Right up front, I must declare that I am partial to lightning detection devices, aka “spherics” receivers, for thunderstorm avoidance. There are two of these airborne devices in common use today, the Stormscope and the Strike Finder. [Since I have no reason to believe all spherics devices don’t have similar performance, I’ll use “Stormscope” the same way my Mom used “Frigidaire,” that is, as a generic term for all spherics receivers.]
About five years ago, I wrote a detailed essay comparing the Stormscope to radar in general and Nexrad in particular. It can be found in the “Members Only” portion of the SAFE website under “Weather.” Here, I will focus solely on the spherics receivers since I believe a lot of instructors and pilots are unfamiliar with these boxes. Have they become “the red-headed stepchild with bad breath” when it comes to weather avoidance?
Consider the thunderstorm environment. Here’s my modification of the FAA’s Figure 11-23 in the Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25A):
It shows a schematic of the three co-mingled thunderstorm considerations: turbulence, lightning, and rain. Very strong up and down drafts (turbulence) are needed to produce the electromagnetic conditions for a lightning stroke. Rain of varying intensities is generally associated with thunderstorms, but clearly we can have rain with no lightning and we can have lightning with no rain. So, there is a very close correlation between turbulence and lightning but less so between rain and turbulence or lightning.
Therefore, I believe the Stormscope gives much more helpful thunderstorm avoidance information than either airborne radar or Nexrad. To set the stage, here are a few personal Stormscope stories. Ten years ago I was going from Huntsville, Alabama to Atlanta in classic warm front conditions: light to moderate rain but a smooth ride. Huntsville Approach was being very helpful and advised of Level 2-to-4 (the terminology then) cells ahead of us and offered vectors around it. The Stormscope was clear so we pressed on through the rain and continued to have a smooth ride.
On another occasion I was heading east on an early morning flight and there were buildups all around. There was one right in front of me that was black as the ace of spades with the sun directly behind the cumulus, but the Stormscope was clear. I kept going and had one small bump as I went through it. More recently, I was going from New Orleans to Dothan, Ala., and there was a weak tropical disturbance over the Gulf that had generated thunderstorms and rain all along the Gulf coast and for some distance inland. As I approached Mobile, Mobile Approach advised me that there was moderate to heavy thunderstorm activity at my 12 o’clock and suggested deviating north or south. The Stormscope was clear ahead but showed a little activity to the north and south so I advised Approach I would proceed straight ahead. He really got insistent and at one point asserted that my Stormscope was broken. Nevertheless, we encountered only light to moderate rain and a smooth ride except for one moderate bump.
Obviously, operating here in the southeast U.S., I have had many opportunities to see the Stormscope become very active. With its 200-nm range, I use it to make strategic decisions on whether to dodge left or right - or land and wait.
Technical Points
I believe the physics of the situation favors the Stormscope for thunderstorm avoidance. It obtains all its raw data at near the speed of light via a simple low frequency receiver and then applies clever on-board processing. The old Automatic Direction Finder (ADF) low frequency radios typically pointed to the biggest thunderstorm (lightning strike) around. So, knowing the direction of the strikes was the easy part. The cleverness was needed to determine how far away the strike occurred. Each lightning strike puts out a tremendous amount of electromagnetic energy. If one could determine the energy given off by the “average” strike, one could make an estimate of how far away that strike was by measuring the energy received at the airplane’s antenna.
I apologize to Paul Ryan and Nicholas Spitzer, who patented the Stormscope in 1976, for my gross oversimplification of their elegant work. Over the years since then, electrical and computer engineers have greatly improved on the original box, but the principles remain the same. According to the Pilot’s Guide for the Stormscope, Lfrom current manufacturer L-3 Communications Avionics Systems, today’s WX-500 “…detects the electric and magnetic fields generated by intra-cloud, inter-cloud, or cloud-to-ground electrical discharges that occur within a 200-nm radius of the aircraft and sends the resulting ‘discharge signals’ to the processor.” The operating handbook recommends using the “cell” mode instead of “strike,” because the latter uses an algorithm that helps identify the locations of storm cells rather than individual strikes. Look at how they differ on the screen:
By definition, then, if there are no lightning strikes, there is no thunderstorm. Perhaps almost as important, where there is no lightning there is also little significant turbulence. The up and down drafts within the cumulus are what stirs up the electrons that then produce an electrical discharge. So, if there are no large up and down drafts generating lightning, there will be no significant turbulence. Those of us who fly in hot and humid places know that even without thunderstorms it will probably be bumpy when you are below the haze level or below the bases of the “fair weather cumulus.”
Graphic Example
The pictures above show how important it is to know the airmass’s environment. When the environment is ripe for thunderstorm activity, one area can seem to energize or flow into an adjacent area. The picture on the left was taken about four minutes prior to the one on the right. In both, the path ahead was clear and the interesting activity was confined to a region extending from about 15 miles to beyond 50 miles (the range set on the Stormscope at the time) at about 1 to 3 O’clock. Eglin Approach was being very helpful and offered headings based on what he was seeing on his radar – moderate to heavy precip. I declined his kind offer and requested on course direct to destination and he cleared me “as requested.” When I was in the middle of his depicted heavy precip, he asked for my flight conditions. I told him I had light rain and light, occasionally moderate, turbulence. I have found this to be a not uncommon reaction from ATC when I elect to use the Stormscope for guidance instead of their friendly recommendations.
The following four images are among my favorite examples. The first shows the Nexrad display from my Garmin 496, the second shows the Stormscope display at the same time, and the third and fourth show the view out the window of a pretty little thunderstorm on the left and then clear ahead.
ATC Precip Levels
Rain can matter. When the weather starts getting interesting, ATC will generally advise of precip levels of Moderate, Heavy, and Extreme. The location of these radar echoes may or may not correlate with thunderstorms and/or turbulence. The Stormscope will tell you where the really bad stuff is. However, another consideration entirely is the fact that even if the ride is pretty sooth, if you are in heavy or extreme rain you can expect paint erosion on the leading edges of the wing, tail and cowling. Think $$$ even if it’s not a bad ride.
A Decision Tree For Consideration
The first question is, who is in the plane with you? When I have my wife with me, I try much harder to avoid virtually all the bumps; when I am alone or with another pilot I am willing to tolerate more roughness. In broad outline, here is my thought process:
Overall Situation
Days in advance of a flight, start becoming aware of the overall weather patterns and get a notion of the environment to be expected. There is truly no good reason to be surprised by a squall line or a hot juicy air mass!
When in VMC
Advise ATC and deviate around the more impressive buildups.
When in IMC
Squall lines (often characterized by bow-shaped lines of red on radar): Avoid at all costs! Be on the ground!
Air mass thunderstorms: Take advantage of the Stormscope’s 200-nm range to make strategic decisions. Remember the 60:1 rule: Each degree of course change will generate 1 nm offset in 60 nm. Use this fact to deviate around Stormcope-depicted cells when they are well out in front. ATC calls of heavy and extreme precip will come when you are much closer and will require larger heading changes to avoid them, if you so choose.
Concluding Thoughts
Thunderstorms can hurt you and/or your airplane. Like Texas, don’t mess with them! I would like to believe that all instrument pilots are a bit timid. If you are like me, I nibbled at cloud flying a little at a time and over the years have become comfortable with flying in most (but not all) types of weather. I tell folks that I don’t do ice and I don’t do lightning! The fact that I live and fly in the Southeast means that I have had the “opportunity” to see a lot of convective activity.
I was also blessed to have a great flying buddy and mentor Mike Scroggins, who provided me with a lot of Stormscope practical in-flight knowledge. I reflect from time to time about the fact that in the “olden days,” unpressurized DC-3s and -4s of Delta and Eastern Airlines plied these same skies at the same altitudes I do on a regular basis. Back then, the common greeting when you went to pick someone up at the airport was, “How was the flight?” The expectation was that it had been bumpy to a greater or lesser extent – but safe.
The guys up front had no Stormscope, no Nexrad, and no ATC guidance, but they knew the weather, how to read weather reports and forecasts, and how to go around the worst of it. But “bumpy” was sort of expected. My thinking now is that if you want to stay out of precipitation bumps, then stay out of Nexrad’s Green; if you want to stay out of thunderstorms, but don’t mind a few bumps and rain, make sure you are using the Stormscope information. To get used to precipitation bumps, plan to fly at least a part of your next hot humid flight below the haze line and/or below the fair weather cumulus. You’ll certainly get bumps there, but no rain or thunderstorms!
-Bill Castlen is an MCFI, Cirrus CSIP and FAA Master Pilot
