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 3M/BF 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 on 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 (Visual Meterological
Conditions), looking out the window will tell you what's
going on; if in IMC (Instrument Meterological Conditions),
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 (Visual Flight Rules), 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.
This article was first published in the October, 1990
issue of IFR Magazine, Copyright © 1990 by
F. E. Potts, all rights reserved.