Rainbows do not appear every time it rains. A rainbow requires a specific geometry between the sun, airborne water droplets, and your eyes, and if any piece of that arrangement is missing, you can stare at a rain shower all day without seeing a single colored arc. The conditions that produce a rainbow are surprisingly narrow, which is part of why spotting one still feels like an event rather than a routine consequence of wet weather.
What Actually Has to Happen for a Rainbow to Form
A rainbow forms when sunlight enters a water droplet, refracts (bends) as it passes from air into water, reflects off the back inner surface of the droplet, and refracts again as it exits. Each wavelength of light bends at a slightly different angle, which is why white sunlight fans out into its component colors. The light that reaches your eyes after this process is concentrated near an angle of about 42 degrees from the line between the sun and the antisolar point, the spot in the sky directly opposite the sun from your perspective.
That 42-degree geometry is the crux of the whole thing. For you to see a rainbow, the sun must be behind you and relatively low in the sky, while rain or mist is falling in front of you. If the sun is higher than about 42 degrees above the horizon, the rainbow arc drops below the horizon and you cannot see it from flat ground. This is why rainbows are far more common in the morning and late afternoon than at midday, and why they are practically impossible to see during a summer noon at tropical latitudes where the sun is nearly overhead.
The Most Common Reasons Rain Produces No Rainbow
Even when it is raining, the sun is often hidden behind the very clouds producing the rain. Overcast skies block direct sunlight, and without a strong beam of sunlight reaching the rain curtain, there is nothing to refract. A passing shower on a partly cloudy day is the classic rainbow-friendly setup precisely because the sun can peek through a gap in the clouds while rain still falls in the opposite part of the sky.
Timing matters as well. Rain that falls entirely after sunset produces no visible rainbow because there is no sunlight to work with (though moonlight can occasionally step in, as discussed below). Rain that falls while the sun is at a high angle pushes the arc below the horizon. Rain that falls in the same direction as the sun relative to you means you are facing the wrong way. You can sometimes fix this simply by turning around, which is why the old advice to look away from the sun during a rain shower is genuinely useful.
Drop size also plays a role in whether a rainbow looks vivid, washed out, or essentially invisible. Droplets around 1 to 2 millimeters in diameter produce the brightest, most colorful rainbows. As droplets shrink well below half a millimeter, the colors blur together and the arc becomes a whitish band called a fogbow. Research on polarized rainbow light shows that smaller droplets fundamentally change the optical structure of the bow: below about 0.3 millimeters in diameter, the intensity pattern and spacing of color fringes shift in ways that reduce the perceived vividness of the arc.1Applied Optics. Polarized rainbow So a very fine drizzle from thin clouds can technically form a rainbow, but one so pale you might not notice it.
Droplet Shape and the Dark Band Between Bows
Most explanations of rainbows treat raindrops as perfect spheres. Small drops actually are very close to spherical, but larger drops flatten into oblate shapes as they fall, squished by air resistance into something more like a hamburger bun than a marble. That distortion subtly shifts the angles at which light exits the drop and changes what you see.
One place this matters is in the region between a primary rainbow and a secondary rainbow, known as Alexander’s dark band. That strip of sky looks noticeably darker than the sky on either side because light is not directed into that angular range by spherical drops. Laboratory work with laser-illuminated oblate drops, however, has shown that when a drop’s width-to-height ratio reaches about 1.08, additional light paths involving multiple internal reflections can send faint cusps of light into that otherwise dark zone.2PubMed. Generalized rainbows and unfolded glories of oblate drops: organization for multiple internal reflections and extension of cusps into Alexander’s dark band In natural rain, large drops routinely reach that degree of flattening, which means the dark band is never perfectly black in a heavy downpour. It is still visibly darker, but trace light from distorted drops softens the contrast.
Why Some Rainbows Are Brighter or More Complete Than Others
Even when all the geometry lines up, rainbows vary enormously. Some stretch as a full semicircle from horizon to horizon, while others appear as just a short fragment. The difference comes down to how uniformly rain is distributed across the sky and how strong the sunlight is.
A full semicircular arc requires rain to be falling across a wide swath of sky at roughly the same distance from you. If rain is only falling in a narrow column, you see only the part of the arc where that column happens to sit. A patchy sky with scattered showers can produce a rainbow fragment that appears and vanishes as different curtains of rain drift through the right geometry.
Brightness depends on both the density of droplets and the intensity of the light source. A rainbow seen against a dark thundercloud backdrop looks vivid partly because of contrast: the bright colored arc pops against the near-black background. On a hazy, bright day, the same arc can look pale because the background sky is already flooded with scattered light. Polarization plays into this too. Rainbow light is strongly polarized, meaning the light waves vibrate predominantly in one plane. If you wear polarized sunglasses, a rainbow can look dramatically brighter or dimmer depending on the angle of your head, because the lenses either pass or block that polarized light.
Double Rainbows and Higher-Order Bows
When conditions are good enough to see a primary rainbow, you can sometimes spot a secondary bow above it. The secondary rainbow forms from light that reflects twice inside each droplet rather than once. That extra bounce reverses the color order (red is on the inside of the secondary arc, whereas it is on the outside of the primary) and spreads the light over a wider angle, making the secondary bow dimmer and broader.
Third-order and fourth-order rainbows exist in theory, formed by three and four internal reflections respectively, but they appear on the same side of the sky as the sun and are almost impossibly faint against the bright sky in that direction. They have been photographed in nature only a handful of times with specialized camera setups. For practical purposes, a double rainbow is the most you will see with the naked eye, and even that requires fairly uniform rain and strong, direct sunlight.
Rainbows Without Rain
Water droplets from sources other than rain can produce rainbows just as effectively. Garden sprinklers, waterfalls, breaking ocean waves, and even industrial mist systems all generate arcs of color when sunlight hits them at the right angle. The physics is identical: light enters a roughly spherical droplet, refracts, reflects internally, and exits at a spread of angles that separates the colors.
Fogbows, sometimes called white rainbows, form in fog or very fine mist. Because fog droplets are tiny, typically well under 0.1 millimeters, the color bands overlap so much that the arc appears white or faintly tinted. Fogbows are surprisingly common if you know what to look for. They show up over mountain passes, along coastlines, and even in your own backyard on a misty morning with the sun behind you. Despite their ghostly appearance, they follow exactly the same optics as a vivid rainfall rainbow; only the droplet size differs.
Spray rainbows from waterfalls are a near-guaranteed sighting in the right conditions because the waterfall continuously generates a thick curtain of mist at a predictable location. Popular waterfall viewpoints are often positioned so that afternoon sun creates a rainbow in the spray for hours at a time.
Moonbows and Nighttime Arcs
A moonbow, or lunar rainbow, forms the same way as a solar rainbow except the light source is the moon rather than the sun. Because moonlight is roughly 500,000 times dimmer than direct sunlight, moonbows are extremely faint. Most appear white or very pale gray to the naked eye because human night vision relies on rod cells, which do not distinguish color well. Long-exposure photography reveals that moonbows do carry the full spectrum of colors, but your eyes simply cannot detect them at such low light levels.
Research into how human vision adapts at low light levels shows that at scotopic (very dim) illumination, sensitivity adjusts primarily by speeding up or slowing down the response of the rod pathway rather than by changing the rods themselves.3PubMed Central. Human scotopic sensitivity is regulated postreceptorally by changing the speed of the scotopic response In practice, this means your eyes are quite good at detecting that a faint moonbow exists but poor at extracting color from it. A bright full moon low on the horizon, combined with rain or waterfall spray in the opposite sky, gives the best chance of seeing one. Even then, most people describe moonbows as ghostly white arcs rather than colorful bands.
Location, Latitude, and Season
Where you live substantially affects how often you see rainbows. The ideal combination is frequent, short rain showers interspersed with strong sunshine and a sun that sits at a moderate angle. Tropical islands like Hawaii are famous for rainbow sightings precisely because trade-wind showers move through quickly and the sun re-emerges within minutes. At high latitudes in summer, the sun stays relatively low in the sky for long stretches, which keeps the rainbow geometry favorable for hours, but long spells of overcast weather can cancel out that advantage.
Deserts and arid regions see fewer rainbows for the obvious reason that rain is rare. But when desert thunderstorms do occur, they often produce spectacular rainbows because the air between showers is exceptionally clear, letting sunlight pass without scattering, and the dark storm backdrop provides vivid contrast.
Altitude changes the picture too. From an airplane or a high mountain, you can sometimes see a rainbow as a complete circle rather than a semicircle, because the ground no longer blocks the lower half of the arc. Pilots and mountain climbers occasionally report full-circle rainbows, also called glory-like bows, that surround the shadow of the aircraft or the observer on a cloud layer below.
How Other Animals Might Experience Rainbows
Humans see rainbows as bands stretching from red through violet because our eyes contain three types of color-detecting cone cells sensitive to long, medium, and short wavelengths. Animals with different visual systems would perceive a rainbow differently, or possibly not at all.
Many bird species have four types of cone cells, including one sensitive to ultraviolet light. A study of 38 bird species found that the wavelength at which their eye media transmit half the incoming UV light ranged from 310 to 394 nanometers, with species carrying UV-sensitive visual pigments having ocular media that allowed significantly more UV through to the retina.4Proceedings of the Royal Society B: Biological Sciences. Ultraviolet vision in birds: the importance of transparent eye media In principle, a bird with strong UV sensitivity looking at a rainbow could see an additional band of color beyond the violet edge that is invisible to humans. Whether birds attend to rainbows in any behavioral sense is unknown, but their visual hardware means the rainbow they could perceive is wider than ours.
Dogs and most other mammals have only two types of cone cells, which limits their color discrimination roughly to blues and yellows. A rainbow to a dog would look like a smeared band of two or three hues rather than the full spectrum. Insects like bees also see into the ultraviolet but have very different spatial resolution, so a rainbow would appear as a diffuse wash of light rather than a crisp arc.
Supernumerary Bows and Other Optical Oddities
If you look closely at a particularly bright rainbow, you may notice faint pastel-colored bands just inside the primary arc, usually in shades of green and pink. These are supernumerary bows, and they arise from the wave nature of light rather than simple reflection and refraction. When two light rays exit a droplet at nearly the same angle but have traveled slightly different path lengths inside the drop, they can constructively or destructively interfere with each other, creating alternating bright and dark fringes.
Supernumerary bows are most visible when raindrops are small and fairly uniform in size, typically below about one millimeter in diameter. In a natural shower with a mix of drop sizes, the fringes from different-sized drops overlap and blur out. A fine, uniform drizzle or the mist from a garden hose produces the clearest supernumeraries, sometimes three or four distinct bands hugging the inside of the primary arc.
The polarization properties of these fringes depend on droplet size. For drops larger than about one millimeter, the supernumerary bands in both polarization components line up at the same angular positions, but for smaller drops the two polarization components produce fringes at different angles, so the bands in one polarization component fall between those in the other.1Applied Optics. Polarized rainbow To the naked eye this just looks like a subtly different color pattern, but it is one reason why supernumerary bows from fine mist can look slightly different from those seen in heavier rain.
Common Misconceptions About Rainbows
One of the most persistent myths is that every rain shower should produce a rainbow if you look in the right direction. As covered above, overcast skies, high sun angles, drop sizes that are too small, and simply facing the wrong way can all prevent one. A second misconception is that rainbows are physical objects located at a specific place. A rainbow has no fixed location; it is an optical effect that depends entirely on where you stand. Two people a few meters apart see slightly different rainbows formed by different sets of raindrops, and neither of them could walk toward the arc and get closer to it.
Another common error is the idea that rainbows always display seven distinct colors. The number seven comes from Isaac Newton’s somewhat arbitrary division of the spectrum, influenced in part by his interest in musical harmonies. In reality, a rainbow is a continuous gradient. Where one color ends and the next begins is a matter of perception and language, not physics. People in cultures with fewer basic color terms perceive fewer distinct bands, and even among English speakers, indigo is notoriously hard to pick out between blue and violet.
Finally, the idea that you need heavy rain to see a rainbow is wrong. Some of the most vivid rainbows form in light showers or fine spray, because small, uniform droplets produce clean optical effects and the air between you and the rain is clear enough to let plenty of sunlight through. A distant heavy downpour can actually reduce visibility so much that the rainbow, though technically forming, is hidden behind a gray wall of rain.