A fogbow is an atmospheric optical phenomenon often nicknamed the “white rainbow” or “ghost rainbow.” This arch of white light appears when sunlight interacts with tiny water droplets found in fog or mist, rather than the larger raindrops that form a traditional, colorful rainbow. Similar to a rainbow in shape and location, the fogbow always appears opposite the sun, but its pale appearance makes it a unique spectacle.
How Light Creates the Fogbow
The formation of a fogbow begins as sunlight passes through the water droplets suspended in the air. Like a regular rainbow, the light first enters the droplet and is bent (refraction). A small portion of the light then reflects off the back wall of the spherical droplet toward the observer.
The light completes its journey by refracting a second time as it exits the droplet and travels toward the viewer’s eye. For the fogbow to be visible, the observer must have their back to the sun, looking into the fog or mist. This geometry ensures the scattered light is concentrated into an arc centered on the antisolar point, which is directly opposite the sun. Most of the light emerges at a scattering angle of approximately 40 to 42 degrees from this point.
The Science Behind the White Color
A fogbow lacks brilliant colors due to the microscopic size of the water particles. Raindrops typically measure 0.25 to 3 millimeters in diameter, but fog droplets are incredibly small, often less than 0.05 millimeters (50 micrometers). This minute size alters how light behaves as it passes through the water.
In large raindrops, refraction cleanly separates the different wavelengths of light, creating the visible spectrum of colors. However, tiny fog droplets are not large enough for this prismatic separation. Instead, their small size causes the light to undergo significant diffraction, where light waves bend around the edges of the droplet.
This extensive diffraction causes the color bands to spread out, overlap, and smear together. The mixture of all visible wavelengths of light results in the colorless appearance perceived as white. Occasionally, a faint red tint may be visible on the outer edge, with a slight bluish tint on the inner edge. However, the overwhelming impression is a broad, pale white arc.
Where and When to Spot a Fogbow
To maximize the chance of seeing this phenomenon, look for conditions where bright sunlight is present alongside a relatively thin layer of fog or mist. The sun must be low in the sky, typically just after sunrise or during the late afternoon before sunset. If the sun is more than 30 to 40 degrees above the horizon, the bow will be too low to be fully visible from the ground.
Fogbows are frequently sighted in environments where moisture is common, such as coastal regions, mountainous areas, or over large bodies of water. Best viewing requires the sun to be shining brightly enough to illuminate the moisture. Look for the bow in the direction of your shadow, as this aligns you correctly with the antisolar point where the arch forms.