A rainbow is a vibrant spectacle resulting from sunlight interacting with moisture in the atmosphere, involving the refraction, reflection, and dispersion of light within water droplets. The term “full rainbow” is often ambiguous, sometimes referring to a double rainbow or the extremely rare 360-degree circular phenomenon. A rainbow is not a fixed object but an optical event dependent on the observer’s position relative to the sun and water droplets. Understanding this optical geometry reveals why the full circle is almost never seen from the ground.
Why Rainbows Appear as Arcs
Rainbows are fundamentally circular, but the view from the ground limits this perception to an arc. The primary rainbow forms when sunlight enters a raindrop, reflects once off the inner surface, and refracts again as it exits toward the observer’s eye. This occurs at a precise angle, consistently around 42 degrees from the path of the incoming sunlight, which is always directly opposite the sun. This geometric requirement means the light rays form an imaginary cone shape, with the observer’s eye at the tip.
The base of this cone is the circular rainbow, centered on the anti-solar point—the spot directly opposite the sun, marked by the observer’s shadow. Because the anti-solar point is beneath the horizon when the sun is above the horizon, the ground physically blocks the lower portion of the circle. The common arc shape is simply the upper segment of the circle cut off by the Earth’s surface. The lower the sun is in the sky, the more of the circle rises above the horizon, creating a taller arc.
The Conditions Required to See a Full Circle
The only reason a rainbow appears as an arc is the obstruction of the horizon, meaning the full 360-degree circle is only visible when this physical barrier is removed. To see the entire geometric shape, an observer must elevate their position high above the ground level where the water droplets exist. This elevation allows the observer to look down on the entire circle, which is centered around the anti-solar point below.
The most common way to witness a full circular rainbow is from an aircraft, as the high altitude places the observer above the rain or mist. From this vantage point, the complete circle of color is often clearly visible on the cloud layer or mist below. The rarity of this view is due to the uncommon requirement of viewing from a high altitude, not a special atmospheric event.
Full circles can sometimes be seen from extremely high vantage points on land, such as mountain tops or tall skyscrapers, provided the atmosphere below is filled with sufficient water vapor like fog or mist. When viewed from an aircraft, the shadow of the plane is often cast directly into the center of the circular rainbow. This phenomenon confirms that the rainbow is an optical projection that moves with the observer, constantly centered opposite the light source.
Clarifying Other Forms of Rainbow Rarity
Double Rainbows
The term “full rainbow” is often confused with the double rainbow, a more common but still striking atmospheric event. A double rainbow, or secondary bow, forms when sunlight undergoes two reflections inside the water droplets instead of just one. This extra reflection causes the light to emerge at a wider angle, between 50 and 53 degrees from the anti-solar point. The secondary bow appears outside the primary rainbow, is fainter due to energy loss, and features a reversed color order.
Moonbows
Another much rarer phenomenon is the moonbow, or lunar rainbow, formed by the light of the moon rather than the sun. Moonbows are created by the same process of reflection and refraction in water droplets. They require a nearly full moon to provide enough light, a low lunar elevation, and a very dark sky. Because moonlight is significantly fainter than sunlight, moonbows typically appear white or silvery to the naked eye, as the colors are not intense enough to activate the eye’s color receptors.