Introduction
A “fire rainbow” is not actually a rainbow or related to fire. Officially called a circumhorizontal arc, this breathtaking optical phenomenon occurs when sunlight passes through ice crystals in cirrus clouds, creating a brilliant spectrum of colors in the sky. Often mistaken for an actual rainbow, a fire rainbow forms high in the atmosphere and appears as a wide, horizontal band of vivid hues.
The Science Behind Fire Rainbows
Fire rainbows occur due to refraction, dispersion, and reflection of sunlight through ice crystals in cirrus or cirrostratus clouds. Here’s how it works:
- High-altitude cirrus clouds (found at altitudes of 20,000 feet or more) contain hexagonal plate-like ice crystals.
- The sun must be at least 58° above the horizon for the light to pass through these ice crystals at just the right angle.
- As sunlight enters the crystal, it is refracted (bent) at a precise angle of 90°, dispersing into its constituent colors—similar to a prism.
- The light exits the crystal, forming a horizontal band of rainbow colors parallel to the horizon.
Where and When Do Fire Rainbows Occur?
Unlike regular rainbows, which can appear after rainfall, fire rainbows are more selective in their appearances. They are geographically dependent and occur only under specific atmospheric conditions:
- Latitude and Seasons: Fire rainbows are most commonly observed in regions between latitudes 55° N and 55° S. This means they are more frequent in places like the United States (especially the southern regions), parts of Europe, China, and India during the summer months when the sun reaches the necessary height.
- Cloud Type: High-altitude cirrus clouds must be present, which is more common in warmer weather.
- Sun’s Position: The sun needs to be positioned at least 58° above the horizon, meaning they are rarely seen in places too far north or south of the equator.
Fire Rainbow vs. Other Atmospheric Phenomena
Fire rainbows are often confused with other colorful sky events, such as:
- Iridescent Clouds: These are caused by diffraction rather than refraction and appear as colorful patches near the sun.
- Haloes and Sundogs: These occur when sunlight interacts with ice crystals but form circular patterns rather than horizontal arcs.
- Rainbows: Traditional rainbows form due to refraction and reflection in raindrops, requiring precipitation rather than ice clouds.
Famous Sightings and Photography
Fire rainbows have been captured in various locations worldwide, often drawing attention due to their striking beauty. Some notable sightings include:
- United States (California & Florida): Frequent sightings during summer months due to ideal sun angles.
- Himalayas (India & Nepal): The high altitude and presence of cirrus clouds make it a great spot for fire rainbow formations.
- Switzerland and France: These countries occasionally witness fire rainbows due to their latitude and high-altitude cloud formations.
Can You Predict a Fire Rainbow?
While rare, fire rainbows can be predicted by checking weather conditions:
- Look for cirrus clouds on a sunny day with the sun positioned at least 58° above the horizon.
- The best time to see them is typically during midday in the summer months.
Conclusion
Fire rainbows are among the most spectacular and rare optical phenomena in nature. Their vibrant, dreamlike appearance results from a perfect alignment of sunlight and ice crystals, painting the sky with a mesmerizing spectrum of colors. While they are uncommon, understanding their science and geography can help increase your chances of witnessing this breathtaking event.