A unique phenomenon: Monochrome rainbows

While rainbows are typically multicoloured, a monochrome rainbow is a rare phenomenon. The above image shows a large monochrome rainbow above Washington, USA.

Not all rainbows are as colourful as their name suggests. Whenever conditions are just right, spectacular monochrome rainbows may be seen.

Rainbows form when sunlight enters water droplets in the air, causing the light to slow down and change direction or bend due to the change in density between air and water.

The light is then refracted and reflected inside the droplet, causing the different colours of light to spread out and become visible as an arc of colours.

A typical rainbow is made up of red, orange, yellow, green, blue, indigo, and violet, with red on the outer edge and violet on the inner edge.

The formation process of a monochrome rainbow is similar to that of a normal rainbow, with light being reflected or refracted in water droplets. The main difference is that a monochrome rainbow occurs when the sun is close to the horizon during sunrise or sunset.

Since the sun is at a lower angle during sunrise and sunset, its light has to travel a longer distance through the atmosphere.

Consequently, the shorter wavelengths of light, such as blue, green, and yellow, get scattered, leaving primarily red light. The monochrome rainbow is most often formed in a low-light environment, which creates a highly dramatic effect.