Are there always rainbows?
August 21, 2016 9:44 PM Subscribe
Is it safe to say that there is always a rainbow visible in the sky somewhere on Planet Earth? Relatedly, is it possible to estimate the average number of rainbows visible across the planet at a given moment?
You can also see rainbows in waterfall spray, so I think it's safe to say that the conditions for a rainbow exist all the time. I guess it's possible that at a single moment no person is experiencing it though.
posted by twirlypen at 2:13 AM on August 22, 2016 [2 favorites]
posted by twirlypen at 2:13 AM on August 22, 2016 [2 favorites]
Best answer: Your assumption seems to be that a rainbow is a physical thing in a specific place. It isn't, as others have explained. It's just an optical effect that relies on an observer being in the right place - in that way it's like the mirage of water you see in the distance on a hot road. Two people standing next to each other might think they're seeing the 'same' rainbow, but technically they're not - they're just seeing a similar optical effect because they're both present under the right conditions. As you move those two people further apart, the rainbows they see will differ more and more in terms of where they appear to begin and end.
I think a lot of people would argue that if there's nobody looking at it, there's no rainbow. Equally, you could argue that anywhere that there's sufficient rain and sufficient light, there's almost certainly a rainbow (or many different rainbows) that can be seen from certain directions within a few miles.
That's even before you get into whether a garden hose can make a rainbow, or whether a 'double rainbow' counts as two.
posted by pipeski at 3:29 AM on August 22, 2016 [2 favorites]
I think a lot of people would argue that if there's nobody looking at it, there's no rainbow. Equally, you could argue that anywhere that there's sufficient rain and sufficient light, there's almost certainly a rainbow (or many different rainbows) that can be seen from certain directions within a few miles.
That's even before you get into whether a garden hose can make a rainbow, or whether a 'double rainbow' counts as two.
posted by pipeski at 3:29 AM on August 22, 2016 [2 favorites]
Interesting concept and explanations. I've always wondered why a rainbow is curved rather than beam-shaped. Most likely it's optics/physics and such; but my favorite explanation every time I'm excited by happening on the temporal phenomenon is: delightful magic
Your question has now added to my inner commentary: delightful magic just for me!
posted by mightshould at 5:11 AM on August 22, 2016 [1 favorite]
Your question has now added to my inner commentary: delightful magic just for me!
posted by mightshould at 5:11 AM on August 22, 2016 [1 favorite]
Best answer: I think that the conditions that need to exist for a person to be able to perceive a rainbow probably exist somewhere on Earth most of the time. There may not be an observer of every potential rainbow instance.
I am inclined to think that it still counts as a rainbow if you could see it if you happened to be in the right place at the right time. I also think garden hose and sea spray rainbows count (and double rainbows count as one instance because they cannot be separated.)
There may be other planets with the conditions for rainbows, or a similar light refraction effect. So if you accept that they exist when people aren't watching, you could say that there's a rainbow happening all the time somewhere in the universe.
posted by blnkfrnk at 7:20 AM on August 22, 2016
I am inclined to think that it still counts as a rainbow if you could see it if you happened to be in the right place at the right time. I also think garden hose and sea spray rainbows count (and double rainbows count as one instance because they cannot be separated.)
There may be other planets with the conditions for rainbows, or a similar light refraction effect. So if you accept that they exist when people aren't watching, you could say that there's a rainbow happening all the time somewhere in the universe.
posted by blnkfrnk at 7:20 AM on August 22, 2016
This is completely anecdata, but I visited Iguazú Falls in Argentina/Brazil last month, and the amount of spray the falling water kicks up is enough to cause constant waterfalls. It was basically impossible to get a photo without a rainbow somewhere in it, especially from the foot of the falls on the Brazil side. So I would say yes, there ARE always rainbows somewhere on the planet!
posted by chainsofreedom at 8:00 AM on August 22, 2016 [1 favorite]
posted by chainsofreedom at 8:00 AM on August 22, 2016 [1 favorite]
Best answer: This is a good question for a guesstimate:
Assuming the average person sees one rainbow per year, and the average observation time is one minute, then one in 500,000 (=365*24*60) persons sees a rainbow. This means right now 14,000 people are seeing a rainbow. If we assume that a rainbow-producing cloud is visible from a 20 km2 area, with a world population density of 50/km2, then those 14,000 people see 14 'separate' rainbows.
In conclusion I think it's safe to assume there's always at least someone seeing a rainbow!
posted by Psychnic at 6:33 AM on August 23, 2016 [1 favorite]
Assuming the average person sees one rainbow per year, and the average observation time is one minute, then one in 500,000 (=365*24*60) persons sees a rainbow. This means right now 14,000 people are seeing a rainbow. If we assume that a rainbow-producing cloud is visible from a 20 km2 area, with a world population density of 50/km2, then those 14,000 people see 14 'separate' rainbows.
In conclusion I think it's safe to assume there's always at least someone seeing a rainbow!
posted by Psychnic at 6:33 AM on August 23, 2016 [1 favorite]
Just wanted to add that rainbows aren't about "interference pattern on your retina", a rainbow is the summation of the effects of refraction of light through multitudes of droplets of water. 1 2 3 4
Theoretically a latent rainbow could be said to exist anytime (a mostly point-source of) light hits more than a handful of droplets. "Latent" describing the fact that everything is there to create one except for the observer, and "light" rather than sunlight because any light source that contains more than one wavelength will be split. i.e. a "rainbow" formed with a laser would have only one band, and one formed from a sodium vapor lamp would be a bunch of distinct, coaxial arcs/circles.
To try to directly answer your questions, it depends on what you mean by "visible". I'd clarify by asking "Does visible mean someone is seeing it, or does it means there is one there to be seen if somebody looked?" Because if you are asking "Is it safe to say that the conditions necessary to produce a rainbow always exist in the sky somewhere on Planet Earth?" I'd say definitely Yes. If you're asking "At every moment in time is there a human seeing a rainbow formed of sunlight and droplets of water falling from a natural cloud?" I'd say "Probably".
Further, if you take the loose definition of "visible" (a latent image, ready for human perception), and a loose definition of "rainbow" (a visual artifact of refraction of different wavelengths of light passing through roughly-spherical droplets of liquid) then I would think that there would be a near-infinite number "visible" at any time.
posted by achrise at 11:11 AM on August 23, 2016
Theoretically a latent rainbow could be said to exist anytime (a mostly point-source of) light hits more than a handful of droplets. "Latent" describing the fact that everything is there to create one except for the observer, and "light" rather than sunlight because any light source that contains more than one wavelength will be split. i.e. a "rainbow" formed with a laser would have only one band, and one formed from a sodium vapor lamp would be a bunch of distinct, coaxial arcs/circles.
To try to directly answer your questions, it depends on what you mean by "visible". I'd clarify by asking "Does visible mean someone is seeing it, or does it means there is one there to be seen if somebody looked?" Because if you are asking "Is it safe to say that the conditions necessary to produce a rainbow always exist in the sky somewhere on Planet Earth?" I'd say definitely Yes. If you're asking "At every moment in time is there a human seeing a rainbow formed of sunlight and droplets of water falling from a natural cloud?" I'd say "Probably".
Further, if you take the loose definition of "visible" (a latent image, ready for human perception), and a loose definition of "rainbow" (a visual artifact of refraction of different wavelengths of light passing through roughly-spherical droplets of liquid) then I would think that there would be a near-infinite number "visible" at any time.
posted by achrise at 11:11 AM on August 23, 2016
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This xkcd 'what-if' has cool rainbow info.
Bonus: Here's a picture of a rainbow I took that kind of illustrates the geometry.
posted by quinndexter at 12:30 AM on August 22, 2016