When a person sees a small patch of color in the sky, they are witnessing one of several distinct atmospheric optical events. While the most familiar is the rainbow, the colorful patch might actually be a completely different phenomenon caused by ice crystals or tiny water droplets. Atmospheric science provides specific names for these localized color displays, which are created through different mechanisms of light interacting with moisture in the air. The answer to what a small patch of rainbow is called depends on where it is located and what kind of particles are bending the sunlight.
The Direct Answer: Rainbow Segments
The simplest explanation for a small, colorful arc is that it is a segment of a primary or secondary rainbow. Rainbows are full circles centered on the antisolar point, the spot directly opposite the sun from the observer’s perspective. Since the horizon blocks the lower half of the circle, we usually only see a partial arc, and if the necessary rain is localized, only a section of that arc may appear.
This vibrant color display is produced by sunlight entering relatively large liquid water droplets, typically raindrops ranging from \(0.25\) to \(3\) millimeters in size. Light refracts as it enters the droplet, reflects internally off the back surface, and refracts again as it exits, separating the white sunlight into its component colors. Because red light deviates slightly less than blue light, the red band appears on the outer edge of the primary rainbow, positioned at an angle of about \(42\) degrees from the antisolar point.
Sun Dogs and Other Ice Crystal Patches
A frequently misidentified “small patch of rainbow” is the parhelion, more commonly known as a sun dog. These bright, often colorful spots appear specifically to the left and right of the sun, always at the same altitude as the sun itself. The scientific name, parhelion, literally translates from Greek as “beside the sun,” which accurately describes their location.
Sun dogs are not caused by liquid water drops but by the refraction of sunlight through hexagonal plate-shaped ice crystals. These crystals are usually suspended in high-altitude cirrus or cirrostratus clouds. The light passes through the \(60\)-degree angle of two non-parallel crystal faces, bending the light at a minimum angle of \(22\) degrees, which is why sun dogs always appear \(22\) degrees away from the sun. Unlike a rainbow, the red color is on the inside, closest to the sun, because red light is refracted slightly less than blue light through the ice prism.
The White Rainbow: Fogbows
Another distinct phenomenon that appears as a bow or partial arc is the fogbow, often called a white rainbow. Fogbows are formed by the same core process as rainbows—the interaction of sunlight with water droplets—but the size of the droplets is significantly smaller. Instead of raindrops, fogbows are created by the microscopic water droplets suspended in fog or mist, typically less than \(0.05\) millimeters in diameter.
This tiny size means that light diffraction becomes a more significant factor than simple refraction and reflection. The result is that the individual colors are smeared and overlap extensively, preventing the formation of the distinct, vivid spectrum seen in a standard rainbow. The arc appears broad and mostly white or very pale, sometimes with a faint reddish hue on the outer edge and a bluish tint on the inner edge. Fogbows also form opposite the sun, just like a rainbow, with the observer’s back to the light source.
Telling the Patches Apart
Identifying which phenomenon you are observing requires noting three simple, observable characteristics: the patch’s location, its color intensity, and the atmospheric conditions.
Location
A true rainbow segment or a fogbow will always be positioned in the sky opposite the sun, meaning the sun is behind the observer. In contrast, a parhelion or sun dog will be located directly beside the sun, at the same height above the horizon.
Color Intensity
The intensity of the color is a strong indicator. A standard rainbow segment features distinct, saturated bands of color. A fogbow appears predominantly white or very pale due to small droplet size and dominant diffraction.
Atmospheric Conditions
The necessary atmospheric conditions are also different. Rainbows require rain or spray, sun dogs require high-altitude ice crystals, and fogbows require ground-level fog or mist.