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F. E. 'Fred' Potts |
This article originally appeared in IFR MAGAZINE.
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| About the Author ... |
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Fred Potts is the author of the award-winning book F.E. Pott's
Guide to Bush Flying — Concepts and Techniques for the Pro.
I heartily recommend this book to all pilots, even if you only fly spam cans
on paved runways! It'll change the way you fly.
Check out Fred's remarkable web site at
http://www.fepco.com/.
Parts of Fred's book are online on his site, as are some of his truly
exceptional collection of Alaska photographs.
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From a mountain clearing, safely ensconced within a strong and comfortable
cabin, a thunderstorm at full throttle is an awesome and beautiful
sight. But from a small aircraft in flight, with thunderstorms
in all quadrants, the sight loses much of its beauty; and if in
IMC with embedded cells, there is no beauty at all.
Thunderstorms are highly developed cumulus clouds (think of them
as having an overdose of testosterone), and cumulus clouds, because
of their very nature, provide a bumpy ride. In their tops there
is often ice, and as they develop they reach a point where precipitation
begins. This precipitation is what airborne radar detects; unfortunately,
precipitation is not a reliable indicator of turbulence and it
is the turbulence in a thunderstorm that can kill us.
This is where the Ryan/3M/Goodrich Stormscopes (and Insight's
StrikeFinder) come into the picture. These instruments detect
and analyze the electromagnetic fields produced by lightning through
evaluation of their individual signatures, a science known as
sferics. And lightning shows a strong correlation to turbulence.
However, to avoid this turbulence, it is necessary to know a bit
about the various stages in the life of a thunderstorm, for both
radars and Stormscopes require knowledge and understanding if
one is to correctly interpret their subtleties and idiosyncrasies.
Of the various weather mapping instruments available, I've found
the Series II Stormscopes to be the easiest to understand and
use. In this article I will be focusing primarily on the WX-1000+,
the unit I chose for my Cessna Turbo 206. Because the manuals
fail to cover the finer points of Stormscope operation, I'll try
to rectify this shortcoming by reporting on my own experience
with the Series II. It should be noted, however, that the Pilot's
Operating Handbook takes precedence over anything written here.
What the Stormscope sees
Put simply, as cumulus clouds develop in the unstable air, they
penetrate the freezing level and the moisture in the updrafts
becomes supercooled. These cooler temperatures, combined with
the weight of the moisture, create downdrafts. Between the updrafts
and downdrafts, an area of convective wind shear develops. Here
you will find strong up and downdrafts (often reaching 2,500 to
3,000 feet per minute), possibly severe turbulence, and icing.
Because precipitation has usually not yet begun, radar is of little
help..However, electrical activity generally has started at this
stage, and while the developing thunderstorm is not yet visible
to radar, it is visible to devices that map the electromagnetic
signals (lightning, either cloud-to-cloud or cloud-to-ground)
that these building storms produce.
At the earliest stages of development a light accumulation of
discharge points will begin to appear on the Stormscope's CRT.
These discharge points represent lightning strikes and are shown
in their calculated positions, both by azimuth and distance. (With
the Series II Stormscopes, distance accuracy—using verified ground
triangulation—has been shown to be within 10 percent.)
At first, there won't be many of these discharge points. If the
"Clear" button is used, they will be slow to return.
But after a while, they will begin to increase in both number
and rate of build-up, taking the form of a small but growing cluster.
This growing cluster (a rough indication of the size and shape
of the storm) is a clear sign that the cumulus is beginning to
mature into a thunderstorm. And it is such clusters that must
be avoided.
Keep your distance
Keep all clusters at least 25 nautical miles from your airplane.
This range is outside the safety circle, which is shown on the
Stormscope as a solid circle when it's in the 360-degree mode.
When using the 120-degree forward display while close to the storm,
keep the clusters not only outside the 25-mile safety arc, but
outside the 30-degree lines as well. This will not guarantee you
a smooth ride, but it will help keep you out of the truly dangerous
turbulence.
As the storm matures, the moisture (whether rain, hail, sleet,
or snow) becomes heavy enough to fall against the force of the
updrafts. This precipitation fuels the downdrafts even more. The
storm (which would now be visible on radar if you had it) is moving
into its most dangerous phase, and at the surface the downrushing
air spreads outward in strong gusts accompanied by a sharp temperature
drop.As the storm gains strength, the discharge points on your
CRT will start to scintillate (flicker): their rate of growth
and scintillation is an indication of the storm's severity.
If random-appearing discharge points seem to "splatter"
around the aircraft symbol with an active cell nearby (perhaps
just inside the safety circle), it is an indication that you are
much too close to that storm. Immediately turn away. Be aware
that any grouping of discharge points within the 25-mile safety
circle is cause for concern.
Eventually entropy, the nemesis and fate of all organized matter,
catches up with the thunderstorm. As the downdrafts cool the
air in the cell, they cut off the flow of heated air to the updraft.
This weakens the updraft, and therefore the convective wind shear,
and gradually the electromagnetic activity dies down. This signals
the oncoming death of the storm. Soon all that will remain are
harmless drifting cloud remnants.
On the Stormscope's screen, the strikes will slowly begin to fade,
and two to four minutes after the electrical discharges cease,
the storm will drop off your CRT. There's often intense rainfall
at this stage, but the Stormscope will not show this, just as
it does not show the presence of hail, or the various forms of
turbulence not associated with electromagnetic discharges.
If you're on top of the situation, you will have been keeping
track of the various cells in your vicinity and marking their
direction and speed of movement. If you are interested in staying
out of the potential drownpour, you will know exactly what area
to detour around even though the cluster has disappeared from
your CRT.
Lines and complexes
The above has been a description of a simple airmass thunderstorm
and its life-cycle, along with a description of what you are likely
to see on a Series II Stormscope's CRT. But, of course, in the
real world thunderstorms, being sociable, like company, and so
are often found in lines and groups in various stages of growth.
And, while the FAA and NWS classify thunderstorms by six levels
(using the precipitation returns from radar, actually an indirect
measurement), and meteorologists like to divide them into four
general classifications, experienced thunderstorm researchers
tend to break them into three classes. Jerry Smith, research pilot
for 3M, classifies them as little, medium, and killer. Dennis
Newton, in his fine book Severe Weather Flying, is a bit more
colorful, calling them Baby Bear, Mama Bear, and (the Big Daddy)
Papa Bear.
As Newton observes, Papa Bears generally travel in gregarious
packs, complete with Mama Bears and Baby Bears, and that would
surely seem to be enough of a family get-together for anyone.
Yet these family groups also have another offspring, a lovely
child we call a tornado.
Radar has a long history behind it, and we have gotten used to
spotting tornados by the infamous "hook" return. But
the Stormscope's history is far shorter (sferics is, after all,
a fairly new science), and the established knowledge base is much
smaller. As a result, we must use indirect means to locate tornados
when relying in sferics devices, just as radar operators must
u se indirect means to gauge the strength of a thunderstorm.
Since I have never had the dubious pleasure of seeing a tornado
on my Stormscope, I am going to give you the description of someone
who has. Jerry Smith, 3M's research pilot, flew thunderstorms
for a living, and one night he described to me what a tornado
looks like on the Stormscope's CRT. To use his words, "It
goes wild." The dots (crosses) are coming in so fast, and
the scintillation (flicker) is so severe, that the cluster stands
out markedly from the normal clusters. However, Smith cautions
that while all the tornados he has seen produced this very rapid
repetition of dots and severe scintillation, he has also seen
violent thunderstorms that did the same, even though tornados
were not associated with them.
If you see an area on your CRT "going wild," press your
"Clear" button. If the pattern snaps right back, perhaps
even intensifying, treat it as confirmation of your worst suspicions
and do exactly what the primitive reptile brain buried deep within
the top of your spinal cord advises. Tornado or not, that's an
area to stay away from.
Understanding radial spread
Radial spread, an artifact of the Stormscope, used to be a real
problem with the early models. It required a fair amount of pilot
experience and skill to interpret correctly. The Series II is
much improved in this respect but radial spread does show up,
generally in three forms.
The most common is a sprinkling of spurious discharge points (crosses)
toward the center of the CRT from the main cluster of a strong
storm This phenomenon is quite obvious, and requires little interpretative
skill.
The next most common is a loose pattern of individual discharge
points off the nose of the aircraft at about the 200-mile circle.
This indicates that either a strong thunderstorm is just beyond
the 200-mile range or that electromagnetic discharges are arriving
by atmospheric skip from a distant storm that's well beyond the
instrument's range.
The least common form sometimes occurs when there is a strong
storm at about the 50-mile range. Suddenly a thunderstorm cluster
seems to pop up between you and it. This could be radial spread
or it could be a new, and fast-growing, thunderstorm. If in VMC,
looking out the window will tell you what's going on; if in IMC,
treat it as real and get it off to the side of one of your 30-degree
lines.
Random discharge and embedded cells
Random discharge points, which are often confusing to those new
to the Stormscope, are usually caused by atmospheric instability
associated with cumulus clouds, or developing/dissipating thunderstorms.
Use your CLEAR button regularly, and monitor the discharge points.
Dissipating storms will disappear; developing cells will build;
and cumulus clouds that are trying to make up their minds will
come and go indecisively.
Some pilots believe that embedded thunderstorms are less serious
than those that are not embedded. This type of thinking is a mistake.
Embedded cells are more than strong enough to get you on the 11
o'clock news and should be treated accordingly.
Embedded storms do not seem to put out as much lightning as airmass
or frontal storms; therefore even very small clusters or scatters
of dots are important. My advice is to keep all of them outside
the 30-degree lines when they are within 50 miles.
The big picture
When VFR, on a hot hazy day when the lifted index is showing minus
and the K-index is in the 20s to 30s, the Series II Stormscope
(set to the 100- or 200-mile range in the 360-degree configuration)
proves valuable for keeping up with the big picture. Some pilots
prefer to use the 120-degree forward range for the big picture,
for they are mainly interested in where they are going.
I feel somewhat differently. I like to know where my outs are
if things begin to get too interesting, and, in my experience,
outs are all too often behind or to the side of my course.
The Stormscope is not designed for storm penetration, it's designed
for avoidance. However, winding through a line or group of cells
is often necessary. Whether this can be done safely or not depends
on how close together the cells are.
When penetrating lines or groups of cells, keep all clusters outside
the 30-degree lines. If it begins to look as if this will not
be possible, then it's time to put Plan B into effect. (You do
have a Plan B, I hope.) My Plan B is usually to land and sit the
storms out in comfort. I'm always monitoring my chart and loran
for nearby airports. However, if matters got serious enough—being
well used to off-airport landings from years in the arctic—I
wouldn't hesitate to use a country road and, if necessary, tie
the plane to a farmer's fence.
Notes on briefings
I noted earlier that I've never had the dubious pleasure of seeing
a tornado on my Stormscope's CRT. That is because I plan carefully
for my flights when thunderstorms are likely, and stay out of
the dangerous areas. I am an avid fan of the radar summary chart
(which also show the severe-thunderstorm and tornado watch boxes),
and make a point of avoiding the critical areas when a watch is
current. I know that even with my Stormscope those areas are trouble
and will be a problem.
If possible, I plan a different route. If that is not feasible,
I scrub the flight until things improve. Most pilots interested
in thunderstorms also become interested in the stability charts
(lifted and K indexes), and it takes very little experience with
them to learn to predict the areas where trouble might arise later
in the day or perhaps during the early evening. Loyal fans of
the stability charts include Dennis Newton and Jerry Smith and
I recommend them highly.
Since thunderstorm research is still in its infancy, new discoveries
are being made daily. One of the more recent flashes of awareness
was that thunderstorms develop not only as individual airmass
storms or in lines, but that they also tend to congregate in large,
roughly circular or elliptical patterns. These groups often have
a diameter of several hundred miles and have a tendency to remain
fairly stationary for extended periods. The NOAA has labeled this
phenomenon the Mesoscale Convective Complex, or MCC for short.
If your briefing indicates that there is MCC activity in the vicinity
of your destination, give serious thought before launching.
Conclusion
It is my opinion that the Series II Stormscope is a safety device
of such importance that when thunderstorms are likely and the
trip is IFR (in a no-radar airplane), it's a go/no-go item. Even
in hazy-VFR weather, it is a valuable adjunct to visual avoidance
and well worth having. When visibility is 5 to 7 miles, trouble
can come up fast at the cruising speed of even a small single
like my Cessna 206, and forewarned is surely forearmed. If used
for its intended purpose, a Stormscope will allow you to complete
many flights safely that would otherwise not be possible.
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