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Thermal Lore - Part 1
by Dennis Pagen (copyright © 2002), published in USHPA’s publication “Paragliding” November
2002.
All illustrations and figures are from USHPA.
Soaring pilots live and dream under the thrall of
thermals. Sure, ridge lift lets us zoom around and play
local hero, waves are a gift from the gods and
convergence is a magic carpet when you find it. But
only thermals are consistently present and readily
exploitable by even newly fledged novice pilots.
Thermals are intriguing because they are mostly
invisible and they can take us to dizzying heights, in
some cases higher than big brother wants our tender
wings to go.
Another aspect of thermals is that they reward the development of certain skills, but involve an
element of dumb luck. Just as with fishing or picking up a new romance at a party, you can never be
100% sure about what you’re going to come up with when you go trolling. It is that element of
expectation and surprise that adds spice to the endeavor. With thermals, we cast our net based on
knowledge and how much height we have to spend, then hope for the best. The fact that it so often
pays off is a tribute to our glider’s performance, the wealth of knowledge that has accumulated in the
flying community and the abundant lift that nature affords. Many of us wish that fishing for seafood or
mates had such a high rate of return.
This series of articles is intended to illuminate the many aspects and peculiar behavior of those
elusive entities we know as thermals. The idea is to promote better flying through knowledge.
Hopefully pilots of all skill levels will find some nuggets to carry with them into the wild blue yonder.
My approach will be to try to avoid too much technical detail, but offer references for those who wish
to delve deeper. I believe this format is appropriate for the vast majority of pilots, since much of
thermal flying is (and should be) intuitive. But we do need a solid groundwork on which to let our
intuitive nature roam free.
Much of what we discuss will come from conversations with the world’s top pilots, but also an
important source has been research papers written on micrometeorology. These papers most notably
appear in the OSTIV publications, which is dedicated to the technical aspects of soaring (sailplanes).
In the last decade or so there has been much interest in micrometeorology because of the development
of drones, surveillance aircraft and other small flying objects. I’m dubious about the uses of these
craft, but grateful for the advancement in understanding.
In the course of this series we will visit the subjects of thermal development, shapes, behavior,
types and ways to exploit them. We will also look at special thermal situations such as the cause of
cloud suck, the “dead zone,” high-pressure thermals, East and West differences and inversion
encounters. Hopefully we will touch on some of the very core material that will make each of us a
better thermal pilot, or at least informed enough to know why we hit the ground while others are
scribing taunting zeros high over our heads.
THE THERMAL DAY
Without going deeper into matters such as lapse rate
and insolation right now, let’s look at the broad
picture of how a thermal day develops. Most of us
know that the air mass sitting over our area must be
relatively unstable for thermals to exist in abundance or usable form. What we mean by unstable is a
certain temperature change in the air with changing altitude. On an unstable day, thermals rise
spontaneously once solar heating gets underway and heats the surface adequately. Here’s the
sequence:
1) The sun’s energy, in the form of visible light and ultraviolet radiation, mostly passes through the
atmosphere and strikes the ground. The solid molecules on the ground catch the solar radiation and
convert it to molecular vibrations and much longer wavelengths — infrared. We detect vibrations
and infrared radiation as heat, and so does the overlying air. It is the transfer of heat from the sun
to the ground and then to the air that allows the creation, birth and growth of thermals. Thus, solar
energy gives rise to all life, including thermals that are born in the heat of the day.
2) In the morning, as the air overlying the surface gets heated, not much happens as a thin layer
thickens and grows warmer. A slight sloshing around may occur here and there, but no real
thermal activity happens until suddenly, all heaven breaks loose — thermals happen everywhere.
What’s going on here? The answer is that ground inversions stop the release of thermals until they
have penetrated to the top of the inversion (we’ll discuss the nature of inversions in a later part).
However, once this penetration occurs, the thermal release comes all of a sudden and from
widespread sources.
3) The abundant release of thermals may continue for half an hour or so, then frequently it shuts
down for a spell before thermals reappear in earnest. After that, a more sparse but regular
production of thermals occurs. This is when the thermal day sets in properly and we are apt to be
successful when we cast our fate to the wind. The mechanism that causes the thermal production
pause, then the more regular succession of thermals is as follows: The warming ground in the
morning heats a large area (almost the entire layer) of air over the surface. Thus, there is a large
reservoir of warm air to go up as thermals. But this air can’t release because of the ground
inversion. When the bonds of the inversion are broken, the thermals release with a vengeance.
These early thermals may
not be all that strong
because the sun isn’t yet
beaming down all that hard,
but they come in rapid
succession and often are
fairly continuous streams as
the warm air on the ground
seeks restitution aloft.
But when the warm air is depleted, it has been replaced by cooler air from aloft that takes time
to heat. So we have a pause. In addition, without the presence of the widespread ground inversion,
the thermals that do build can release when they grow to a certain size, or they are induced to do so
by triggering mechanisms. The most common triggers are downdrafts impelled by the rise of other
thermals in the area. Thus, we have a picture of a steady-state growth and release of thermals as
long as the sun’s heat continues in sufficient strength. The size of the thermals depends on (among
other things) how long they sit on the ground and grow before release. The initial release, then
pause in thermal production, is often seen in the ridge and valley systems in the eastern U.S.
4) As the day progresses, thermals tend to climb higher and peak in strength just a bit after the peak
solar heating. Then they dwindle in strength and frequency but still retain their maximum height.
Finally, only a few anemic old-maid thermals rise as the sun wanes and our soaring prospects dim.
In the end, only dreams of the day’s glory remain unless special situations occur that continue to
result in the artificial release of heat from the surface. (The artificial matters may be buildings with
internal heat sources, fires or water heated by some means other than the sun’s rays.)
5) As evening falls, the moon rules and the earth loses what it has taken from the sun. The heat
re-radiates off as infrared, and this effect sustains the warmth of the air for a while, but with no
new solar heat to tickle the earth’s surface, the surroundings soon grow colder. Then, the air stills,
chills, and a ground inversion layer develops. This layer thickens throughout the night until the sun
again peeks over the peaks and warming begins again. The cycle is complete.
ADDING DETAILS
Ground inversions can be anything from a few feet to a few thousand feet in thickness in extreme
cases. The thickest inversions occur in deep valleys in desert conditions. The reason for this situation
is that desert conditions result in rapid and extensive radiation of heat from the surface because of the
clear, dry air, and thus a much colder overlying layer. The high mountains surrounding the valleys
drain these layers of cold air down into the valleys all night long until a blanket of cold air is pooled
deeply in quiet repose.
The thicker the ground inversion layer in a given area, the longer it takes to reach trigger
temperature, which is where thermals break through the inversion in the morning. However, in desert
conditions the sun’s heating is comparatively more intense, so trigger temperature is reached relatively
sooner than in humid areas. In addition, thicker inversions often result in a longer initial release of
thermals, and in this case there may be no pause between initial release and the onset of regular
thermal production. The reason for this latter factor is that the thermals developing in a thick inversion
are already rising high enough to promote the vigorous downdrafts that can trigger other thermals
building on the ground. Thus, once the thermals begin their initial rise to full potential, the process
continues unabated. This situation is often noticed in the Owens Valley and the Alps.
THERMAL STRENGTHS
There are a number of factors that affect thermal strength. These are in two main categories: the
temperature profile of the air and the intensity of the solar heating. Let’s look at the heating factor
first.
The more readily a surface on the ground is heated, the more it imparts this heat to the overlying
air. Thus, we should expect to get good thermals above such surfaces. Take a barefoot walk across the
landscape on a sunny day and see what you feel. Did your feet get burned on that blacktop? Did you
enjoy the cool of the grass? How about the medium heat of bare dirt or fields in crops? We know from
experience and common sense that the surfaces that heat most are more likely to produce the best
thermals. But we also know that no surface stands alone. Everything is affected by everything else
surrounding it, because the atmosphere is a dynamic system. It is moving and three dimensional, so
sometimes an area that would normally be excellent for thermal production is constantly being swept
with cooler winds or stable air and thus does not live up to its potential. One such situation is beach
areas. We have all burnt our feet on those littoral sands, but beaches are rarely great thermal producers
because the constant inflow of the cool, stable sea breeze attenuates the effects of the heated surface.
A big factor in intensity of heating is the humidity in the air. When the atmosphere is dry, the solar
influx goes right to the ground with nearly its full power. But in humid conditions, a good portion of
the solar radiation gets scattered by the suspended water molecules, so the air itself takes up heat and
less is available to heat the surface. You might think, “That’s okay, what we want is heated air and we
just bypass the surface exchange in this situation.” Unfortunately, that’s not true. What happens in the
case of humid air is that the sun’s beaming heat is scattered deeply throughout the air’s layers, so we
don’t have the potentially unstable situation of a warm blob at the bottom of cooler overlying air
pressing down. In fact, the hot, humid, summer doldrums are what we Eastern pilots dread, because
the few thermals that do develop are weak. In the case mentioned here it should be apparent that there
are many factors that affect both the amount of surface heating and the lapse rate.
Two more factors that affect the solar heating of the surface are the sun’s position and the amount
of cloud cover. We acquire an almost unconscious knowledge of the sun’s diurnal variation. We all
know that only mad Englishmen and dogs go out in the heat of the day in the heart of Africa. So we
know that the peak heating at the peak of the day provides peak thermal production. But put a little
fudge factor in there because there is a lag in the whole process, so peak thermal production usually
occurs a half hour to an hour after maximum sun height. Speaking of sun height, we should all be
aware that June 21, when the sun is at its peak height, and December 21 when it is at its low point, are
the acme and nadir of thermal production, all other factors being equal.
Clouds affect solar heating of the surface and thus thermal production simply by blocking the sun’s
rays and scattering or absorbing the energy. Cumulus clouds denote thermals rising, so we are happy
to see them around as long as they don’t throw a wet blanket on our fun by overdeveloping into
sunshine-robbing shrouds. Clouds in general reduce the strength of thermals, as well as their
abundance. They also alter thermal behavior. A broad, weak, stratus cover may make the day less
punctuated with thermal exclamation points, but also make the thermals more regular as the thermals
spend more time building on the ground and are less interrupted by vigorous, cool downdrafts. We’ve
also seen it happen that the approach of a stratus layer is accompanied by pre-frontal unstable air, so
the thermals actually get stronger even as the solar insolation weakens. So, you can never talk
absolutes in this game, which is what makes it a game in the first place.
WHAT YOU CAN USE
This article speaks mostly in generalities in order to set the stage for our later discoveries. However,
we can still glean a few straws of learning from the general discussion. Perhaps the main point to
recognize is that at many sites it is a normal process for the first thermals of the day to happen in the
morning, anywhere from 10:00 AM to 11:59 AM. Then, after a flurry of thermal activity, things die
down and nobody stays up until a bit later when the thermal day begins in earnest. It is important to
recognize this occurrence, because you don’t want to be the early bird who gets to be in the landing
field feeling like a worm. Learn to understand the behavior of your own site(s) in this regard. Does it
happen nearly every good thermal day? Does it never happen? What are the conditions when it does
happen? (Hint: Clear nights with little upper wind, so a deep ground inversion forms. Note that these
are the same conditions conducive
to dew and frost formation.)
Once you have figured out your
sites, carry your newfound
awareness with you when you visit
other sites. As you gain knowledge
and experience you will perhaps
be able to predict thermal behavior
at other sites. It is this type of
understanding that helps create
great pilots, for after all, a great
pilot is just like you and me but with more skill, more knowledge and more luck. I just wish there were
some way to work on the luck factor.
We bypassed the discussion of lapse rates to avoid over-complicating this first installment. But
next time we will give the subject its due, because it is important to the understanding of how thermals
really work. For more information on the matter of solar heating, and thus thermal production daily
variation, see Understanding the Sky, beginning on page 189.
.
Thermal Lore - Part 2
Thermals are children of the sun. They cavort and leap
about on a bright day like a schoolboy full of piss and
vinegar after long sessions of math, Latin and etiquette.
They dance in the hall of the sky to the tune called by
the wind and clouds. To understand the dervish tempo
and raucous nature of their dance, we must know
details of their upbringing and surroundings. To
illuminate our metaphor, we must comprehend the
meaning and effects of the lapse rate.
I have tried to put off discussing the subject of
lapse rate for as long as possible, for you have no doubt heard the expression, “He who lapse last,
lapse best.” But we can no longer dodge the inevitable: To really know the heart and soul of thermals,
we need to grasp their intimate involvement with lapse rate.
MEANING REVEALED
It is not hard to understand the basics of
the lapse rate if we simply realize it is a
graph of the air’s temperature at heights
from the surface upward. We can also call
such a graph the air’s temperature profile.
A typical morning lapse rate or
temperature profile may appear as in
Figure 1. Let’s look at some details to see
what we can learn. First, we notice that
near the ground the air’s temperature is
cool (55° Fahrenheit in this example). As
we go higher, the temperature actually
gets warmer up to the 1,000-foot level
(70° F), in this example. Then the air
cools off with altitude up to 3,000 feet.
Following this, it cools rapidly with
altitude until we reach an altitude of 5,000
feet where it actually gets warmer with
altitude again. Finally, above 5,500 feet,
the air again cools off with increased
altitude.
The air’s normal temperature change
with increasing altitude is to get cooler.
That’s because nearly all the heat in the
air comes in at the bottom from surface
heating. But moisture, movement and
pressure systems serve to alter this “normal” picture as we shall see next month. For now, note that
when the air cools off rapidly with height, the conditions are known as unstable because they promote
the rise and continuation of thermals. In other words, the atmosphere is folding itself inside out as
thermals climb and cool air plummets.
The opposite atmospheric condition — when the air warms with height, or doesn’t cool
significantly — is known as a stable condition. In this case, thermals are suppressed faster than tort
reform talk at a lawyer’s convention. Any wayward upward puff quickly dies out so the atmosphere is
not active vertically (even though a wind may be blowing). It lies there limp as an overly liquored
lover and is about as disappointing to a soaring pilot.
Why do thermals rise in an unstable environment and fall back to earth when it’s stable? For an
answer free of numbered (or numbing) details, note that the air pressure falls off with increasing height
because there is less air above pushing down. You can feel that effect when your ears pop as you go up
in an airplane, a tall elevator or ski lift. A rising glob of air (a thermal, for example) experiences the
reduced pressure as it rises as well, and thus it expands. Its head swells so to speak. As it swells it
cools off because the same amount of heat energy is distributed throughout a greater volume.
But the reason it began its rise in the first place was because it was heated at the surface, expanded
and became less dense than its surroundings. Consequently, the glob will rise as long as it remains less
dense than the surrounding air, which in general means warmer. Thus, when the air’s temperature
drops quite a bit with increasing altitude, the glob always remains warmer or less dense than its
surroundings and continues to rise, even though it is cooling as it is rising. The glob in this type of
environment is what we call a thermal.
In the opposite condition — when the surrounding air doesn’t cool off at the rate the glob is
cooling with increased height — the glob eventually reaches the same temperature and thus density as
the surrounding air and is no longer buoyant. This condition is the stable situation.
Note 1: Lapse rates can be quite varied, so conditions can be anything from extremely unstable to
neutral to extremely stable. You can imagine the different buoyancies or upward impetus of thermals
in these varied air conditions. The amount of available heating and wind in combination with the lapse
rate is what determines the nature of the thermals of the day.
Note 2: For details on actual temperatures and changes of thermals with height, see Understanding
the Sky.
Now let’s return to our figure. In view of our previous discussion, we see that the lower layer (to
500 feet) is very stable because it gets warmer with height. We call such a layer an inversion because
the situation is the inverse of the normal cooling with altitude. The inversion that occurs at the ground
is called (what else?) a ground inversion.
A bit higher in our figure, the air cools off quite a bit with altitude, and is labeled unstable. Higher
still its cooling is reduced to the point that the air is stable. Then we come to another layer that warms
with altitude (at 5,000 feet) which is another inversion.
Finally, above that the air is again unstable.
LAPSE RATE CHANGES
Since we know the thermal
prospects change from day to day,
it doesn’t take an Airistotle to
figure out that the lapse rate itself
changes. Let’s see how these
changes occur and how they affect
thermals.
The most obvious change in the
lapse rate is caused by the sun’s
great variation in heating as the 24hour day progresses. At night, as
the earth’s heat radiates off, the air
at the surface is cooled so the
ground inversion of Figure 1 is
formed. The thickness of this
inversion depends on the extent of
cooling (how clear the air is and
whether or not clouds block the
radiation process). Also, in
mountainous areas, additional cool
air will cascade down the mountain
sides to increase the pooling of cool surface air.
In the morning, this surface air begins to heat from the bottom up. Small plumes of warmed air rise
a bit and mix the warm air upward. This process is shown in Figure 2. In our example, the lapse rate is
being changed from the bottom and appears as a dashed line at various times in the morning. By 11:30
AM, we see that the spreading of warm air upward by convective mixing has wiped out the ground
inversion. In fact, it no longer exists once the surface temperature has reached about 73° F (in this
example). As the surface heats even more, any warm blob rising from the surface finds itself warmer
than its surroundings well past the 1,000-foot level and continues on upward. The warmer the blob, the
higher it goes, as shown.
When the surface temperature reaches 73° F in Figure 2, thermals rise rapidly in height. This
magic number is called the trigger temperature. (Of course, the trigger temperature varies daily and
from place to place, depending on the thickness and temperatures of the ground inversion.)
THERMAL HEIGHTS
How high do the thermals go? Figure 3 illustrates the possibilities. In case 1, they rise until their
cooling results in their temperature equaling (approximately) that of the surrounding air. The warmer
the surface gets, the higher they go. You can see one reason why thermals in the Western U.S. are
much taller than their Eastern counterparts.
In case 2, the thermal never cools to the point of equaling the surrounding air temperature, but
gradually erodes to nothing as it climbs higher. The erosion process is due to the mixing caused by
friction at the thermal’s edges and entrainment of outside air into the thermal (more on this process in
a later installment). The situation in this case often occurs when there are weak thermals (those
produced under a layer of high cirrus, in hazy conditions, over water or in winter). In this case, the
larger the thermal, the higher it will climb.
In case 3, the thermals reach an inversion layer and get the brakes put on as if they were trying to
penetrate molasses. We have also shown one situation where the thermal enters an inversion layer,
gets slowed, but bursts through the top, then continues rising. In this case it may continue upward until
it erodes away, meets another (higher) inversion or forms a high cloud. Only the strongest thermals
will burst through a thick inversion in this manner, so the clouds and thermals above an inversion will
usually be few and far betwixt.
Finally, we have case 4, in which the thermal reaches the dew point level and forms a cumulus
cloud. The dew point is the temperature at which the water vapor contained in the thermal condenses
to water droplets. The millions of water droplets are visible as cloud. Once cloud forms, great reserves
of heat energy are released (this energy is the latent heat of vaporization stored up when the surface
moisture evaporated) so the thermal becomes roiled and mixes rapidly with its surroundings. This
mixing with cool ambient air soon spells the demise of the lift in that particular area unless a
continuous font of thermals is feeding the cloud.
What determines the height of the dew point, and thus cloud base? The answer is the humidity of
the air and the actual air temperature (warm air can hold more water vapor than cool air). With talent
we can learn to predict the height of cloud base (or whether or not clouds will form) by taking the
surface dew point displayed on a chart called a skew T or a tephigram. By running the surface relative
humidity value up a line called the constant energy line, we find the dew point where it crosses the
lapse rate. However, this technique is beyond the scope of this series.
We should note that cases 1, 2 and 3 result in blue days (no cumulus clouds forming from ground
sources). Thankfully, case 4 occurs often enough to give us frequent skies marked with soft, white
stepping stones guiding our aerial paths.
Hopefully the above concepts give you the idea that we can predict the thermal height of the day if
we know the air’s temperature profile (now available on the Web for most of the country), the
maximum predicted surface temperature, the surface dew point and how much a thermal cools as it
rises. This latter value is 5.5° F per 1,000 feet of rise. (You knew we’d slip numbers in eventually,
didn’t you?)
Now here are a couple of wrinkles in the process. As the day progresses, the earth’s surface tends
to dry out as thermals wick moisture upward. Consequently, cloud base rises higher (the dew point
moves up) since the thermals’ water vapor content is less. A typical daily process is shown in Figure 4.
We have already seen that thermals rise higher as the surface temperature rises, so these combined
effects produce higher rising lift until a peak at about 3:00 PM. Of course, this typical cycle may vary if
heating is delayed, or excess clouds reduce heating. Next month we’ll see the effect such a process has
on inversion layers.
The final point to understand is that thermals are like massive hot-air balloons that have great
inertia. It takes a while to get them started and they are reluctant to slow down once they are buoying
upward. As a result, they penetrate quite readily as much as 1,000 feet above the height where they are
no longer warmer than their surroundings. They always penetrate a distance into inversions (while
usually becoming broken) and may punch through a weak one. In fact, many studies have shown that
by the time a thermal has reached two-thirds to three-fourths of its maximum climb, it is no longer
warmer than its surroundings, although it still rises by virtue of its inertia and water vapor content
which makes it a bit less dense than its surroundings. As a result of these factors, determining the
height of the lift based on the lapse rate chart will always underestimate the actual experienced level. It
takes a practiced predictor to guess the correct values.
WHAT YOU CAN USE
Think about how the lapse rate changes from night to day and realize that just because the night is
clear and cold doesn’t mean that the next day will be one of great thermal production. What is
important is the (in)stability of the air above the ground inversion. I recall one day during the East
Coast Championships in the Sequachie Valley. Everyone thought we were going to have a great day
since the air was clear, crisp and cool. A few anemic morning thermal currents came up to stir our
juices, but the air mass was stable and we sled rode all day long. Someone could have written a thesis
on group depression that day.
Part of the fun of flying engine-free is taking what you can find and making the most of it.
However, if you are a pilot with limited time resources (read family obligations), and you have to
choose your days, it behooves you to learn to read the lapse rate diagrams to predict the days with
good conditions. In order to do this effectively, you must understand the principles.
Another useful point is to note that the nature of thermals of the day is greatly determined by the
nature of the surrounding air. So, the sooner you figure out what the thermals are like (wide, narrow,
short, tall, turbulent, smooth, tilted, shifty, multi-cored, strung out downwind, continuous, short-lived,
pumping, infrequent, etc.), the better you will be able to exploit their gift of lift. The lapse rate changes
slowly during a day’s progress, so the nature of the thermals changes slowly as well.
Finally, when you begin your flight, it is very wise to scope out the type of “top conditions” you
encounter. If you determine that an inversion is stopping the thermals, you can work hard to punch
through it and possibly get hundreds or thousands of feet above those bouncing up against the ceiling.
The technique for doing this will be discussed in our final installment.
Thermal Lore - Part 3
My first thermal encounter was unwitting, as in, “What the hell is this?” The experience occurred
at a dinky 450-foot former ski area in the spring of 1975. We flew that northwest slope nearly every
weekend and I eventually opened a school there. I was flying a standard hang glider (four poles and a
rag wing) that weighed only 35 pounds. We would hump the gliders up the hill and take as many sled
rides as our youthful exuberance could stand. I had learned to ridge-soar the previous winter and knew
that I needed a stiff breeze to stay up, given the typical 400 fpm sink rate of our gliders. So on that day
it was sled rides in the 10-mph wind wafting straight in. It was my turn to launch and I edged to the
edge. Just as I began my launch lunge, a major gust caught my wing and carried me and the whole
ensemble upward. Naturally, I was going too slow and was turned. In a jiffy I was aiming back at the
mountain, but lifted above the tops of the trees. Yikes!
I recovered from my surprise and completed the 360 to fly out to the front of the mountain. The lift
was so strong, that by the time I cleared the ridge I was easily 100 feet above and climbing. Wanting
no part of this robust air, I white-knuckled the bar and flew straight ahead to safety. The funny thing
was, I kept climbing and climbing under a long roll of clouds. The farther I went, the higher I got.
Eventually, I climbed to about 2,000 feet above the ridge and continued forward to land more than a
mile upwind. By turns, I was surprised, then scared, then relieved, then full of bravado by the time my
friends arrived to marvel at the flight. We were sure it was a record of some sort. Looking back, I
realized that we all learned a lot from that experience, and someone even mentioned the possibility of
something he had heard about called thermals. Now I know I was under a street, strung out for miles.
In hindsight I also know that it was one of the best-looking X-C skies I have ever seen in these parts.
My first witting (as in intentional) thermal flight came in June of 1976. 1 was attending the first
invitational meet at Grandfather Mountain in North Carolina. Several of us were soaring in light air on
the northwest-facing cliff. The wind wasn’t strong, but plenty of buoyant lift was sliding up the
mountain from the warm valley. Most of us were running back and forth along the half-mile ridge, but
I noticed that two pilots, Steve Moyes and Rollie Davies, were flying out in front and turning a series
of 360’s. By the time they were back near the mountain, they were hundreds of feet above us. The
light bulb went off in my head. I realized exactly what they were doing, and the next time they went
looking for a thermal, I followed. We didn’t have varios back then, but could feel the surge of lift and
tell I was climbing by watching the mountain. I tried to match their circles and, miraculously, I was
thermalling. I can’t describe the thrill of that moment. I repeated the experience for nearly an hour, but
can honestly say that I learned to thermal in the first 15 minutes of realization and exploration and
have been a devotee of augering upward ever since.
Sometimes it only takes forming the correct model in your head to let you catch on to a skill or
concept. In fact, the main thing we are trying to do with this series of articles is to form a good
working model of thermals in our image databank. The better images we hive to work with, the better
we will perform when globular lift beckons. So, we continue with our exploration of the world of
thermals where we left off last time.
INVERSION BEHAVIOR
Last month we investigated lapse rate and inversions, as well as their effect on thermals. We’ll
begin here with a bit mire about inversions and then look at some details of thermal creation. The first
question we should answer is, How are inversions formed?
As we learne4 in the previous installment, inversions are layers of air in which the air temperature
does not decrease with increasing altitude, at least to the degree necessary for instabi1ity to occur.
We also saw how this feature most readily occurs near the ground through the process of nighttime
cooling. But we also encounter inversions aloft. At competitions, the air’s sounding (lapse rate
graph) is often presented. It is not uncommon to see three inversion layers at different levels up to
the altitude of common local cloud base. (Whether or not the clouds reach that altitude depends on
whether or not the thermals can punch through the various inversions.) These inversions are very
important for thermaling and cross-country prospects. They can gradually disappear or intensify.
Many inversions higher in the air come from the sliding of warmer air over cooler air that inhabits
an area. This is the case when a warm front approaches. But even with a cold front, a layer of warmer
air aloft is usually left as the cooler air plows under the warm. Cold fronts are typically limited in
vertical extent, so on top they are capped by a warmer flow. If you look at charts or the wind flow near
a front at different levels, you will see that aloft, the warm air is not being pushed out of the way by
the cold air as much as it is at the surface.
Another cause of inversions is sea breezes moving inland. Usually these sea breezes act like mini
cold fronts and move cooler air in under the existing warm air. Multiple sea breezes on succeeding
days can cause inversions at different levels. Next we should mention the effect whereby
mountains block lower flows and allow warm air moving into an area to pass over the mountain
and thus above the cooler air on the other side. All of these causes may come into play in certain
areas so that a complicated lapse-rate profile with multiple inversions of different strengths and
thicknesses occurs. Below we’ll describe how thermals create inversions, which is perhaps the
most important cause.
INVERSION CONVERSION
There
are two
factors that
affect
inversions.
The first is
the
widesprea
d vertical
movement
of the air,
and the
second is
thermals
themselves
. There’s a
general
rule that you can bank on: The air rises in and around a low-pressure system, and it sinks in and
around a high-pressure system. In most of the U.S., the passage of a cold front means the arrival of
cooler, unstable air driven by a high. Typically, one to three days of good thermal production follows
the front and then things get more stable as the high approaches. What’s happening here?
First we should note that although thermals may be rising vigorously in a high-pressure system, the
general air mass is sinking at a rate of about an inch or two per minute. This sinking is caused by
the air at the bottom of a high-pressure system flowing outward. This effect is shown in Figure 1.
As a layer of air sinks, it becomes warmer due to compression caused by greater pressure. It also
becomes more stable. When the opposite happens — a layer is lifted by some mechanical process,
such as due to frontal movement or over a mountain — it becomes cooler and less stable. The
cause of “mackerel sky,” with its array of alto-cumulus or cirro-cumulus clouds, is the result of
thermals that are born
high in the sky due to
the lifting of a layer
until it auto-convects.
But right now we are
interested in highs (the
weather kind, so we can
achieve the glider kind).
The effect of the sinking
air is to compress the
layers of air (as they move
lower they have more
weight above them), alter
the lapse rate and narrow
the inversion layer while at
the same time lowering it
and essentially intensifying
it. These effects are shown
in Figure 2. Here we see
the lapse rate on three
successive days. On the
second day, the general
lapse rate isn’t as sloped as on the first. That means it has become more stable. Also, it is moved to the
right which indicates that the temperature is warmer at any given altitude (this warming is exaggerated
for clarity). We can clearly see that the inversion has moved lower, and become narrower and
intensified. This intensification is the result of it having become more stable (slanted more to the
right).
Most of us know that high-pressure systems bring stable air and usually weak, nonexistent or
punchy thermals. Now you see why. The mass is stable, and normal thermals simply die out in the
stable air, or are stopped by the inversion layers that get lower and lower. Particularly strong heating at
the ground may produce a thermal that rises for a good ways, but it will be quickly eroded, so only the
strongest portions push upward and these portions will be well mixed (read turbulent). These highpressure thermals should also seem familiar to coastal pilots, since they are very similar to thermals
after a sea breeze has passed. The sea breeze is a thick layer of stable air moving inland from the sea.
The reason this mass is stable is precisely for the same reason high-pressure masses are: the air has
sunk from aloft to the surface (out to sea in this case). Sea breezes are beyond the scope of this series,
so those who want to know more about this important facet of our flying should consult Understanding
the Sky.
Readers with good memories may recall the story we told last month describing the day a hopeful
flock of pilots expected the clear, crisp weather to deliver them a cornucopia of thermals. All they
found was a bright, sunny, dreary day of dead air. This occurrence was precisely due to a big fat high
squatting over the Eastern states. The air was cold and heated well from below, but since it was stable,
thermals didn’t rise very high. It should be clear to us that lingering highs are a bane to good,
wholesome thermal flying.
But there is some solace in high-pressure systems. The fact is, since they lower an inversion layer
within the high, eventually the inversion reaches the ground and becomes part of the ground inversion,
to be wiped out by the next day’s surface heating. In that way, a couple of days after a high-pressure
system has hung over an area, conditions may suddenly get better again. Of course, we have described
a weeklong process, considering the one- to three-day good soaring followed by stable air, then the
return of good unstable conditions. The one thing we are all aware of is the variability of the weather,
so the scenario we described is only a common possibility, not something on which you can rely.
Often there is only one day of good soaring post cold front. Equally often, the inversions don’t have a
chance to reach the ground because some other weather disturbances move through to start the cycles
of warm front, cold front, warm front, etc., all over again.
THERMAL EFFECTS
As indicated above, thermals also have an effect on inversion layers as well as lapse rate. Think
about the curriculum vitae, the résumé
of a thermal. It is designed solely to
wick heat away from the surface on a
sunny day. Without thermals, heat
would build up to an unbearable level
(our northern climes would be like the
steamy tropics, which themselves
would be unbearable without
thermals). So where does all this heat
go? Up in smoke, of course. It gets
transplanted to the air at various
levels. Let’s start from the bottom up
to get the picture.
We saw in the last installment of
this series how the warming of the
surface and convective stirring
eradicates a ground inversion, usually sometimes in the morning. Then, when thermals trigger, the
mixing begins higher and higher as the thermal ceiling rises. So, thermals distribute heat upward, mix
with the surrounding air as they rise, and thereby alter the lapse rate. But we learned last time that
thermals are no longer warmer than their surroundings after they rise to two-thirds or three-fourths of
their maximum height. Thus, the heat redistribution doesn’t go as high as the thermals. In Figure 3 we
have illustrated some of the principles described.
It should he clear that the lower few thousand feet above the surface will be warmed by the
constant passage of thermals. The presence of downdrafts bringing cool air from aloft toward the
surface spreads out the heating and mixes the air, so the change in the lapse rate is not as great as it
would be if this mixing did not occur. But the net effect is to warm the lower atmosphere and actually
make the lapse rate more unstable as shown. But the rub is, a thermal must be heated to a greater
temperature in order to begin rising in this more unstable environment. So the thermals take longer to
heat, become farther in between, but rise more vigorously once they do rise. This effect and the change
in heating as the sun moves accounts for the difference in thermal strength and frequency as we go
from morning abundance of weak thermals to afternoon increase in strength but decrease in frequency.
The sudden evening cutoff of thermals occurs when the sun’s radiation no longer can raise the ground
temperature above the trigger temperature. Residual heat may still release an occasional late thermal if
something can trigger an initial rise. That something is usually cool air sliding down a slope in shadow
or out of a canyon.
When thermals
enter an inversion
layer they can
intensify it, if it is
sufficiently low
(so the thermals
still have excess
heat) and strong
(so the thermals
don’t punch
through it). On
the other hand,
thermals can wipe
out or reduce the
strength of an
inversion. To see
how this happens,
look at Figure 4.
Here we see some
thermals strong
enough to pass
through the
inversion, and
some being stopped in its clutches. The strong ones pass through, entrain air with them and
produce a general mixing that can thicken an inversion and thus make it less intense. Even the
thermals that are halted produce some mixing with the layers of air above and below the inversion,
so the inversion is rendered less intense if the thermal isn’t warmer than its surroundings.
But the major effect that thermals have on inversions is to create them in the first place.
Remember, we noted that thermals lose much of their excess heat as they rise and may simply erode
away to nothing. However, often they reach the dew point or condensation level and form cloud.
When cloud forms, the water vapor changing to water droplets releases a good deal of heat energy
(called the latent heat of vaporization). This heat raises the temperature of the surrounding air as the
cloud mixes with it vigorously. Now, this heat is not free money in the bank, but is only on loan, for as
soon as the cloud starts evaporating, heat is again taken in the evaporation process and the surrounding
air cools, then often sinks. That would be the end of the story, since the cooling would be as much as
the initial heating, except for our good friend, the sun. Water vapor is greatly invisible to the sun’s
rays, but water droplets are not. The sun heats the clouds itself and thus provides added heat energy to
the area. So there is some residual heat left when the cloud evaporates. This heat builds up at the cloud
formation level throughout the day and, voila, we have an inversion layer.
You can readily see that an inversion layer formed in this manner will persist through the night
(there is nothing to cause an exchange of heat) and into the next day. If thermals don’t reach as high
the next day (perhaps the mass has moved over more moist ground so cloud base is lower), a separate
and lower inversion may be formed. In this manner, multiple inversion layers will be created.
No doubt inversion layers, like most things in the atmosphere, are more complex than we normally
think, but they are extremely important to successful and excellent flying, so it behooves us to
understand them as much as possible.
OUR WESTERN FRIENDS
We have been speaking of fronts, relatively low cloud bases and multiple inversions. The last two
factors are often rare in the high desert area of the U.S. West, so let’s see what modifications are
needed for our model to apply. To be sure, near the West Coast, you can encounter inversion layers
accompanying the sea breeze, producing the famous LA smog which contributes to road rage and the
genetic defects that result in Valley Girls. But further inland the inversions occur mainly when
mountains trap cool evening air in a layer so thick that the day’s heating cannot produce thermals
strong enough to bust through. This effect happens most often in the winter with weaker sunshine.
Picture Salt Lake
City for a model of
this behavior.
For the most
part, Western
conditions create
what is known as a
heat low. This
process is very
similar to that
which takes place
in the sea breeze.
A local area gets
heated. The air
expands and flows
away high aloft
due to the “bulge” effect (see Figure 5). Once air flows away aloft, the pressure at the surface is
reduced (thus the term “heat low”) and a lower-level inflow occurs. The process continues as long as
the. heating continues. There is a net effect of slowly-rising air over a widespread area. This slow
rising would be an anathema to flying in the moist East, since cloud would soon form and block the
sun. However, in the thirsty West, the rising air produces little cloud and thermals are greatly
enhanced.
The general rising air, in combination with the dryness of the air (more solar heat), and the often
bare ground is what accounts for the vigorous (and sometimes violent) thermals compared to their
Eastern kin. There are few inversions formed in the high desert because the thermals don’t often reach
a dew-point level, and if they do, the gradual rise of the air mass weakens them or puts them out of the
reach of the next day’s thermals. Note that heat lows can be as small as a single field, or multi-state in
expanse. In the summer, a heat low typically sets up that covers the entire front range of the Rockies,
for example.
WHAT YOU CAN USE
Perhaps the main idea to take away from this installment is that inversions are a common enough
occurrence that we should understand their cause and effect. If you only fly in the Owens Valley in the
middle of summer, then perhaps you can ignore inversions, but the rest of us need to study them so we
can avoid their worst disappointments. By learning how inversions change from day to day, we know
better what to expect on a given day according to what went before. If you have access to the
soundings (lapse rate) for your area, you can look at what was displayed compared to what you
experienced. In time you will be able to see how thick and intense an inversion is, and figure how
likely it is to stop thermals at its level. This judgment in turn lets you know whether or not it is worth
your effort to try to punch through the inversion.
If you are in the area of the country where fronts and high-pressure systems affect your flying, you
are also in prime inversion territory. Learn how the high changes the stability of the air as it lingers
in the area. Watch for the times when instability returns to the area and judge where the high is and
how long it took for the change to occur. You don’t have to go flying to detect these changes since
you can judge thermal production by the gustiness on the ground, as long as strong winds aren’t
around (which they probably aren’t since a high typically brings light winds). Now you have a
good reason to be staring out the window while at work. Tell your boss I said it was okay.
Almost all thermal pilots have flown through inversions. That’s what’s happening when the
thermal slows down and things start to get bumpy. There are real useful techniques for staying with
the thermal and hopefully punching through the inversion. We’ll describe those techniques when we
get to the flying part of this series.
We have struggled through perhaps the driest part of thermal lore. But a good basis in how all this
works will help you figure things out on the fly so you can make good decisions when things go awry.
In the next installment we will moisten matters up by looking at real thermals.