Seeing rainbow colors in the sky without any rain is surprisingly common, and it almost always has a straightforward explanation. Rainbows need water droplets and sunlight, but rain isn’t the only source of water droplets. Ice crystals high in the atmosphere, fog, waterfall mist, and even thin clouds can all split sunlight into its component colors. What you saw likely falls into one of several well-documented optical phenomena.
How Rainbows Actually Form
A traditional rainbow happens when sunlight passes through water droplets and gets bent, or refracted, separating white light into its familiar color bands. The key ingredient is water suspended in the air, not rain specifically. Any collection of droplets or ice crystals in the right size range, hit by sunlight at the right angle, can produce rainbow-like colors. This is why so many different sky events can look like a rainbow even on a perfectly dry day.
Ice Crystal Halos
The most common rainbow-without-rain sighting is a halo around the sun or moon. These form when light passes through hexagonal ice crystals in thin, high-altitude clouds called cirrostratus. The light bends twice as it enters and exits each tiny crystal, deflecting it exactly 22 degrees from its original path. The result is a faint ring of light, sometimes with a rainbow tint, circling the sun at a consistent distance. These 22-degree halos can appear on completely clear-looking days because cirrostratus clouds are so thin they’re nearly transparent.
Circumhorizontal Arcs (“Fire Rainbows”)
If what you saw was a vivid band of color running roughly parallel to the horizon, it was likely a circumhorizontal arc, sometimes called a fire rainbow. These occur when the sun is very high in the sky, at an elevation of 58 degrees or more, and its light passes through plate-shaped ice crystals in high-altitude cirrus clouds. Because the sun needs to be so high, these arcs are mainly visible during summer months and at latitudes closer to the equator. In northern parts of the U.S. or Europe, the window for spotting one is limited to a few months around the summer solstice.
A related phenomenon, the circumzenithal arc, appears as an upside-down rainbow near the top of the sky. According to the World Meteorological Organization, this one only forms when the sun is below 32 degrees elevation, making it more of a winter or early-morning sight. It’s often described as the most beautiful of all ice crystal arcs because its colors tend to be pure and vivid.
Iridescent Clouds
Sometimes you’ll see patches of pastel color clinging to the edges of clouds, almost like an oil slick in the sky. This is cloud iridescence, and it works through a different mechanism than a rainbow. Instead of refraction (light bending through droplets), iridescence is caused by diffraction, where very small water droplets or ice crystals scatter sunlight around their edges. NOAA’s satellite service describes it as a distinct phenomenon from traditional rainbows. The colors tend to be softer and less organized, appearing in random pastel bands of pink, green, and blue rather than the orderly red-to-violet arc of a rainbow.
Iridescent clouds are most visible when the cloud is thin and positioned near the sun. You’re more likely to notice them if you block the sun itself with your hand or a building, letting your eyes adjust to the surrounding sky.
Fogbows and Mist Bows
Fog produces its own version of a rainbow, called a fogbow. The physics are identical to a rain rainbow, but fog droplets are much smaller, rarely exceeding 100 microns in diameter compared to raindrops that are typically over 1 millimeter. That size difference matters because smaller droplets spread and overlap the color bands, washing them out. The result is a broad, ghostly white arc with only faint hints of color at its edges. If you’ve ever seen a pale white arch in foggy conditions and wondered if it was a rainbow, it almost certainly was, just one made by droplets too small to produce vivid colors.
Droplets in the 100 to 500 micron range, common in waterfall spray, produce bows that fall somewhere between a fogbow and a full rainbow. Yosemite Falls and Bridalveil Fall in Yosemite National Park are famous for this effect. When wind catches the spray and sunlight hits it at the right angle, a full spectrum of color appears in the mist. The display depends on time of year, sun position, wind conditions, and how much water the falls are carrying.
Sprinklers, Fountains, and Garden Hoses
You don’t need anything atmospheric to see a rainbow without rain. Any fine spray of water in sunlight will work. Garden sprinklers, car washes, fountains, and pressure washers all produce droplets in the right size range. The sun just needs to be behind you while the mist is in front of you, ideally with the sun at a moderate angle (roughly 42 degrees matters here, as that’s the angle at which light exits a water droplet after internal reflection). Morning and late afternoon tend to produce the best results because the sun is lower in the sky, putting the rainbow arc higher and easier to see.
How to Tell What You Saw
- Full arc near the horizon with vivid colors: likely a circumhorizontal arc, especially if the sun was high overhead
- Ring around the sun or moon: a 22-degree ice crystal halo
- Upside-down arc near the top of the sky: a circumzenithal arc
- Pastel patches on cloud edges: cloud iridescence
- Broad white or faintly colored arc in fog: a fogbow
- Standard rainbow shape near a waterfall or fountain: a spray bow, working on the same principle as a rain rainbow
All of these are caused by sunlight interacting with water in some form, whether liquid droplets or ice crystals. The variety comes from differences in particle size, crystal shape, and the angle of the sun. None of them require rain, and most of them are far more common than people realize. Once you know what to look for, you’ll start noticing them regularly.