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Flying Friday: the one with thunderstorms

I lived in the Bay Area for about five years. Among the many weird things I experienced there, one that particularly sticks with me is the way people acted when we got a thunderstorm. They’re quite rare in that part of the country and the arrival of even faint thunder was considered quite an event.

Of course, here in the southeastern US, thunderstorms are as common as pickup trucks, especially in the spring and summer. As I write this, here’s what the airspace near me looks like. When I look out my window, there are plenty of building cumulus clouds, but the real action is off towards Chattanooga.

It’s not necessarily intrinsically harmful to fly into, through, or under falling rain or snow. (Hail isn’t great, though!) However, when rain falls, it displaces air, and the result is that you get updrafts and downdrafts. Those can be harmful. In fact, the common rule of thumb is to avoid flying within 20 nautical miles of the boundary of a thunderstorm (like the one just northeast of the PRONE intersection above). That’s because, in the FAA’s words, “All thunderstorms have conditions that are a hazard to aviation. These hazards occur in numerous combinations. While not every thunderstorm contains all hazards, it is not possible to visually determine which hazards a thunderstorm contains.”

Notice that I was careful to use the word “thunderstorm” and not “rainstorm” or “rain cloud” in the preceding paragraph. That’s the crux of the problem: your eyeball alone can’t tell the difference. Thankfully, we have radar, which is where the image above comes from. There’s lots to say about modern weather radar, and all the modes and capabilities it has, but the best way to think about it from an aviation perspective is that it can show you two important things: is there precipitation in the area you’re going towards, and what are the winds doing?

If you shoot a beam of radar energy into a cloud, some will be absorbed, some will scatter off in various directions, and some will be reflected back to the receiving antenna. By magic, it’s possible to figure out quite a few things about a storm cell based on this reflection and a few other parameters, like the tilt of the antenna. For example, if you look at the Doppler shift of the returned reflections, that tells you something about the relative movement of air and water masses in the beam, which you can use to figure out which way the storm’s moving and, oh yeah, if it is showing signs consistent with the formation of tornadoes.

Anyway, enough about that. What I wanted to talk about today is something radar can’t tell you.

Before I get into that, though, I should spill a dirty little secret. Most of us don’t even have radar in our planes. The FAA broadcasts radar images through a ground-to-air datalink system known as FIS-B. This is worlds better than not having any radar imagery in the cockpit, but it’s super important to know that it’s not a real-time picture. FIS-B datalink images can be up to 15 minutes delayed, which means that they show you where the weather was. That means that what you see out the window is king, not what your FIS-B receiver shows. This is extra true because what the FIS-B radar shows you is a composite picture that tells you there’s precipitation (and if so, roughly how much). It doesn’t tell you at what altitude the cloud based or tops are, how much precipitation is reaching the ground, or much else of use.

With these limitations in mind, you can’t depend on ground-based weather data to distinguish between a rain shower and a giant thunderstorm, the more so because that ground-based data won’t show you where there’s lightning.

In the image above, you can see little blue lightning icons. Each one indicates a lightning strike picked up by what is basically a bare-bones radio receiver– lightning strikes make a hell of a lot of radio-frequency noise (as any AM radio listener can tell you). This noise is in the form of radio waves called sferics. With the right receiver you can pick those sferics up and triangulate their source– even better, you can do that in flight and get accurate, instantaneous real-time lightning data.

Why do you need to know where the lightning is? Because that’s where the thunderstorms are. Thunderstorms can have lightning (duh), extreme turbulence, hail, wind shear, and/or icing– and the only one of these that is easily detected from a distance is lightning. So it’s a pretty good proxy: you won’t ever see lightning if there’s no thunderstorm.

The picture above shows a live display from a recent flight I took from Decatur to Auburn. Each one of those little crosses is a lightning strike. It doesn’t take a Ph.D. to figure out that the more little crosses there are in an area, the less you want to be there. This screen is from a BFGoodrich (yes, them) Stormscope, which is basically this small LCD screen, a small box with a primitive computer in it, and a small array of antennas inside a flat enclosure on the outside of the airplane. The antennas pick up sferics, the computer estimates distance and bearing, and the screen shows you a +. There are newer, fancier models than this– mine was made in 1991– but they all work essentially the same way. In this case, I get a real-time, 360° view of lightning activity at up to a 100-nautical-mile radius, which is pretty great.

Compare what you see on the Stormscope view above with what the FIS-B picture looked like, below. On the bottom display (which is set to “track up”– so that the airplane’s southerly track is towards the top of the display) you can see a bunch of awful-looking red and yellow. I took these pictures a few minutes apart, so they don’t line up precisely, but they’re close enough to get the point across.

Of course, the best solution would be to have real-time in-flight radar and lightning data and ground-based FIS-B. Why?

  • Radar shows you what’s in front of you now, with good resolution and detail
  • A Stormscope shows you whether is lightning (and therefore, thunderstorms) embedded in the clouds you see via eyeball or radar
  • FIS-B feeds can show you radar imagery from the area where you are (including beyond immediate radar range), or over all of the continental US, which is really handy when you want to look ahead towards your destination.

Remember that earlier I said lightning is a good proxy for the presence of thunderstorms. The absence of lightning doesn’t mean you’re good to go, though. You can still have a thunderstorm with no lightning. That’s why a Stormscope alone isn’t enough to keep you out of trouble.

I don’t yet have radar, although this airplane did at one time and still has a good-condition nose radome. Until I equip a radar (which, let’s be honest, probably won’t ever happen), having the Stormscope along makes it much easier to decipher what’s happening in those clouds so I can give them an appropriate berth.

Thunderstorms are a hell of a lot more fun to watch from the ground than to fly through.

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