A fogbow is a captivating atmospheric phenomenon that appears as a luminous arc in the sky, often evoking a sense of wonder due to its ethereal quality. Unlike the vibrant spectacle of a rainbow, a fogbow presents a more subtle display, making it a unique and rare sight. This optical event forms under specific atmospheric conditions, transforming ordinary mist into a canvas for light.
What Exactly is a Fogbow?
A fogbow resembles a rainbow in its arc shape but differs significantly in its appearance, most notably by its predominantly white or very faintly colored hue. This has led to its common nicknames, such as “white rainbow” or “ghost rainbow.” The arc of a fogbow also tends to be wider and appears less distinctly defined compared to the sharp, colorful bands of a rainbow.
While sometimes appearing entirely white, a fogbow can occasionally display very muted colors, typically with a faint reddish tint on its outer edge and a bluish tinge on its inner edge. Its broad and diffused nature results from the way light interacts with the minute water droplets present in fog or mist. This diffused appearance distinguishes it visually from its more colorful counterpart.
The Science of Its Formation
The formation of a fogbow hinges on the interaction of sunlight with incredibly tiny water droplets suspended in fog, mist, or clouds. These water droplets are significantly smaller than the raindrops that create rainbows, typically measuring less than 0.1 millimeters in diameter.
The primary physical process responsible for a fogbow’s appearance is diffraction, where light waves bend and spread out as they pass around these minute droplets. This phenomenon differs from rainbows, which are formed mainly by the refraction and reflection of light within larger raindrops. The extensive diffraction in fogbows causes the various colors of light to overlap considerably, “smearing” them together and resulting in the characteristic white or pale appearance.
Where and When to Spot a Fogbow
Observing a fogbow requires a specific combination of atmospheric conditions: the presence of fog or mist and bright sunshine. The sun must be positioned directly behind the observer, casting their shadow towards the fog bank. For optimal viewing, the sun should also be relatively low in the sky, ideally less than 30 to 40 degrees above the horizon.
These conditions often occur during early morning or late evening hours, when the sun’s angle is low. To locate a fogbow, an observer should look away from the sun, aiming their gaze approximately 35 to 40 degrees from the shadow of their head. Fogbows are frequently sighted in areas prone to fog, such as coastal regions, mountainous terrains, or during cold mornings in valleys. They can also be seen from elevated positions like aircraft, where they are known as cloudbows, or from ships, sometimes called sea-dogs.