Embedded - 268: Cakepan Interferometry

Episode Date: November 15, 2018

After many bouts of lightning round, we finally got our lightning questions answered by Eric Brunning (@deeplycloudy). Eric is a Professor of Atmospheric Sciences at Texas Tech University specializing in storm electrification and lightning . You can hear some of Eric’s field adventures by listening to his episode of the Don’t Panic Geocast show. The Wikipedia page for lightning will lead you down many strange pathways. Though the Wikipedia Lightning Energy Harvesting page may convince you that it isn’t feasible (though some math might as well, as discussed on this show). For more about lightning interferometry, check out Michael Stock’s in-depth site. You can hear lightning on Jupiter if you listen to the right bands. Neat video of the Milky Way in radio waves reflecting off the moon Elecia really enjoyed The Cloudspotter’s Guide by Gavin Pretor-Pinney.

Hello, and welcome to Embedded. I'm Elysia White, here with Christopher White. I remember I've been asking guests for lightning facts. It hasn't been going so well. But this week, I expect it to go very well. Our guest is Eric Bruning, and he's an expert on lightning. Be prepared for a shocking episode. Hi, Eric. Welcome to the show.

Hi, thanks for having me. Could you tell us about yourself, as though we just met after John Lehman's Geocast podcast? Yeah, sure. I work at Texas Tech on the faculty. I teach in an atmospheric science program and grew up in Wisconsin originally and did my graduate work and undergraduate as well at the University of Oklahoma and spent a couple years in Maryland after finishing my PhD and wound up in sunny Lubbock, Texas. Studying lightning. Studying lightning. Studying lightning and meteorology, yeah. Cool. Texas is probably a good place for that. Yeah.

It is, yeah. It's nice to live in the place that has the thing you want to study and lets me run an instrument here and get graduate students involved in that instrumentation work. So now we want to do the thing that has been getting us into trouble, which is lightning round, where we ask you short questions and we want short answers. Okay. Christopher, do you want to start?

Sure. Benjamin Franklin and the kite. Thought experiment or write-up of scientific apparatus? Write-up of scientific apparatus write-up of scientific apparatus back to the future thought experiment or write-up of scientific apparatus thought experiment ball lightning fact fiction or the coolest thing ever fact probably probably sprite a diminutive ethereal creature a two-dimensional bitmap that is integrated into a larger scene on computer graphics or a weird thing that happens above clouds yes what is the most shocking fact that you know

oh boy um that's what you get for letting me write these do i go with a pun or not? I guess that's the question here. Let's see. Lightning has many thousands of volts. Oh, that is a shocking fact. It's good that somebody knows that. Yeah.

If you were playing Legend of Zelda, and it was lightning around you, would having the rubber pants actually help you as you were standing in the lake? Or would rubber pants not really do anything? Probably not help you too much. Okay, we've got the rubber pants, the rubber shirt, and the rubber boots. The helmet, too. There's a rubber helmet. I'd rather be in a metal cage all right okay so let's get on to some more detailed and possibly useful questions

uh let's see do you want to talk about your career first or do you want to go through the list of many listener questions about lightning. Well, we can, I guess, start with career since we just did the meet and greet. Okay, so you work on lightning detection as well as meteorology. Yeah, that's right. My degree is really in meteorology, and that makes me a little bit unusual among scientists that study lightning. A lot of folks, I think, get into it from a physics or electrical engineering background because the instrumentation is relatively straightforward to build. And, you know, it's a kind of fun phenomenology to study from that point of view. So my degrees are all in meteorology and worked in a lightning research group at the National Severe Storms Laboratory. So I kind of got into it sideways and almost by accident

as an undergrad, there was a research position available and I got involved and never really left. Okay, so how do you detect lightning, assuming it doesn't hit you and it's more than five miles away? listen for lower or higher frequencies with a radio receiver, or there's even simpler versions of that simply involving digitizing charge that comes off of a metal plate. So the radio receiver, I have heard like more AM than FM, but AM radio gets really glitchy and staticky in a lightning storm. Yeah, it absolutely does. Lightning's a very broadband emitter in the radio frequency spectrum. I always think of it when I read those FCC warnings on the back of electronics. And because AM is allowing you to listen directly to the amplitude of the radio signal in that band, you can very readily pick up then the noise produced by lightning.

And so if you want to do lightning detection scientifically, it's a matter of digitizing that noise fast enough to be able to resolve processes. And it's a pretty high velocity impulse. I mean, we look at lightning and it flashes and it's very big. And when we hear it, it's a big bang if you're close enough. So it's very impulsive. Do you see that all across the spectrum? Yeah, it's impulsive all across the spectrum. So the thing that you just referred to is what's called the return stroke that we see. It's the big bright flash of light when a lightning channel connects to the ground. So there's a whole lot of charge that's moved over a very long distance, heats up the air, and you see that bright flash of light and the percussive sound that goes with that very impulsive heating. So there's more to it than just that. Lightning is not just that flash I see. Yeah, exactly. So if you go to the, say, the very high frequency part of the electromagnetic

spectrum, so tens of megahertz and above, kind of right where the old analog TV channels sit, at that frequency band, the stepping processes are on a very small scale, tens to hundreds of meters in terms of step length. And so if you sense in that band, you can map out the entire shape of the lightning channel. But that too is also an impulsive process and essentially acts as a point emitter. Okay, I sort of understand this. I used to work on a system that would find gunshots by listening to audio waves and locate them based on time difference of arrival to the different sensors. And one of the things that was really kind of pounded into me is sound moves at a certain speed. And if we get the sound speed wrong because of humidity or whatever, then we get the location wrong.

And there's a very constant proportion to it. And so what you're doing is looking at a very high frequency, which would give you a finer grained picture of what was happening. Is that right? Am I getting the right? Yeah. Okay. Yeah, that's right. And the time difference of arrival is the primary method that's used to do lightning detection. It's the instrument that I use mostly uses that method. And so we don't have to worry quite as much about the propagation speed. There are some applications where that does matter, but certainly up in the VHF where it's propagating through the atmosphere, it's essentially treated as a constant speed. How do you, so is it a pretty distinct signal or

are there kinds of other terrestrial interference that can get confused? Yeah, so the instrument works by very naively digitizing the peak pulse that arrives in typically about an 80 microsecond window. window and that uh um and so if you have a local noise source say a power line or something that's sparking um that's going to raise your local noise floor and reduce your your sensitivity to lightning okay i'm gonna ask an off the wall question now uh my dad uh used to do ham radio and i kind of do ham radio in the sense i have a license but i don't do anything with it um but i remember as a kid tuning in to a particular band and having my dad tell me we were listening to Lightning on Jupiter. And I looked it up this morning before the show, and it's all true. You can do that.

It's a shortwave. There's a shortwave band. Really? And I was wondering if it's, you may not know this because it's kind of a weird thing, but is that in the same region or is it somewhere else i could we be confusing jovian lightning for earth lightning oh that's a good question uh shortwave so what uh what wavelengths is that uh 10 to 25 megahertz is the frequency yeah um it's interesting i'm so that must somehow make it through the uh the ionosphere as well so i wonder it's kind of an interesting thing that that's even possible um uh so yeah that's

that's close enough that i guess you could be picking it up um but ours would be louder that's true terrestrial would be a lot yeah more imp. Yeah, I'm sure it's pretty weak by the time it gets here. Even though it's much bigger lightning, I'm sure. Yeah, so that's interesting. Although our antennas are probably not that much more sensitive than ham radio. Maybe they're actually less sensitive. I've never done any ham stuff,

so it's possible you guys are picking up a a smaller signal that way yeah okay as long as we're on the topic of of lightning in outer space can you do you get lightning effects from the moon mean, do things bounce off the moon? I have never looked into that. I, who asked that? One of our listeners. Yes. Interesting. I don't, I don't know if that would work or not. You'll certainly get radio frequency signals that'll propagate from the Earth upward to where you could detect them from a satellite. So it's likely that if you were listening up on the moon, you'd pick that up.

And so I suppose a signal could bounce off the moon again it probably depends on if uh if there's some is there some other signal that's that's known to bounce off like yeah uh there's there's i think there's there's been other moon bounce stuff that's been done um well and then there's that neat video of mil Way in radio waves reflecting off the moon. That was fantastic. So the Milky Way's own radio waves reflect off the moon, and if we point radio receivers towards the moon, we can image the Milky Way in radio spectrum, and it's just magical.

Here it is again. Ham radio operators utilize Earth-Moon-, Earth for two-way communications. So people do use them. That's really neat. That's crazy. Yeah. Would you like us to go back to asking things you know about? You have to ask to figure out what it is that I do know about.

We're just giving you suggestions for papers. Yeah. I wish I had the time for all the ideas that are out there. Okay, so you have a metal plate, if I heard right, and you measure very high frequency, and then what happens? Okay, so the one that I was talking about in the very high-frequency band, that's a regular dipole antenna, actually. Okay. Yeah, so that's a different thing from the metal plate system,

but we can talk about that metal plate if you want. I think you wanted to just hear about the whole detection system, right? Well, actually, no, the metal plate's kind of cool, because I heard from someone that I could build my own lightning detection antenna with like a salad bowl and a pizza pan. Who have you been talking to? John Lehman.

Oh. Yeah, so John and I have actually been working on a sort of prototype for this, and it's based on a design that really dates back to the 60s or 70s. It's a really standard atmospheric electricity instrument. I went out to my local restaurant supply store and looked for a pizza plate that you might use in a restaurant to serve or heat pizza, and found one that fit inside the opening of a salad bowl. And if you hook up a charge amplifier to the pizza plate and digitize that, you can detect lightning that way. And salad bowl is ceramic or insulated? It's a stainless steel or aluminum.

Okay, so they're both metal. Yeah, that's right. So the salad bowl is really to, you can think of it as giving it a nice ground plane, just a nice sort of smooth shape that isn't going to concentrate the electric field too much. And I usually hang it upside down, so it also serves as a rain shield. Okay, so now I have a salad bowl with the opening pointed down and a metal pizza pan. Also, now it's pointed towards the ground, but it's attached to the salad bowl through bolts, glue? It's electrically isolated from the salad bowl. So hot glue. Yeah, hot glue, whatever your favorite adhesive or any sort of insulator that'll just keep it separate electrically.

And then the electric field in the atmosphere, there's always some background electric field. And then when charge moves around inside a thunderstorm, let's say there's one of those big ground strokes and a bunch of charge is moved very quickly that changes the electric field very quickly that induces a change in the charge on that metal plate because the electric field changed and then the charge amplifier circuit pulls off that charge that's been induced

and that's proportional to the electric field change that sounds sort of like a capacitor I mean you have two metal bits that charge that's been induced, and that's proportional to the electric field change. That sounds sort of like a capacitor. I mean, you have two metal bits and then an insulator between them, and then you kind of just read what the charge is on this capacitor. Is that a reasonable mental model? That's not a bad model at all, no. I think that sounds good. And so what I'm measuring there is pretty simple. What does my amplifier have to look like then? The audio on your question just dropped out there. Oh, what does my amplifier circuitry have to look like um it's a uh it's an op amp and an rc circuit and uh a little bit of signal conditioning um you know to reduce noise um around that but

it's about the simplest circuit you could uh you you could imagine and then an adc and a little controller and then a little warning inside my house that says lightning is coming? Yeah, that's right. So, yeah, of course, you've got to digitize the voltage that's produced by that little circuit. So an ADC and then, you know, depending on how fancy you want to get, you know, we've this version we've been testing is running at about 100 hertz which is uh um fast enough to slow mind numbingly slow um but if you get up into tens of kilohertz you can uh you can resolve most of the important processes and lightning stuff that's happening inside the cloud um before a connection is made to ground.

And the sort of research-grade sensors right now are in the sort of 1 to 10 megahertz digitizer range. Okay. And you've said this before, the lightning strike that hits ground, that's not all of what is happening. Can you walk me through how lightning happens from when there's not a cloud in the sky to when it hits the ground? Yeah, sure. So once a cumulus cloud starts to build, the atmosphere gets colder as you go up.

So as that cloud gets taller, you eventually start to get production of ice in the cloud and its collisions between precipitating ice particles called called grapple and regular smaller ice crystals that are sort of like snow very small snow crystals those collide with one another in the presence of super cooled liquid water and the combination of all of that is enough to allow positive and negative charge to be separated, where the precipitating particle gets one sign of charge and the non-precipitating particle gets another sign. Wait, wait, wait. Yeah, go ahead. Why don't they stay balanced? I don't understand why they get separated. bit of the ice surface gets torn off in the process of that collision. And the result of that is that one of the particles retains more of one of the signs of charge. And that's actually one

of the great mysteries is exactly what's going on at that ice surface collision. And we have some idea from laboratory studies how to predict which sign of charge we're going to get, but there's PhD-level work in refining that understanding. So someday they're going to find out that little tiny, tiny ice cubes are walking around in their bathrobes, shuffling their feet against each other, and magic happens. I think I'm going to teach it that way from now on. That's a great picture. What's interesting, though, is it's fundamentally an ice-based process, right?

You think about thunderstorms as this summer event, you know, barreling across the hot southwest, but there's a lot of very complicated things happening in those thunderclouds with ice and rain and hail and lightning. Yeah, that's exactly right. And yeah, if you can't make the ice, you typically don't get lightning out of those storms at all. There's been some very sketchy observations of lightning in what are called warm clouds where there's no ice phase precipitation. But those are more anecdotal reports, and so it's really a requirement to have that

deep cloud that gets into the ice phase. And that's why with severe thunderstorms producing a lot of hail, that's okay. You're in deep danger of lightning. Yeah, exactly. So the more of that precipitating ice you have, the more collisions you get, the more charges separated. And the consumption of that charge and reduction of the electrical stress produced by that charge being concentrated into regions that are perhaps a kilometer apart in the cloud is what leads to lightning. So is it the ice cubes that are falling that are positive and the ice cubes that are staying that are negative or the other way?

So there's actually a reversal line. It's about minus 15 or 20 Celsius in the cloud. And below that level, the grapple tends to charge positive, and the ice crystals charge negative. And above that level, the ice crystals tend to charge positive, and the grapple charges negative. And that leads... And the grapple's what's falling, right? The grapple's the precipitating one, yep. Okay, okay. Sorry, that leads. Yeah, so that leads then to the basic electrical structure of a thunderstorm.

It's called the normal tripole model, and it's a positive, negative, positive charge stacked in the vertical. And between each of those, you can think of them in a simplified form as charge layers. Between each of those charge layers, there's a large electric field. And when the electric field reaches a certain threshold, then you can get a lightning flash to start. And so this is where we get lightning to lightning or cloud to cloud lightning. Yep, that's right. And yeah. Where does the earth come in? I mean, why does it go to earth? What is earth? What is ground? charge that's available inside the cloud. And so that's what leads to these ground strikes then, is that it's energetically favorable to move all that charge to the earth

in order to neutralize the electrical energy in the cloud. Okay, so you said energetically favorable, which actually leads to one of the listener questions kbx81 fake name clearly a fake name not on the birth certificate we've been talking about harnessing energy from lightning for a long time and it's chaotic in its nature but there's a lot of energy there why why aren't we catching lightning to charge batteries so there's a lot of current there but there's not a lot of energy um and so i would just – Very short, right? Yeah, it's a very short time process. Like you said before, it's very impulsive. So there might be, in a typical ground strike, tens of kiloamps for no more than a few milliseconds.

In certain discharges, you can get something called continuing current, which might last on the order of 100 milliseconds or so. But the total amount of energy that's actually moved when you add up all that current over time is very, very small. Okay, let's take the question the other way. Why is anybody working on harvesting lightning? There are people working on that? I've read about it. Really? Huh. It's certainly not something I run into on a regular basis.

And I think if you would survey the scientific community, they would say that's not a scientifically – just from scientific principles of what we know about how the energy is moved around and lightning is not a real uh real likely to be a successful thing yeah it seems like we'd be better off harvesting an entire thunderstorm well yeah just put up a bunch of uh giant windmills that go inside them yeah yeah that's uh wind solar lots of lots more direct ways to get uh get the energy out of the sun converted into electricity but that's the thing i remember working on lasers is a confusion between energy and power like a very powerful event might not dissipate a lot of energy not comprise a lot of energy because it only happens over the course of a tiny tiny tiny fraction of a second yeah exactly exactly so lightning can still be very damaging

um so uh for a wind turbine blade in fact they uh there's a whole lot of charge that's uh that's moved across the surface of the blade or through the interior of the blade um in a in a very short amount of time and so that leads to leads to heating just like it doesn't uh in the air that leads to thunder um and that can rupture a blade but the the total energy is not that large okay um i believe that now and and having gone back to the wikipedia page that led me that is actually called harvesting lightning energy the first half is all about how people are doing it the second half is all about how stupid that is so i clearly should have read the whole thing. Well, if you think about it,

people get struck by lightning, right? If it was something worth harvesting energy from, it would vaporize people instead of killing them just most of the time. Yeah, it's good. Yeah, yeah. How often do people actually get struck by lightning as opposed to being near a lightning strike

and believing they were struck by lightning? Have you ever been struck by lightning? Or do you just believe you've been struck by lightning as opposed to being near a lightning strike and being and believing they were struck by lightning have you ever been struck by lightning or do you just believe you've been struck by lightning um i can definitely confirm that i have not been struck by lightning although it's been close when we were launching balloons into thunderstorms um and uh that's another whole fun story but um yeah there, there's typically tens of deaths part of the channel, not the main channel to ground, is what actually winds up affecting the person. And so the actual direct strike part of it is relatively rare. And that's why it's very dangerous to be standing under a tree, because if you're standing under a tree and that tree gets struck by lightning, there's a big potential difference between you and the tree. And so you get sort of involved in it by association.

That's a good euphemism. I was involved in a lightning strike by association. are there off the shelf uh lightning detection systems that people interested in it can can purchase um there there's certainly are devices out there um that will look at things like the electric field or they'll look at the the size of the radio frequency spheric which is going to be proportional to current. And so if you assume that a typical lightning discharge has this much current, you can sort of infer the distance from that. If you're in the back country or something, it's great to be informed that there's an electrified storm nearby so that you can,

for instance, move to lower ground. For a lot of folks that are going about their day-to-day business, you can find a variety of weather apps that you can buy. Lightning ground strike detection maps is part of a subscription, and that's fairly affordable to do so that you can actually see exactly how far you are from the nearest, nearest ground strike. How far do the lightning strike signals travel? I mean, here in California, we don't get a lot of lightning. Can I, can I watch your Texas lightning with the right antenna? With the right antenna, you could. So there's

a technique that can detect the very largest ground strikes that take place, the very highest current discharges. And you can build a worldwide network to detect lightning that way with about six sensors globally. So the lightning signal is at the right wavelength to propagate in the Earth ionosphere waveguide and can make multiple trips around the Earth. And from that, you can geolocate then where that took place.

And the ionosphere's waveguide only works on certain frequencies. This goes back to what you were saying about VHF and it being a broad-spectrum impulse. Yeah, exactly. Right, yeah. So those big high-current discharges, those are moving a lot of charge over distances of kilometers, and so they have a low-frequency component that's very large and that can then um the channel actually acts as a as an antenna that is transmitting a radio signal at the the right wavelength to uh to be caught in that that wave guide um when you when you get up to the what's called the stepped leader it's the

the process of the lightning channel itself developing before a lot of current flows in it. That stepping process then is a smaller segment of channel. So just like with an antenna, the smaller the antenna, the higher frequency that radio frequency noise is emitted at. Can I offer to build an antenna at my place and then find five other people to make a system? they wanted to build one of these flat plate antenna systems. And so they needed to build that network, and they designed about 10 or 12 of those and set them up in backyards of graduate students over a county-type area. And that's enough to then locate where those processes were taking place. Is this what's known as lightning interferometry,

or is that something else? No, lightning interferometry, you have to change your culinary equipment there. That's usually done with a cake pan. Is this how science works? Yes. I've heard that cooking is basically chemistry, so that's close.

Cooking pans are basically meteorology. Yeah. Yeah, I mean, it's a nice, well-manufactured, flat, cheap way to get a shape that you need. And so instead of having some custom part machined for you, you just go shopping at your favorite local restaurant store. And so yeah, lightning interferometry, that's done with a very short baseline. Maybe, you know, 100 meters would be a very large spacing between the antennas. And the way you do lightning interferometry is it's sort of an extension of the time of arrival idea, except that you're digitizing very, very quickly, like at, let's say, 180 megahertz,

the charge that's being induced on your cake pan. And so you do that very carefully with a very fast digitizer, and then you run that, and then you look at the interference pattern produced by the signals from each of the antennas um for the bright spot that was produced by a lightning channel as it as it develops so that essentially becomes then a high speed uh camera view of uh how lightning develops um in the as depicted in the vhf band why okay this is i know we're in the, as depicted in the VHF band. Why, okay, this is, I know we're in the details, but I'm going to ask a meta question. Why study lightning at this level of scrutiny?

Other than it's really damn cool. Yeah. Right. I mean, and I think that's what motivates a lot of us, you know, to get interested in it is that it's a basic science thing that's really interesting and, you know, to get interested in it is that it's a basic science thing that's really interesting and uses all that stuff from physics, too, that, you know, you were wondering what you were going to do with since everyone else was talking about Newton's law. And so, yeah, I mean, we're just interested in it as curiosity to start with. But, you know, if you want to talk about understanding

lightning safety or lightning protection for a building, you need to know how much charge is moving in that lightning discharge and is likely to be transferred to the building. And that allows you to then spec out a system for protecting the building. Some of the more recent applications, now that we can, really as computers have gotten faster,

it's allowed us to detect lightning much more regularly and much more easily. And so that allows us to then do continuous monitoring of lightning. And that allows us to get into meteorological applications. So we can infer that the storm is intensifying because if we see more lightning, we wind up seeing, we can infer then that the amount of ice production in that storm is larger. And that allows us to infer that the storm's updraft is stronger. And all of the rest of how lightning changes, lightning production changes with time as a storm's processes evolve throughout its life. Can lightning help you predict when a storm is coming?

Or are other methods more reliable? Yeah, I think the data has gotten really quite nice now to where we can map out the entire extent of the lightning in the cloud and even all of the branch channels and make a distinction between these really big extensive flashes and really tiny flashes that are discharging tiny pockets of charge in the storm's updraft. And you can make pictures from lightning data that look very much like a weather radar. And so you can certainly do storm tracking that way. And in fact, for instance, in the western U.S. where there's mountainous terrain, it's very difficult to locate a weather radar there in a way that will allow you to, you know, comprehensively see where all the storms are. And so NOAA, the U.S. weather agency, and as part of the Department of Commerce, just launched a geostationary weather satellite that has an optical lightning detector on

it that is going to allow us to, allow us to really serve that purpose of detecting all thunderstorms comprehensively over that satellite's full disk field of view. And then that will basically, to answer Christopher's question, predicting weather saves lives, protects crops. There's a reason why we pay for it. Yeah, that's right. So, you know, your forecast model needs to know where thunderstorms are taking place in a weather forecast model. And one of the applications of lightning data there for improving routine weather forecasts is to ingest that lightning data to say the atmosphere was overturning in this part of the the world and that's uh you know that's a sort of part of the atmosphere's energy budget that we need to account for and so you can pull that

lightning data into a into a forecast system and use that to refine that system's depiction of the state of the atmosphere. Okay, I want to go to a question from Nick, the Exploding Lemur. What characterizes a lightning strike? What looks like a lightning strike but isn't? There's more to it, but I think we should start there. What? I mean, we talk about lightning strikes and and all of that but there are there's light there's all kinds of lightning and there's all kinds of lightning

processes that happen before the strike hits the ground oh i see yeah what what is lightning again could we do the start there yeah i'll go all the way back uh right so it's um it's a spark but there's a lots of different lots of different parts of that spark and so um you know we talked about the the lightning channel developing and those are uh called stepped leaders and um as those stepped leaders develop um even before they make a connection with the ground. And this is how the in-cloud flashes work all the time. Once you have a channel network that's developed sufficiently, it's kind of a double-ended tree structure, and charge can move back and forth between one end of the tree or another

to rebalance how the charge is configured and minimize the energy imbalance. And so any sort of larger charge flow along that channel network is going to light up the cloud. And you would, you know, you would see the cloud, you know, light up and we would call that lightning. And sometimes you can even hear thunder from that and then you know as that channel sometimes will step down towards ground and it wait when it makes that connection then you get these ground strikes and that's another you know another part of the lightning process that that we would characterize i've i've heard from you know pop science occasionally that oh lightning doesn't go from the cloud to the ground it goes from the crowd to the cloud does that have any meaning at

all or yeah so that's that's a super fascinating super fascinating story in uh uh in terms of scientific communication so um when you have all, all ground strikes that you see start by a channel that's, um, that's developed somewhere in the cloud and it's stepping down towards the ground. And when that channel connects to the ground, then there's a potential wave that develops from the ground and goes back up into the cloud. And that's what results in the heating and thunder and what you would sort of consider the human sensible phenomena that's associated with a ground strike but um you know there was uh when that study was was done i believe it was sometime in the 90s um that that sort of demonstrated that there was this this potential wave that propagated back upward once that connection was made um that got sort of picked up by the by media and uh described as oh lightning goes from the ground up to the cloud but that left out the entire part of the story where the lightning channel developed

to make the connection with ground in the first place. I'm going to make that my ringtone, what he just said. And then I'm going to have another one that says, well, actually. I asked for it. No, no, no. It was great because we do get pop science miscommunication exactly i always pick something hooky yeah to try to and often the the real science gets lost in the oh did you know that what you think is really this is that so back to the cake pan interferometry. Mm-hmm. Words I never thought I'd say together.

You said 100 meters apart. That's not very far when you're talking about light or high frequency. I mean, it's far for a high frequency signal. It just doesn't seem very far apart if you're doing time difference of arrival sort of things.

Right. And for a time difference of arrival, you want, you know, 20 kilometers or something like that to really, um, uh, to make that, you know, work the easiest. Um, but when you're, when you're doing that a hundred meters spacing, that's going to be, you know, you're, you're doing that 100-meter spacing between those uh those peaks or because you're digitizing so fast um you don't need as much separation in order to distinguish um distinguish processes from one another yeah in fact it works against you to have too much distance with interferometry you want to have a few multiples of a wavelength which 100 meters sounds about right. Yeah, yeah, exactly. And if you were trying to do interferometry and you had a storm on two sides, not that you were in the middle of a storm, but you had one storm off to the left and one storm

off to the right, would it just give you garbage data? No, you would. So the way the data is often displayed is it's, uh, you can see processes that are happening simultaneously. It's easier if there's only one source that's emitting at a time, but you can actually look at techniques from radio astronomy, and they have methods for doing multiple source detection at the same time. So, for a lightning discharge, a typical lightning discharge, really, you even tend to see that lightning discharge cover the entire sort of hemispheric field of view of the interferometer from right to left. Yeah. Or just sort of spatially covering its sort of azimuth elevation field of view. Yeah. You don't want it to saturate, you know, in terms of amplitude.

No, that's how you burn things. Yeah. Or just, you know, you avoid being able to cross correlate the peaks and the fluctuations. And so cross-correlate is one of the things I was hoping you would say, because in order to find things in time from different directions, usually the cross-correlation shows you which direction you're coming from. Is that kind of right? Let's see. That's not how I'm familiar with processing the time of arrival stuff that I normally work with, but you can certainly frame it as a cross-correlation. Maybe I should ask the question the correct way. What kind of signal processing do you do on your data? Oh, yeah. So time difference of arrival processing with the non interferometer data is done with a Levenberg-Markhardt algorithm. It's a nonlinear least squares retrieval, given a bundle of time of arrivals that you know come from the same point, and a sort of test solution that you iteratively refine to match the observed arrival times. And so when it comes down to it, it's a least squares retrieval problem.

And then you also have the trouble of matching up which lightning pulses or which detected pulses go together. And we get around that. Computers are fast enough that you can kind of get around that in a brute force way without even being too smart about it and just checking all the combinations. What about the impulse detection? Do you have special techniques for that? Or is it just before it was quiet and now it's loud? Yeah, so there's an instrument that was the instrument that I use regularly was invented at New Mexico Tech in the late 1990s. And it combines a radio receiver that's digitizing that AM noise we talked about earlier. It does that very fast at about 25 megahertz.

And every 80 microseconds, it records the time of the peak pulse that arrived at the antenna in that 80 microsecond window. And so what allowed that instrument to work well is the ability to combine a digitizer, which has been around for a while, with a GPS timing reference that's precise enough to allow us to work on lightning timescales. And so there's some custom electronics that that do the peak detection within that 80 microsecond window. And then that gets sent along with the GPS time into a little Linux computer that is at every site and writes the data to a hard drive

using a very sort of conventional approach from there. And then those are connected by the internet and we get the data back. And we can actually watch this in real time. So this is really fun when I have a thunderstorm here in West Texas. There's a server I have that collects data from each of the stations streamed in real time.

And that least squares retrieval process we just talked about runs fast enough that we can watch on a display. The way I like to describe this is like you see a lightning flash happen outside, it'll show up on the display with its branched

channel structure and then you hear the thunder from it. That would be kind of a fun Saturday afternoon. It really is, yeah. You can sort of play a game like, is that loud one I just heard? Is it a really large lightning flash?

Oh yeah, there it is. So it's a fun find-the-flash game. So it's a fun Find the Flash game. So switching gears for a second, Ball Lightning, which we brought up in Lightning Round, but Nick has another question about it, kind of the same as mine is, is it real and what is it?

And I guess the real question I have is, if it is real, what is it? I don't think we know. It's one of those phenomena where we have reliable reports of it that date back hundreds of years, and yet it's rare enough that we can't really design an experiment to go out and observe it in any sort of scientifically organized sort of way. There's been attempts in laboratories to send a bunch of current through things like silicon plates or other materials that lead to these sort of stable plasmas that kind of roll along the floor. And so you can produce some things in light in laboratory settings that behave a bit

like ball lightning, but, um, precisely what is going on there that, that leads to the, uh, you know, the, the full scale phenomena that we observe in the atmosphere, um, is not real clear. Um, I'm, I'm not even sure we could, we could say definitively exactly what scale it is. Um, although you, you know, you hear reports of, uh, for instance, the ball lightning moving through,

uh, uh, a window or, um, you know, so it's maybe, maybe about that size. They're just ghosts.

Could be ghosts. Could be ghosts. Um, ghosts with a love of physics. But there are weird, uh, lightning phenomena beyond normal lightning that that have been reliably observed such as the things at the top of right thunder clouds that have been uh

photographed from space station yeah yeah that's right yeah there's uh sprites and blue jets and elves and halos uh this whole uh this whole uh collection of of creatively named things. So sprites take place when you have a whole lot of charge moved around in a regular, a regular thunderstorm discharge. So one of these very largest discharges and that moves so much charge that the electric field in the ionosphere changes enough that the ionosphere breaks down and you get a secondary lightning discharge in the ionosphere. And so that's what a sprite is.

So it's lightning that goes up. It's not even connected. The sprites aren't even connected to the regular thunderstorm except through the field aloft changing because the thunderstorm changed. Now, the other things like us, like blue jets and gigantic jets, those are lightning discharges that go up. And you can think of those as like a ground strike, except the thing that's being struck is the ionosphere. Is the ionosphere grounded? I mean, why would it start? It's a differently charged region.

Yeah, I guess so. It's just, you don't think about that, because you're like, oh, well, it hits ground, and ground is important, because it's ground. Well, but there's cloud to cloud lightning, too. Yeah, and so is cloud to ionosphere lightning. All right, I sort of buy it, but I think you might be making it up. I could be, I could be. Next time, when AGU returns, the American Geophysical Union has its annual meeting every year in San Francisco, and you should come hang out with all the lightning people. There's a set of sessions there every year in San Francisco.

Yeah, you definitely want us there to heckle. Absolutely. Absolutely. One of the things that got me interested in lightning and how it was formed was this book called The Cloud Spotter's Guide. Are you familiar with it? I have seen it. I'm looking behind me on my bookshelf here. It's by Gavinavin pretor piney pinny luckily he doesn't listen to the show so i can totally butcher his name

yeah yeah i don't think i see a i don't think i see a copy of it but i've definitely heard of it yeah uh one of the stories it had in that book and in another book that was similar, was of a pilot who ejected from his plane after hitting a storm unexpectedly and was in his pilot chair, his military pilot, so jet chair. And he fell, and the updraft kept lifting him up up and then he would keep falling and going up and back and forth for 40 minutes that's how plausible that have you that's extreme no doubt i've heard this story yeah and uh it's i i tend to believe pilots when uh when they've they've gone through something like that um they i would yeah i would I would consider that as reliable of a lay report as you could get. And I don't know the case in a great amount of detail, but the idea that someone could be carried up and down through a thunderstorm for that amount of time yeah that that uh that's not

implausible and uh certainly sounds miserable you don't want to try it no no i mean it did sound kind of miserable because i guess he almost drowned at one point and he broke some pieces it's also very cold yeah frostbite too but if you were prepared would didn't sound like fun no only me no i sounds sounds dangerous um there there is uh there was uh actually a storm penetrating aircraft it was an old uh t28 uh uh piston uh engine plane that um was that had had all of its leading edges on the wings and things, uh, up armored. And so they would fly that through up to, I think, one inch hail and fly right through the updraft in a storm. Um, one of the, uh, one of the pilots of that now is, uh, um, uh, is, uh, also still an active scientist and, uh, does a bunch of work in high-speed

lightning photography, um, and, and videography and videography, which is useful for studying lightning physics. So there is actually ways to go get inside the thunderstorm. That aircraft has since been decommissioned, and there's been talk of bringing an A-10 aircraft into service for that role, but budgets being what they are, it's maybe tricky to get that project completed. You're going to have to start telling people how important it is to study lightning and how it will save lives

and stop telling them you're doing it because it's so damn cool. Yeah. Yeah, you know, the ability to measure inside a thunderstorm is so important because you can do some things with remote sensing with radars, but there's nothing like an in-situ verification where you can actually look know how much of the clouds are a big component of the Earth's radiative energy budget. And so if you want to understand climate change, you need to understand how clouds are comprised and exactly how longwave and shortwave radiation is going to interact with those clouds as part

of the climate system. You've said super cooled water a few times. And so super cooled water is water that is cold enough to freeze, but it can't because water kind of needs a catalyst to freeze, where a catalyst is less of a chemical here and more of just like a dust particle or a little tiny molecule it can glom onto and then it starts building its crystal but super cold water won't crystallize on its own is that all right yeah you got you got that exactly right that's um until the uh a water droplet that's pure water gets down to about minus 38 to minus 40 Celsius. It'll remain in liquid form, and then once it gets that cold and colder, it'll homogeneously freeze. And so it's important that we keep the atmosphere very dirty so that we get lots of our clouds back?

I wouldn't exactly advocate that. I would – you could ask folks that lived in London at the – you know, during the start of the Industrial Revolution. If they enjoyed the atmosphere being kept dirty, it led to a lot of fog. We're doing quite well here right now. Yeah. I can't see the tree across the street so oh you've got yeah the big uh big wildfires we're not anywhere near them but we are getting the smoke i feel a lot more for the people who are actually near them yeah yeah it's uh those are

some some scary fires and real real tragic situation there okay let's go back to discussing fun things yeah uh for a little while and so one of the things i wanted to ask you about was this weather balloon thing you go out in the middle of a storm with the weather balloon and i assume there's a key attached to it like like reenacting ben franklin's thing yeah it's uh it's it's ben franklin but we let go. And that allows us to keep doing this in a university setting. In Oklahoma, Dave Rust, who passed away a couple years ago, was the sort of leader of this effort to do atmospheric electricity ballooning in Oklahoma, along with Don McGorman. And yeah, these are big balloons. They're about 20 feet long when we launch them. Usually we would launch polyethylene balloons. It's a very thin polyethylene, basically a large garbage bag that we inflate with helium, and it gets launched out of a launch tube. We inflate it in the back of a U-Haul truck and chase storms with the U-Haul truck, which is its own interesting experience. Why are you renting our truck today?

Yep. And then there would be the service call like, no, seriously, we really do need good tires on this. We're not just moving to bed today. And yeah'd we'd chase with that and a couple other vehicles for data collection and so yeah i've launched this big balloon into the into a cloud it has a what's called a radio sonde on it which measures pressure temperature and humidity and gives us position information as the balloon flies through the cloud and then at the very very tail end of the train of instruments hanging off the balloon is an electric field meter that we use to measure the vector electric field inside the cloud. And by electric field meter, you mean basically a metal marble? It's two marbles attached to a spinning rod. And those marbles are made of aluminum.

And just like our flat plate lightning antenna, those metal spheres are attached to a charge amplifier circuit. spin in the thunderstorm's electric field, charges induced on those spheres back and forth, and the charge amplifier digitizes that signal, and that gives us a measurement then that's proportional to the electric field in the cloud. Does it try to radio the signal to you, or do you collect the whole apparatus afterwards? It does both. So the original version of the instrument actually sized those spinning spheres so that they were the right size to radiate in the 400 megahertz radio band that's used for meteorological instruments. That's since been re-engineered to put a proper transmitter separate from the spinning spheres. So we collect the data in real time that way, and then the data is also recorded on board, and that's a perfectly clean signal that's recorded and doesn't have any transmission noise and things in it. So we also try and retrieve those those instruments that way so the the two spinning balls uh i'm assuming do they they don't go at a constant rate because

they're being essentially pushed around by the electric field of the storm is that right no they're uh they're driven by a motor so they, okay. They do spin at a constant rate, and that helps us in doing the signal processing, because we can also then record a reference for the spin rate and then use that to, it's called demodulate, the electric field signal that's recorded. And by demodulate, that is every time it goes in a circle, it gets a different view of the world. And so it kind of forms a sine wave. If you had charge on one side and none on the other, it would be high, getting lower, lower, lower, low, and then back to high, and it'd be going around in a circle.

And so your data would look kind of like a sine wave on one channel and on the other channel it would like a look like a sine wave that was exactly 180 off yeah exactly um and so you you've described that perfectly it's uh um and then we we record another sine wave from an accelerometer that's inside the spheres that tells us the position of the spheres but we do that as a um as a demodulation problem where we multiply the reference sine wave with the electric field sine wave and uh and use that to uh and then do some filtering to pull out the the varying electric field and the accelerometer is because you might not be in a single plane. You're not spinning on a desk.

You're in a 3D space. And so the accelerometer will tell you the tilt? So the accelerometer is actually how we get the in-phase and 90 degrees out-of-phase orientation of the spheres. 90 degrees, yes. Yeah. Yeah, I guess 180 would just cancel us all out. So we have a 0 and 90 degree reference from the accelerometers.

That gives us the position of the spheres. They are spinning in a plane. And then the whole instrument platform also has, that also rotates usually around a vertical axis. There's paddles on the end of that spinning rod that has the spheres attached to it. And that spinning, the aerodynamic forces on those paddles cause the instrument to rotate around at a slower rate than our spin rate. And so that's what allows us to get the vector electric field out of the charge that's induced on the spheres as it spins around. And the end effect of this is you are taking a picture of the electricity in the whole cloud, and you end up with tree-like looking structures or more general pictures? It gives you, yeah, the electric field variability in the cloud. You can use Gauss's law from our beloved physics two class to infer charge density from how the electric field varies with height.

And so that's how we know and verify what we think the charge structure is in thunderstorms. That normal tripole plus minus plus structure is something that you see in the fluctuations of the electric field. And then the other thing it tells us is the maximum electric field that we uh we observe in clouds and that maximum electric field is uh is much much less than the actual breakdown dielectric strength of the atmosphere and so um that's another one of the the sort of enduring mysteries in lightning is that we know roughly at what electric field lightning starts at. But it's not just a conventional dielectric breakdown that allows the lightning flash to get started. And so there's some high-energy physics explanations for what might be going on there.

Relativistic things, even. Really? Yeah. Okay, we're going to start the show over now for another hour where we talk about tell us about relativity and lightning uh yeah um so now we're this is where it would be great if i didn't have a meteorology background in fsx1 but um what i learned from my but from my colleagues over the year um you you do observe uh X-rays and gamma rays that are emitted by lightning flashes. These are actually a source of noise for astronomers that are trying to do satellite- the initial breakdown stage of the lightning discharge.

And there's electrons that are accelerated in the electric field up to relativistic speeds, and that results then in these higher energy emissions that are due to relativistic processes. Cool. Did you get all that? Can you explain it to me later? Sure. Okay, cool. Lightning, thunderstorms are particle accelerators. Oh, all right. Yeah, that's cool. Yep, that's excellent. Yeah.

Is there such a thing as lightning tourism lightning tourism i've never seen lightning with a samsonite yes covers a lot of miles i mean and uh does so with uh low carbon, so that's a plus. There are certainly storm chasing tours that you can get. A lot of those are based around trying to find a tornado and see some storm structure. There's folks that also do that from a more photographic point of view where you want to be a little bit further away from the storm and, you know, get some good photos of it. Lightning's a part of that, and you could certainly, I'm sure there's good lightning photographs that are taken as part of those things. Is there some other kind of tourism you've heard of?

No, I just wanted to know where I could go to see lightning. Yeah. The web. The web. Christopher, last time there was a storm here and there was lightning, Christopher made me come inside right away. It was very sad. No, that's the right reaction. Yeah, that's probably a safe move in the US. doing the initial instrument checkout,

there happened to be a thunderstorm over California on the day when they were doing some of those initial checkouts. So they actually got the rare local lightning event right overhead where the instrument was developed. That is pretty rare. If I recall, in the case you're talking about, we were at the beach and lightning was striking the water. Yeah, but I'd already gotten out of the water.

From Indrik, he says there is an Estonian proverb that goes, lightning keeps striking the same house. This is opposite to the English speakers, lightning never strikes the same place twice. Which one is more true? Oh boy. Fascinating how these, how science and culture sort of interact

or physics and culture interact to give these different impressions of things. So I wonder if that house that keeps getting struck is in a you know, if it were sitting on top of a hill, it would behave just like tall TV towers or radio towers that

tend to wind up concentrating any lightning strikes in the vicinity to the location of those towers. And so perhaps it's a, it's a house in an unlucky location that, that winds up getting struck. But overall,

I would say lightning, I, I would say is a fairly random process. And if there's nothing else to drive at any given direction, it's, it's gonna, it's gonna move around pretty randomly. Okay. Okay. So, great answer. That was an odd but interesting question. I have one more question

for you before we let you go about your weekend. This one is from Stony Monster. Who's that? What powers will I develop if I get struck could you be specific specific and what is the best way to get struck in order to get these powers i highly do not recommend that um so the uh that i knew that would be the answer dang it but there are um there are major sort of – if you survive a lightning strike, and people do, there are persistent neurological effects that include – X-Men powers? Not X-Men powers, unfortunately. No, it's –

I shouldn't make fun. It is pretty bad. I mean, yeah, it really does leave people with debilitating injuries or depression or other things that really has a significant impact on their quality of life. And there's even sort of things that the death statistics don't capture in terms of the impact of lightning on folks. So even as we've done a good job of reducing lightning deaths, those lightning injuries can still be really quite severe. All right. I'm chastened. Sorry to be a buzzkill. Lightning-ing.

Well, Eric, thank you for being with us. Do you have any thoughts you'd like to leave us with? I would say that if any of this sounds interesting to you, it would take as much math and physics as you can if you're, say, a college student listening. And we always love having people that have an interest in instruments and building hardware that enlist in graduate programs to study this kind of thing. And, you know, the ability to make things and build instruments is just such an advantage and maybe a skill that's a little bit too rare. So, you know, if you have that kind of interest, I'd encourage you to go for it.

Cool. Well, our guest has been Eric Bruning, Professor of Atmospheric Science at Texas Tech University. Thank you again for being with us, Eric. You're welcome. I really enjoyed talking. Thank you very much to our Patreon subscribers, both for Eric's mic and their awesome questions, including Nick, Tom Anderson, and Indrik, and KBX81. And I guess Stoney Monster, although I feel a little weird because he's going to get thanked again. Special thanks to John Lehman of the Don't Panic Geocast for connecting us to Eric and for handing me some more questions. Wow, I didn't have to do hardly any work this week.

Eric's episode on the Geocast was a lot of fun to listen to. Finally, thank you to Christopher for producing and co-hosting, and thank you for listening. You can always contact us at show at embedded.fm or hit the contact link on embedded.fm. Now, a quote to leave you with. This is from Bob Ross. In painting, you have unlimited power.

You have the ability to move mountains. You can bend rivers. But when I get home, the only thing I have power over is the garbage. Embedded is an independently produced radio show that focuses on the many aspects of engineering. Thank you.