Why Is Our Lightning Purple?

Transcript

ERIC: From the Museum of Science in Boston, this is Pulsar, a podcast where we answer our favorite visitor questions. I'm your host, Eric. And one of my favorite things about my job is presenting a lightning show in our Thomson Theater of Electricity. It has the world's largest Van de Graaff generator, musical Tesla coils, and it inspires some great questions. One of the most common we get after a show is: why is your lightning purple? Joining me to dig into this question is a fellow zapper, Becca from our education team. Becca, thanks for joining me.

BECCA: Thanks for having me here, Eric.

ERIC: So the first time I got this question, I had some guesses. But it's my favorite kind of question. Because by researching the answer, I learned a ton.

BECCA: Yes, this question is one that we get all the time. And it's one that there isn't a really good clear cut answer to, but I still love to talk about it.

ERIC: Yeah, there's a whole lot going on. And it's not like other questions where we have a simple answer. Honestly, sometimes that's the most satisfying when you can be like: that's the biggest animal. But it's complicated. This one's really complicated. So let's just start with what lightning actually is.

BECCA: Sure. So lightning essentially is just when charges get separated and come back together. And in doing so they end up heating the air where these charges get put back together to pretty extreme temperatures. Up to 50,000 degrees Fahrenheit, maybe even more. And so it is really cool that it creates this amazing light, this flash that we see. Of course, it can be a little bit different depending on the conditions.

ERIC: Yes. So we explained that in the show: what lighting is. What we're seeing is light from superheated air. And the color of that light depends on the air, really.

BECCA: Yeah, there are so many factors that really go into what produces a certain color of lightning. And one of those conditions is what is in the air because air is made up of different things, different elements, pieces of dust or moisture in the air. And depending on where you are, that may be different. And so having lightning kind of going through this one spot of air may have it look a little bit different, just depending on what's going on.

ERIC: Yeah, there's no such thing as pure air because air is a mixture of gases. The average across the Earth's atmosphere is like 70% nitrogen, 20-something percent oxygen, but there's carbon dioxide, there's argon, there's a bunch of other gases, there can be more of some in certain places, there can be moisture in the air, there can be dust in the air. it really depends on what the air is made of, and what's in the air. And our indoor lighting is the same thing as a lightning bolt outside in a storm. It's just smaller, and it's going through air. But it's indoor air. So our air is more controlled than in a storm.

BECCA: Yeah, we definitely control our humidity quite a bit. So having a little bit less moisture in the air inside will cause our lightning to be so specifically consistent in that it's always kind of purple. And that's one of the questions that we get all the time. But it also has to do a lot with kind of how close you are to the lightning. Since light moves through air, the further you are away, the more air it has to go through. The closer you are to the lightning, the less air has to go through and often will appear more white if you're really close. I know Eric, you probably have experienced this too. But when we are in the cage during the presentation in our Theater of Electricity, the lightning appears white because we're actually right next to it. But further away in the audience, it appears a little bit more purple because that light has gone through more air to get to your eyes.

ERIC: Yeah, so it's the same as looking at a sunset. The sun looks like a different color because the sunlight has to go through more air. Some of the light colors get separated out so that sunset sun looks more red. The same thing with lightning, even a couple feet of air can take away some of the colors and affect its appearance.

BECCA: Yeah, absolutely, it can definitely change the way that that light is hitting our eyes, it can bend it in a way that we end up getting different colors. Another reason why people have seen pink or green lightning before is because it often gets bent by, really specifically, snow in the air. And that snow can cause that light to bend in ways that really no other gases can. So snow is sort of important to be able to get those really cool colors.

ERIC: Yeah, it's all about your viewing conditions, what kind of air you're looking through how much air you're looking through what's in the air, all that stuff adds up. And it makes it even harder because everybody's eyes are different. And cameras are different. I mean, when I was researching this, I saw plenty of things on the internet that said if you take a picture of a lightning bolt with film versus digital, it comes out different somewhat. Sometimes it can look more yellow or brown depending on your camera settings.

BECCA: Yeah, and even, it might look different to each human in the exact same location, just depending on how each human is able to perceive color.

ERIC: So lightning out in a storm, it can look white. If you see a lightning bolt it can look blue, maybe purple. There's some colors that are more rare. Like you said, maybe snow lightning can look a little bit more green. But some reddish or orange or yellow lightning tends to be associated with volcanoes, which I know you're just really excited to talk about today.

BECCA: Absolutely. I love volcanoes. So combining two of my favorite subjects is excellent. I love this question about volcanic lightning, because it's something that we often don't think about. Usually people associate it with, maybe there's a storm. But actually, the volcanoes themselves in their eruptions can create lightning. They occur when there's a really large ash cloud, all of these sort of small particles of kind of cooled, volcanic, well, magma when it comes out, comes out in a big column, because it's being sort of ejected all at once. And in that column, there's a lot of ash that tends to rub against other particles of ash. And in doing so it separates the charges within this column, causing lightning to actually form through the entire column, but also even going outside the column to really anywhere else that can sort of rebalance those charges. And because there's so much ash, and often so much dust and dirt, inside this column, all of those particles get heated too, and they can burn off, which is why that lightning can look a little bit more red, and a little bit more orange and yellow. Also, there's just different gases coming out of the volcano. You get a lot of sulfur dioxide, and several other pretty intense gases that cause this lightning to look so different, but so cool.

ERIC: So it's caused by a lot of the same things that storm lightning is caused by with droplets kind of rubbing up against each other or separating charge. It's the same thing we do in our theater on a much smaller scale with our machinery. And it can happen in a volcano with all that ash. But the conditions are way different because you have all these volcanic gases, you have the dust, you have the really distinct lightning bolts that are like no other colors anywhere else in the world.

BECCA: Absolutely. And that's why a lot of volcanoes have been correlated with sort of other deities in mythology because of these intense bits of power and energy coming out of them. Not only are they impressive on their own, but when they create their own lightning, well, that's just another layer on top of it.

ERIC: Well Becca, thanks so much for chatting about different colored lightning.

BECCA: Of course. Thank you. This was so fun.

ERIC: On your next visit to the Museum of Science, don't miss our daily lightning shows. check mos.org for the daily schedule. And while you're at home, you can take a virtual tour of the inside of our 30 foot Van de Graaff generator in our Sparks of Science video series. Until next time, keep asking questions.

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