From a mountain clearing, safely ensconced within a strong and comfortablecabin, a thunderstorm at full throttle is an awesome and beautifulsight. But from a small aircraft in flight, with thunderstormsin all quadrants, the sight loses much of its beauty; and if inIMC with embedded cells, there is no beauty at all.
Thunderstorms are highly developed cumulus clouds (think of themas having an overdose of testosterone), and cumulus clouds, becauseof their very nature, provide a bumpy ride. In their tops thereis often ice, and as they develop they reach a point where precipitationbegins. This precipitation is what airborne radar detects; unfortunately,precipitation is not a reliable indicator of turbulence and itis the turbulence in a thunderstorm that can kill us.
This is where the Ryan/3M/Goodrich Stormscopes (and Insight’sStrikeFinder) come into the picture. These instruments detectand analyze the electromagnetic fields produced by lightning throughevaluation of their individual signatures, a science known assferics. And lightning shows a strong correlation to turbulence.
However, to avoid this turbulence, it is necessary to know a bitabout the various stages in the life of a thunderstorm, for bothradars and Stormscopes require knowledge and understanding ifone is to correctly interpret their subtleties and idiosyncrasies.
Of the various weather mapping instruments available, I’ve foundthe Series II Stormscopes to be the easiest to understand anduse. In this article I will be focusing primarily on the WX-1000+,the unit I chose for my Cessna Turbo 206. Because the manualsfail to cover the finer points of Stormscope operation, I’ll tryto rectify this shortcoming by reporting on my own experiencewith the Series II. It should be noted, however, that the Pilot’sOperating Handbook takes precedence over anything written here.
What the Stormscope sees
Put simply, as cumulus clouds develop in the unstable air, theypenetrate the freezing level and the moisture in the updraftsbecomes supercooled. These cooler temperatures, combined withthe weight of the moisture, create downdrafts. Between the updraftsand downdrafts, an area of convective wind shear develops. Hereyou will find strong up and downdrafts (often reaching 2,500 to3,000 feet per minute), possibly severe turbulence, and icing.Because precipitation has usually not yet begun, radar is of littlehelp..However, electrical activity generally has started at thisstage, and while the developing thunderstorm is not yet visibleto radar, it is visible to devices that map the electromagneticsignals (lightning, either cloud-to-cloud or cloud-to-ground)that these building storms produce.
At the earliest stages of development a light accumulation ofdischarge points will begin to appear on the Stormscope’s CRT.These discharge points represent lightning strikes and are shownin their calculated positions, both by azimuth and distance. (Withthe Series II Stormscopes, distance accuracyusing verified groundtriangulationhas 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 numberand rate of build-up, taking the form of a small but growing cluster.This growing cluster (a rough indication of the size and shapeof the storm) is a clear sign that the cumulus is beginning tomature into a thunderstorm. And it is such clusters that mustbe 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 theStormscope 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, butoutside the 30-degree lines as well. This will not guarantee youa smooth ride, but it will help keep you out of the truly dangerousturbulence.
As the storm matures, the moisture (whether rain, hail, sleet,or snow) becomes heavy enough to fall against the force of theupdrafts. This precipitation fuels the downdrafts even more. Thestorm (which would now be visible on radar if you had it) is movinginto its most dangerous phase, and at the surface the downrushingair spreads outward in strong gusts accompanied by a sharp temperaturedrop.As the storm gains strength, the discharge points on yourCRT will start to scintillate (flicker): their rate of growthand 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 (perhapsjust inside the safety circle), it is an indication that you aremuch too close to that storm. Immediately turn away. Be awarethat any grouping of discharge points within the 25-mile safetycircle is cause for concern.
Eventually entropy, the nemesis and fate of all organized matter,catches up with the thunderstorm. As the downdrafts cool theair 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 signalsthe oncoming death of the storm. Soon all that will remain areharmless 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 rainfallat this stage, but the Stormscope will not show this, just asit does not show the presence of hail, or the various forms ofturbulence not associated with electromagnetic discharges.
If you’re on top of the situation, you will have been keepingtrack of the various cells in your vicinity and marking theirdirection and speed of movement. If you are interested in stayingout of the potential drownpour, you will know exactly what areato detour around even though the cluster has disappeared fromyour CRT.
Lines and complexes
The above has been a description of a simple airmass thunderstormand its life-cycle, along with a description of what you are likelyto see on a Series II Stormscope’s CRT. But, of course, in thereal world thunderstorms, being sociable, like company, and soare 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 indirectmeasurement), and meteorologists like to divide them into fourgeneral classifications, experienced thunderstorm researcherstend to break them into three classes. Jerry Smith, research pilotfor 3M, classifies them as little, medium, and killer. DennisNewton, in his fine book Severe Weather Flying, is a bit morecolorful, calling them Baby Bear, Mama Bear, and (the Big Daddy)Papa Bear.
As Newton observes, Papa Bears generally travel in gregariouspacks, complete with Mama Bears and Baby Bears, and that wouldsurely seem to be enough of a family get-together for anyone.Yet these family groups also have another offspring, a lovelychild we call a tornado.
Radar has a long history behind it, and we have gotten used tospotting tornados by the infamous "hook" return. Butthe Stormscope’s history is far shorter (sferics is, after all,a fairly new science), and the established knowledge base is muchsmaller. As a result, we must use indirect means to locate tornadoswhen relying in sferics devices, just as radar operators mustu se indirect means to gauge the strength of a thunderstorm.
Since I have never had the dubious pleasure of seeing a tornadoon my Stormscope, I am going to give you the description of someonewho has. Jerry Smith, 3M’s research pilot, flew thunderstormsfor a living, and one night he described to me what a tornadolooks like on the Stormscope’s CRT. To use his words, "Itgoes wild." The dots (crosses) are coming in so fast, andthe scintillation (flicker) is so severe, that the cluster standsout markedly from the normal clusters. However, Smith cautionsthat while all the tornados he has seen produced this very rapidrepetition of dots and severe scintillation, he has also seenviolent thunderstorms that did the same, even though tornadoswere not associated with them.
If you see an area on your CRT "going wild," press your"Clear" button. If the pattern snaps right back, perhapseven intensifying, treat it as confirmation of your worst suspicionsand do exactly what the primitive reptile brain buried deep withinthe top of your spinal cord advises. Tornado or not, that’s anarea to stay away from.
Understanding radial spread
Radial spread, an artifact of the Stormscope, used to be a realproblem with the early models. It required a fair amount of pilotexperience and skill to interpret correctly. The Series II ismuch 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 strongstorm This phenomenon is quite obvious, and requires little interpretativeskill.
The next most common is a loose pattern of individual dischargepoints off the nose of the aircraft at about the 200-mile circle.This indicates that either a strong thunderstorm is just beyondthe 200-mile range or that electromagnetic discharges are arrivingby atmospheric skip from a distant storm that’s well beyond theinstrument’s range.
The least common form sometimes occurs when there is a strongstorm at about the 50-mile range. Suddenly a thunderstorm clusterseems to pop up between you and it. This could be radial spreador 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-degreelines.
Random discharge and embedded cells
Random discharge points, which are often confusing to those newto the Stormscope, are usually caused by atmospheric instabilityassociated 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 willcome and go indecisively.
Some pilots believe that embedded thunderstorms are less seriousthan those that are not embedded. This type of thinking is a mistake.Embedded cells are more than strong enough to get you on the 11o’clock news and should be treated accordingly.
Embedded storms do not seem to put out as much lightning as airmassor frontal storms; therefore even very small clusters or scattersof dots are important. My advice is to keep all of them outsidethe 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 minusand 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 pilotsprefer 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 areif 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 designedfor avoidance. However, winding through a line or group of cellsis often necessary. Whether this can be done safely or not dependson how close together the cells are.
When penetrating lines or groups of cells, keep all clusters outsidethe 30-degree lines. If it begins to look as if this will notbe possible, then it’s time to put Plan B into effect. (You dohave a Plan B, I hope.) My Plan B is usually to land and sit thestorms out in comfort. I’m always monitoring my chart and loranfor nearby airports. However, if matters got serious enoughbeingwell used to off-airport landings from years in the arcticIwouldn’t hesitate to use a country road and, if necessary, tiethe plane to a farmer’s fence.
Notes on briefings
I noted earlier that I’ve never had the dubious pleasure of seeinga tornado on my Stormscope’s CRT. That is because I plan carefullyfor my flights when thunderstorms are likely, and stay out ofthe 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 iscurrent. I know that even with my Stormscope those areas are troubleand 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 interestedin thunderstorms also become interested in the stability charts(lifted and K indexes), and it takes very little experience withthem to learn to predict the areas where trouble might arise laterin the day or perhaps during the early evening. Loyal fans ofthe stability charts include Dennis Newton and Jerry Smith andI recommend them highly.
Since thunderstorm research is still in its infancy, new discoveriesare being made daily. One of the more recent flashes of awarenesswas that thunderstorms develop not only as individual airmassstorms or in lines, but that they also tend to congregate in large,roughly circular or elliptical patterns. These groups often havea diameter of several hundred miles and have a tendency to remainfairly stationary for extended periods. The NOAA has labeled thisphenomenon the Mesoscale Convective Complex, or MCC for short.If your briefing indicates that there is MCC activity in the vicinityof your destination, give serious thought before launching.
It is my opinion that the Series II Stormscope is a safety deviceof such importance that when thunderstorms are likely and thetrip is IFR (in a no-radar airplane), it’s a go/no-go item. Evenin hazy-VFR weather, it is a valuable adjunct to visual avoidanceand well worth having. When visibility is 5 to 7 miles, troublecan come up fast at the cruising speed of even a small singlelike my Cessna 206, and forewarned is surely forearmed. If usedfor its intended purpose, a Stormscope will allow you to completemany flights safely that would otherwise not be possible.