The Rainbow Connection—To Physics

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Who among us doesn’t stop and marvel at a rainbow? It is really a wonder of nature. And if you don’t believe me, just ask the double rainbow guy.

SPEAKER 1: Whoa, that’s a full rainbow all the way. It’s a double rainbow all the way. Oh my god. Woo!

IRA FLATOW: Whoa, a lot of enthusiasm there. And the double rainbow is just the beginning. Did you know there are moonbows at night? And yes, there are even white rainbows. And how about this? Each of us has our own rainbow. The one I’m looking at is different than a rainbow you’re looking at, standing next to me. We’ll get into all of that, the science, the magic behind these majestic multicolored arc forms.

My next guest is here to be our rainbow connection– get it– and to tell us all about rainbows and why Hawaii might be the rainbow capital of the world. And there’s an app that will let you become a rainbow hunter if you’d like. Let me bring on a man who had a double rainbow for breakfast, he tells me. Steve Businger, professor of atmospheric sciences at the University of Hawaii in Manoa, welcome to Science Friday.

STEVE BUSINGER: Thank you, Ira. I’m thrilled to be here. I’m a big fan of your show. Amazingly, I woke up this morning and looked out the window, and there a spectacular double rainbow right outside my window.

IRA FLATOW: Thank you. Nice to have you. Just a quick note to our listeners, our segment is being recorded with a live Zoom audience. And you all can learn more about joining a future live recording at sciencefriday.com/livestream. I think it’s great that you had a double rainbow for breakfast. Because I love rainbows. They are works of nature, but they’re also works of physics, right? Tell us how a rainbow works. Where did the colors come from? How does all of this happen?

STEVE BUSINGER: Well, you need sunshine, and you need some rain. And that’s the combination, which we get a lot in Hawaii. The sunshine, it goes into the raindrop. It refracts as it goes in. It reflects off the back of the drop, and it refracts again as it comes out. And in the process, the light from the raindrop has an angle of 42 degrees from the incoming sunlight. And that is the angle of the rainbow. And it is this refraction that is different for different colors or wavelengths of light that separates out in the process to produce this gorgeous rainbow.

IRA FLATOW: And so things have to be very specific. You have to get that 42 degree angle, right? You have to have the sun behind you.

STEVE BUSINGER: That’s right. You have to have the sun behind you. And if you look at the shadow of your head, when you’re standing looking away from the sun, the shadow of your head, that’s the point where you measure the 42 degrees from. And the funny thing is, your friend is standing next to you, and the shadow of his head is the center of his rainbow. And that’s why no two people see exactly the same rainbow. Because each rainbow comes out from the shadow of your head at 42 degrees.

IRA FLATOW: So the raindrops that are falling down as they’re making the rainbow are different for me than they are for you. That’s what you’re saying.

STEVE BUSINGER: The colors are coming from a different set of droplets, and therefore it’s two different rainbows essentially.

IRA FLATOW: That really is cool. So if the rain droplets are falling, we get that ROYGBV sort of color, right?

STEVE BUSINGER: Sir Isaac Newton, who explained that dispersion of the colors and actually came up with spectrum, that word, and he decided that there were seven colors in a rainbow. And it turns out that the seven colors is a bit arbitrary. But it does follow along with the seven days of the week and the seven main major planets and the seven musical notes in an octave. So he felt that that was really compelling, that there ought to be seven colors. But the human eye can see many, many more colors than seven.

IRA FLATOW: Let’s talk about the ingredients for a rainbow. What are the environmental conditions that you need for a rainbow to form and for anyone to see it?

STEVE BUSINGER: Right, it’s interesting because a place like Seattle or London gets lots and lots of rain, but they don’t get as many rainbows as Hawaii. And the reason is that you need to have rain falling out of relatively isolated showers with lots of ability for the sunshine to get in on the side.

And another thing that we have in Hawaii, plus the fact that we have these isolated showers– trade wind showers they are called– is that we have mountains. And the mountains enhance those showers as the air comes down on the lee side, it causes the cloud to evaporate and have sunshine. So in the town of Honolulu, which is on the lee side, you look back towards the mountains in the afternoon, and there are lots and lots of these showers creating rainbows.

IRA FLATOW: Let’s get into this double rainbow question. We started out the segment on double rainbows. Let’s go to our first listener question. Barry has a question about the rarity of double rainbows.

BARRY: Yes, double rainbows– how common are they? And can you have rainbows that are triple or even more than that?

IRA FLATOW: Oh, yeah, good question. What do you say, Steve?

STEVE BUSINGER: That’s an excellent question. It turns out that double rainbow is the product of two reflections at the back of the drop. The primary rainbow has one reflection, and the double rainbow has two reflections. And it comes out at 51 degrees instead of 42 degrees. So it’s a little higher above the primary bow. And because there’s an extra reflection, the colors are reversed, with red on the bottom and indigo on top.

There are possibilities for more internal reflections. And therefore, there is that third rainbow and the fourth rainbow and a fifth rainbow, et cetera. But with each reflection, you’re losing a bit of light out the back of the drop. And so it’s very rare to be able to see these other rainbows.

The third rainbow creates a big circle around the sun. And then the fourth rainbow goes back in on the other side. And in the last few years, it turns out, with all these digital cameras and people taking photographs, that there actually have been some photographs, some photographic evidence of these tertiary and quaternary rainbows. It’s really fascinating.

IRA FLATOW: Wow. There’s so many things about rainbows that are fascinating. For example, I hear that you can have rainbows that are a single color. Is that right? How does that happen?

STEVE BUSINGER: Well, let’s say that there’s just a little bit of volcanic emission that happens in Hawaii. We get a little bit of foggy aerosols in the air that scatter the blue light and the green light out. And so, what’s left is this reddish orange light. And when that hits the rain, it produces a red rainbow. Now, on the other side, there is a possibility to get a white rainbow, too. And we call that a cloud bow or a fog bow, depending on what the cloud is that’s producing it.

So, in that case, you have the droplets are small enough so that there’s enough Mie scattering going on, and there’s a blurring of the colors. And since they all kind of blur together, they become white light again. That gives you a white rainbow. And the interesting thing is that the Hawaiians had a saying that if you saw a white rainbow, it was good luck. It meant prosperity.

IRA FLATOW: That’s quite interesting. And rainbows at night, do we have a special name for them?

STEVE BUSINGER: Well, moonbows, of course.

IRA FLATOW: Moonbows.

STEVE BUSINGER: Yes, the moonbow is more restricted because we don’t always have a full moon. And you need a lot of light from the moon in order to create a moonbow. The human eye is less able to see color when the light is very dim. Think about a car driving by at night. It may be red, but it kind of looks black. So the moonbow has less distinct colors than the rainbow does. However, digital cameras now can create some beautiful photographs of moonbows, so it’s definitely worth going out looking.

IRA FLATOW: Now I understand that you have a study that makes the case that Hawaii is the rainbow capital. Tell me what conditions make Hawaii the capital for rainbows.

STEVE BUSINGER: It turns out that there are lots of reasons why Hawaii is special when it comes to rainbows. I’ve already talked about some of them. The trade winds blow seven days out of 10 and bring these isolated showers, which are very perfect for rainbows. The mountains produce updrafts on one side and sinking motion on the other, so you get rain over the mountains and clear skies in the lee– another great opportunity for rainbows.

Then we have very clean, clear air here– most of the time. We’re very far from any pollution sources, from dust and pollen that you might have on the mainland. And so the air is pretty clean. And that results in very strong sunshine that produces a brilliant rainbow. Now the last thing, which is related to all that, is that the clouds here because the air is so clean have fewer cloud droplets.

And what that means is that the moisture in the air is going to be divided over fewer individual droplets. And this makes the clouds more efficient at producing coalescence, where the rain comes together and produces rain droplets out of relatively shallow, small clouds. And so we have lots and lots of this rain coming out of small, shallow clouds with bright sunshine coming in. So it’s a combination.

IRA FLATOW: I love these details. Who knew? This is rainbow geek heaven–

STEVE BUSINGER: It is indeed.

IRA FLATOW: –that you’re talking about. We talked to Pukea Nogelmeier, who studies Hawaiian language and you worked with. And he talked about how Native Hawaiians describe rainbows.

PUKEA NOGELMEIER: There’s a really high level of specificity in Hawaiian. In English, I think we use rainbow a great deal. There’s a rainbow over the ocean. There’s a rainbow there, where a Hawaiian would distinguish that more and say it’s a pillar of color. It’s a band of– so it’s a different specificity, I think, so. Also, the notion of ROYGBV, that’s sort of culturally defined. Those distinctions of color are sort of uniquely English. Yeah, blue, indigo, and violet actually merge into a single sort of [? [HAWAIIAN] ?] in Hawaiian. [HAWAIIAN] is the baseline of the arc of color. It has as much to do with intensity as with color distinction.

IRA FLATOW: And he had this to say about these observations as science.

PUKEA NOGELMEIER: It kind of overlaps almost in the sense of citizen science, right? Isn’t science just observation that’s verified over repetition and collection of data? This is science data, whether it’s scientifically approached or simply as an observer approach, is invaluable stuff.

IRA FLATOW: Yeah, Steve, how did you work with all that scientific citizen science data?

STEVE BUSINGER: The ancient Hawaiians were keen observers of the natural environment. And their survival really depended upon that kind of very careful observation. They had names for winds coming down different valleys and from different directions. So they have hundreds of names for different winds. They have many, many names for rainbows. I mean, I just went to the dictionary and pulled out 20 names. But it turns out there are many, many more.

And from these citizen science articles that were published in the newspapers, we discovered that there was a category 4 hurricane that hit the Big Island and Maui in 1871. We could see exactly what the track was. We knew what the damage was by the descriptions of what happened to the trees and so forth. And it made a difference in how we think of hurricane risk in the Hawaiian islands. So these citizen science observations have real implications.

IRA FLATOW: That’s amazing. That’s really cool. I’m Ira Flatow, and this is Science Friday from WNYC Studios. Let’s go to a next question from our listener Renoir has a question about rainbows on other planets. Yeah, I’d like to know that. Go ahead.

RENOIR: I just had a question about how a rainbow might look on another planet. I guess, you wouldn’t really know because we haven’t really seen one, I don’t think. But I was curious about how the sun– various suns and their wavelength frequencies might affect how a rainbow would look. And also, do you need water droplets, or could you have other chemicals or whatever in the atmosphere that would create a different type of rainbow? Thanks.

IRA FLATOW: Great question. What do you say to that, Steve?

STEVE BUSINGER: Oh, that’s a wonderfully inventive question. And it shows a lot of creativity. I haven’t really thought about that, so this is just off the cuff. Certainly, if the sun or the star that this other world is orbiting around has a different color than our sun– and it also depends on the eyes, how they have developed in the life form of this other planet.

Of course, if we traveled there, then it would be our eyes. But yeah, there certainly could be other kinds of precipitation and different refractive index associated with that. One could imagine, maybe do a little bit of research on this to make a case for a different look of a rainbow on a foreign world. I think it’s really intriguing.

IRA FLATOW: Yeah, so if it’s raining nitrogen or something, it’s going to look a whole lot different there– or methane or whatever it is. And we’re running out of time, but I want to get to something I’ve been dying to talk to you about. I know you’re creating an app called Rainbow Chase to help people track where a rainbow might be. It sounds to me like tornado tracking, but you’re doing it with rainbows– finding a rainbow forming.

STEVE BUSINGER: Yeah, a friend of mine, Paul [? San, ?] he’s absolutely nuts about rainbows. He’s actually originally from South Korea. And in Korea, when they see a rainbow, it makes the nightly news. And so, he and I dreamed up this idea of photographing a circle rainbow from a helicopter. And we needed to know where to go with the helicopter. So I said to myself, well, why don’t we create an app for a smartphone where we can give the helicopter pilot some guidance as to where he should fly to get the circle rainbow? And that’s sort of described in this paper that I published.

But the rainbow app is actually available, and you can download it. Just type in Rainbow Chase, and you can download it for free. And it will remain free, because our goal is to bring more rainbows into people’s lives. Unfortunately, at the moment, it’s only available in Hawaii in terms of– and so if you download it, you’ll see Hawaii. And you can look at where the rainbows are in Hawaii. But we are planning to expand it to the mainland. And that’s our goal.

IRA FLATOW: So if you want that rainbow connection on your app, you’ve got to go to Hawaii to use it.

STEVE BUSINGER: That’s right– for now. But in the future–

IRA FLATOW: For now. All right, I’m looking forward to that, and I’m very happy that you took time to talk with us about rainbows today, Steve.

STEVE BUSINGER: It’s been a whole lot of fun.

IRA FLATOW: Dr. Steven Businger is professor of atmospheric sciences at the University of Hawaii in Manoa. And if you want to see photos of these double rainbows, the 360 bows and more, you can get your rainbow fill at our website at sciencefriday.com/rainbows.

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