).
That example is for illustration, and I am not suggesting that every time the
ground absorbs 8.2 million calories of sunlight, a usable thermal will be
formed. Instead, this energy (when conducted to air) creates one SG
worth of buoyancy. If we could contain the warm air in a perfectly insulated,
weightless bag, it would be just enough to suspend one glider and pilot. In
reality, we can only utilize a small fraction of a thermal's lifting power. On
the other hand, in sufficiently unstable air, a relatively small amount of warm
air could evolve into a thermal capable of lifting a larger load than the raw
energy input would indicate. But without solar heating, such a good lapse rate
cannot last for long. The essential point: 8.2 million calories is a bare
minimum for simply balancing one glider and pilot.
How much solar energy might be available? At maximum, during June and July in
the southwestern
This raw buoyancy does not tell us how large or frequent the resulting thermals
will be or how quickly they will ascend. To make such predictions would require
a sophisticated fluid dynamics analysis beyond the scope of this article, but
we can make some observations:
- Each 3° C (5.4°F) temperature increase will expand a volume of air
by one percent, making it one percent less dense than the surrounding air. For
example, heating 3,000 tons of air by 6° C (10.8°F) will produce a
modest-sized thermal (or say, 80 m diameter and 700 m height) with 60
tons (480 SGs) of buoyancy. This requires four billion calories,
roughly the output of four acres in a half-hour under ideal conditions.
- For the same temperature change, a larger thermal will have a higher
buoyancy-to-drag ratio and will therefore accelerate more quickly and reach a
higher terminal velocity.
- For the same total buoyancy, a smaller, hotter thermal will have less drag
than a bigger one, and will ascend faster.
In light of the physical dimensions of thermals, how could tiny objects such as
tractors, gliders, or rabbits have any effect on them? Only if thermals are
somehow stuck to the ground, yearning for the open sky, but tethered like hot
air balloons. What kind of force could restrain a thermal with buoyancy in the
tens of tons? Dixon White, according to "Bird Man," describes
thermals in terms of surface tension: ".surface bubbles of warm air [that]
eventually exceed their inherent ability to swell, then they burst and
rise." So does Gadd, who writes that rocks are ".good wicks and
passive triggers, as they tend to pierce the surface tension." In his
book, Pagen implicitly accepts the surface tension idea by describing a thermal
as "a bubble.that remains on the ground for a period of time before it
releases in a sudden rush."
Bubble Theory
Sorry to burst all these bubbles, but.surface tension is strictly a liquid
phenomenon! It cannot occur within or between gasses. When I mention this, some
people say, "Maybe it's not really surface tension, but it's something
like surface tension." That's an unsatisfying explanation and it is quite
an understatement. Water has relatively strong surface tension, but it cannot
support a drop of condensation weighing more than about 0.15 gram, or 1/200
ounce. A force capable of holding down a thermal would need to be some 10,000
times stronger than the surface tension of water. Anyone who demonstrates the
existence of such a novel effect can earn a slam-dunk PhD, if not a Nobel
Prize, in physics. But research efforts are probably better spent in pursuit of
cold fusion.
What supports the Bubble Model of thermals? First, the analogy of air or steam
bubbles on the bottom of a heated pan of water is appealing because it occurs
for similar reasons; it's neat and clear; and we can see it happen! Second, we
want to explain the cyclical nature of thermals. Discrete bubbles seem to
behave the same way. As they grow, first they stick to the surface, then they
pop loose and float upward. While a bubble is stuck to the pan, in unstable
equilibrium, perhaps a tractor the size of a grain of salt could bump it loose.
A fundamental principle of science says that we should only look for novel
explanations when established theory fails to explain a phenomenon. Surface
tension between masses of air certainly qualifies as a novel hypothesis,
previously unknown to science, so the burden of proof should be on those who
propose it. On the other hand, can we explain our experience with thermals in
terms of conventional fluid dynamics and thermodynamics?
While some thermal sources produce lift more or less continuously for hours at
a stretch, most of them are periodic, with bursts of lift alternating with
lulls. Something must hold the air close to the ground while it warms and gains
buoyancy, but surface tension is not required. The more prosaic forces of time
and inertia can do the job.
Let's look at two models in parallel sequences of snapshots. In the Bubble Model,
pools of air form above thermal generators or collectors. The pools evolve into
domes and then spheres of warm air that is trapped or stuck to the ground by
surface tension. If a "triggering" object or event comes along, it
can break loose the bubble, which will suddenly "bloop up into the
atmosphere," according to "
In the Realistic Model, thermals are not so neatly defined. Air above heated
ground gradually warms, expands, and begins to rise. Meanwhile, it mixes to
some extent with surrounding air, and it might be diluted by wind. As the air
slowly rises, it forms an indistinct dome. Near the surface, cooler ambient air
moves in to replace the rising air, and is in turn heated by the ground. This
process continues in a gradually accelerating manner. When the rising, fuzzy
blob of air attains a sufficient vertical speed, the heated ground below it
cannot produce warm air fast enough for a continuous supply. The incoming air
also ventilates and cools the warm ground. At this point, which might be a few
minutes to as long as 30 or so minutes after the beginning of the sequence, the
thermal cycle ends, and we go back to Frame 1.
What provides this leisurely cycle time? Remember that significant heating (in
the 2-3° C range) of a modest-sized thermal produces lifting power on the
order of 30 to 60 tons. We can call this the thermal's absolute buoyancy.
However, the same thermal's relative buoyancy, compared to an equal volume of
surrounding air, is only one or two percent. If we hold a chunk of wood,
waterlogged so that only two percent of it shows above water when it floats, on
the bottom of a pool, it will rise slowly at first. The same applies to our
several-thousand-ton thermal. It's in no hurry. Here's one place where the
bubble analogy falls down. An air bubble in a pan of hot water has a relative
buoyancy of about 78,000 percent! A far better-but more difficult to
see-analogue is the warm water that forms actual thermals in a heated pan,
visible against shiny metal in good light. That water makes indistinct,
turbulent columns that climb far more slowly than the nearby frantic bubbles.
Consider Frame 3 in both sequences, and imagine that the heat is switched off
by a dense cloud shadow. In the bubble model, the thermal will stick to the
ground indefinitely if nothing triggers it. After all, the surface tension that
holds it to the ground also isolates it from the surroundings, so it can only
lose heat slowly through conduction. In the realistic model, the thermal will
not wait around, but will lift off as a small, weak runt. Sound familiar?
Other thermal behavior can also be explained without resorting to exotic
surface tension theories. Yes, thermals tend to rise from sheltered bowls, but
not because the air pools there or because thermals bump into tree lines or
houses and are jostled into the sky, but because these so-called collectors are
sheltered from the mixing and cooling effects of wind. Ridges can perform the
same function, on their lee and windward sides. A ridge or other high ground
reliably generates lift, not because it wicks air skyward, but because it
usually has faces that are more perpendicular to the sun than flat ground, it
tends to be sparsely vegetated, and it is well drained and DRY (more about
the role of water later).
The Roots of Myth
With such weak (I would say non-existent) scientific support, why is
the surface tension/bubble theory of thermals so popular? The main reasons are:
1) Appealing Explanation for a Mysterious Phenomenon We
humans seek neat, clean mental models, and if the process in question is
largely invisible, we are quick to invent something to hold in our
imaginations. ["That thunderstorm came out of nowhere! There must be
gods up there, throwing lightning bolts and waiting for us to ask for
favors."]
2) Anthropocentrism Humans have a natural inclination
to see themselves at the center of the universe. Will Gadd writes, "How
many times have you landed in a likely field only to watch someone else climb
out above you?" The implication: "Since there was a thermal just
after I landed, I must have caused it!" ["It rained after we
danced, therefore."]
3) Sampling Bias A pilot who believes in tractor
thermals will fly from tractor to tractor, thus meeting more thermals that seem
to come from tractors than if he believed in parking lots or yellow barns or
pinto ponies. [We perform our most intense rain dances after we've been
afflicted by a long drought. And what defines the end of a drought? Rain!]
4) Selective Memory People tend to remember data that
supports their conceptions and forget what doesn't. If you press someone who
believes that flying low over a field can "release a thermal," you'll
find a pilot who has landed plenty times after flying low over a field and
triggering.nothing! ["Sure we danced last week and it hasn't rained
yet.but we didn't dance hard enough."]
5) Coincidence with Reality If a false theory leads one
to correct decisions most of the time, it can be almost as good as an accurate
understanding. When other factors can produce similar results, it's easy to
forget that correlation does not imply causation. This is a key element of a durable
myth, so it is worth exploring further with some examples.
Case 1: Tractors stir up dust, so if a thermal lifts off
nearby, the dust can help make it visible. If the dust lies on the ground in a
long trail, we ignore that tractor and fly toward the next one. This gives us
insight into a key role of the tractor. Since they move rather slowly, why
wouldn't a parked tractor in a 2 m/s (5mph) wind work just as well as
a moving one? Or how about a barn in a light wind? Those also disturb the field
of moving air. But the best tractors are the active ones, because they raise
dust. If I have to choose between two equally attractive dry fields, one with a
tractor, one without, I'll favor the tractor-not because I believe it will kick
a thermal loose, but because it might provide useful information. On the other
hand, opting for a shaded field with a tractor instead of a sunnier one without
can end a flight prematurely. Correctly understanding the benefits and
limitations of tractors can sometimes tip the balance between a low save and an
early landing.
Case 2: Ridges are usually good thermal producers, not because
thermals stick to them and drip upward off their crests, but for the reasons
previously noted. The problem with the ridge wicking and dripping model is that
if it's over-applied, it can lead us to misinterpret conditions where ridges
are worse thermal generators than the surrounding flatlands. One common example
is an east-west ridge late in the day, in a west wind. It no longer faces the
sun as well as the flats, the wind ventilates and cools it, and we give up
precious ground clearance by flying over a ridgeline rather than over the
flats.
Case 3: Pagen writes: "One site in
Can tractors also help us if they don't bump thermals loose from the ground?
Not enough to bother with. Farm tractors come in the range of about 100 to 400
horsepower. Let's assume that an average one is 250 HP, and (generously)
that it operates at 200 HP constant output. Because the engine and drive train
are only about 25% efficient, such a tractor actually creates 800 horsepower
worth of heat, so let's assume that all 8.6 million calories per minute go into
heating the air. That's equivalent to about 0.7 hectar (0.3 acre) in
full sun. A typical field in eastern Washington covers 1/4 square mile (65
hectars ) so adding a tractor might increase the field's thermal output by
1/5 of one percent. This contribution could be more than balanced by the
cooling effect of damp soil that the tractor turns over.
Returning to the anthropocentrism and selective memory angles, if we believe in
thermal triggers, why do we imagine that they generally work in our favor? What
would cause a tractor to bump a thermal loose just when we happen to need it,
rather than few minutes too early or late? And, if a trigger did release a
thermal, wouldn't it be one that hadn't ripened to the point of bursting its surly
bonds on its own? In other words, triggered thermals should be weaker than the
ones that aren't so favored. As we've seen, it's all about energy, and
human-scale vehicles or objects cannot contribute more than a minuscule
fraction of the required heat. A freight train, possibly, but not a tractor,
glider, or rabbit.
Psychology professor Gregory W. Lester explores the roots of myths in a
fascinating article, "Why Bad Beliefs Don't Die" (Skeptical
Inquirer, Nov./Dec. 2000, www.csicop.org/si/2000-11/beliefs.html). Lester
illustrates the crucial survival value of beliefs (conceptions of the world
that do not rely on immediate sensory data), and he gives compelling
reasons for resistance to changing our beliefs, even in the face of
contradictory evidence.
Hydro-powered Lift: New Myth in the Making?
Pagen writes: "Ground that is moist after a rain is generally a poor
producer of thermals because of the cooling effects of evaporation." Gadd
writes: "Moist ground cover absorbs the sun's energy and uses it to
evaporate water, a cooling process that kills thermals." Disputing this,
Jim Palmieri (letter, April 2001 Hang Gliding) claims that water vapor
is lighter than air, and is therefore good for creating lift. For example, a
benefit of plowed versus flat fields is that the "furrows allow moisture
to rise.and then vaporize." Then, "the heated water vapor will rise,
not so much because it is warmer but because water has a low molecular weight
and is less dense than the rest of the atmosphere." In fact, water vapor
has less than 2/3 the density of air. To equal the buoyancy of water vapor, we
would need to heat an equal volume of air from 26.7°C (80°F) to 210°C
(410°F)! Sounds good, huh?
Ample flying experience indicates that Gadd and Pagen are right about this one,
but why? The key is energy. Evaporating a quantity of water requires 300 times
more energy than raising its temperature by one degree Fahrenheit. Remember
that making 113 kg (250 pounds) of air buoyancy consumes 8.2 million
calories. Using the same energy to evaporate water produces only 9.6 kg (20.4
pounds) of lift, which makes water vapor less than 1/12 as effective!
Also, the higher heat capacity of water vapor means that more energy is needed
to raise its temperature (and volume), so it is about 13% less
effective than air for producing lift after it evaporates. Yes, humid air is
somewhat more buoyant than dry air at the same temperature, but it only reaches
the same temperature at a tremendous energy cost-energy that could have gone
into far more efficient dry-air lift production. There's a good reason that
sweating works so well.
I predict that the water-vapor-benefit notion will fail as a myth, despite its
ostensible grounding in physics, because it doesn't meet the Coincidence with
Reality test noted above. After a few blunders into territory that I had
forgotten was assaulted by thunderstorms the previous day, I'm not inclined to
make the same error on purpose. It's no coincidence that few world's records
are set in regions that get more than 50 cm of annual precipitation.
Better Soaring through Physical Chemistry
While no one needs a formal education in thermodynamics to fly cross country, a
basic sense of the interactions of air and energy can't hurt. I have already
found that knowing about the sheer tonnage of thermals helps me understand how
they are affected by time, wind, and terrain. Most of our myths do not
drastically cut our performance, or we would soon abandon them. However, a
sharper sense of what to search for and what to ignore can make the difference
when we're at unzip-the-harness altitude, desperately scratching for one more
climb. I hope this article is a step in that direction.