A fog bow is a meteorological phenomenon that appears as a white or pale arc, resembling a rainbow but formed by tiny water droplets in fog rather than raindrops. Because these droplets are significantly smaller than those in rain, they cause diffraction rather than refraction, resulting in a mostly colorless appearance.

What is a fog bow?

Why a fog bow happens

A fog bow occurs when sunlight interacts with the minute water droplets suspended in a bank of fog. Unlike standard rainbows, which rely on larger raindrops to refract and disperse light into a spectrum of colors, fog droplets are typically less than 0.05 millimeters in diameter. This small size causes the light to undergo diffraction, where the waves bend around the droplets, effectively washing out the vivid colors associated with larger-scale refraction.

The resulting arc is primarily white, though it may occasionally exhibit a faint red tint on the outer edge and a bluish hue on the inner edge. Because the droplets are so small, the diffraction patterns overlap, blurring the distinct color bands that define a traditional rainbow. This phenomenon is often colloquially referred to as a "white rainbow" or a "cloud bow" depending on the altitude and density of the moisture in the air.

Where to see a fog bow

To observe a fog bow, you must position yourself correctly relative to the light source and the fog bank. The sun must be behind your back, low on the horizon, while the fog must be directly in front of you. Because fog is often localized in valleys, near coastlines, or over large bodies of water, these areas provide the best vantage points for viewing.

Find a location with a clear, unobstructed view of a fog bank.
Ensure the sun is shining brightly behind you and is at an angle of roughly 40 degrees or less.
Look toward the fog bank where the light is being scattered back toward your eyes.
Keep your gaze steady, as the bow can be faint and easily missed against a bright, hazy background.

How to photograph a fog bow

Capturing a fog bow requires specific camera settings because the phenomenon is often quite subtle. Since fog bows lack the high-contrast colors of a rainbow, a camera’s light meter might struggle to find the correct exposure. Using a polarizing filter can sometimes help enhance the visibility of the arc against the background, though it may also diminish the effect depending on the angle of the light.

Use a tripod to ensure stability, as the bow may be faint and require a slightly longer exposure time.
Shoot in RAW format to allow for post-processing adjustments to contrast and clarity.
Focus on the fog droplets themselves rather than the distant background to ensure the arc is sharp.
Avoid using an automatic flash, as it will illuminate the fog immediately in front of the lens rather than the distant arc.

Common mistakes when viewing

One of the most frequent errors observers make is looking for vibrant colors. Many people expect a full spectrum of red, orange, yellow, green, blue, indigo, and violet, and they assume they have failed to see a fog bow if the arc appears white. Understanding that the lack of color is a defining characteristic of the phenomenon is essential for a successful sighting.

Another common mistake is choosing a location where the fog is too thick or too thin. If the fog is too dense, the sunlight cannot penetrate deep enough to create the arc; if it is too thin, there are not enough droplets to scatter the light effectively. Aim for a "light" or "moderate" fog that still allows you to see some distance, as this balance provides the optimal density of water droplets for the diffraction process to occur.

How it differs from a rainbow

While both rainbows and fog bows are optical phenomena caused by the interaction of light and water, their underlying physics differ significantly. A rainbow is primarily a result of refraction and reflection inside larger raindrops, which act like tiny prisms to split white light into its component colors. The larger size of the raindrop prevents significant diffraction, allowing the colors to remain distinct and separated to the human eye.

In contrast, the fog bow is a diffraction-dominated phenomenon. The tiny droplets in fog are so small that they do not act as individual prisms. Instead, the light waves interfere with each other as they pass through the droplets, creating a broad, white arc. While a rainbow is usually seen against a dark, rainy sky, a fog bow is almost always seen against a bright, white, or gray background of mist, making it a much more subtle and delicate sight to behold in nature.

Key facts

A fog bow is an optical phenomenon that appears as a white or pale arc in a bank of fog or mist.

Unlike a rainbow, which is caused by large raindrops, a fog bow is created by tiny water droplets typically less than 0.05 millimeters in diameter.

Because these droplets are so small, diffraction dominates over refraction, causing the colors to overlap and wash out into a white appearance.

To see one, you must stand with your back to the sun and look toward the fog bank.

A fog bow is significantly broader than a rainbow and often lacks a distinct color spectrum.

If the droplets are slightly larger, you might observe a faint reddish tint on the outer edge and a bluish tint on the inner edge.

The bottom line

A fog bow is a captivating meteorological display that transforms dense mist into a shimmering, colorless arch. By understanding that these arcs rely on the diffraction of light through microscopic water droplets, you can better identify them during foggy weather. Remember to keep the sun behind you and look toward the mist; with patience and the right light, you will witness this subtle, ethereal atmospheric wonder.

Can you see a fog bow at night?

Yes, a lunar fog bow can occur when the moon is bright enough to cast light onto a dense fog bank. These are rare and appear as ghostly white arches against the dark sky.

Why are fog bows white instead of colorful?

The colors in a rainbow are separated because different wavelengths of light refract at different angles. In a fog bow, the droplets are so small that the colors overlap, resulting in additive color mixing that appears white.

Is a fog bow the same thing as a halo?

No, a halo is an optical effect caused by light interacting with ice crystals in high-altitude clouds. A fog bow is strictly a water-droplet phenomenon occurring in low-level mist or fog.

What is the best time of day to spot a fog bow?

The best time is when the sun is low on the horizon, such as during the early morning or late afternoon. This position provides the necessary angle for light to scatter back toward your eyes.

Can I take a photo of a fog bow?

Yes, but because they are faint, you should use a camera with manual settings to adjust the exposure. Using a polarizing filter can sometimes help enhance the contrast against the background fog.

Why do fog bows seem to move when I move?

Like all rainbows, a fog bow is a subjective optical phenomenon that depends entirely on the observer's position relative to the light source and the water droplets. As you move, the specific droplets reflecting the light change, making the arc appear to shift.

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