The rainbow is a commonly known meteorological phenomenon. It is formed through the refraction, reflection and dispersion of sunlight on water droplets present in the atmosphere (rain, mist). While passing through water drops, sunlight is split into the colours of which it is formed. A rainbow is always visible as a colorful arch on the opposite side of the sky from the Sun. The colours of the rainbow are red, orange, yellow, green, blue, indigo, and violet. However in certain conditions a rainbow can consist of only one colour – red.

Sunlight is dispersed as it passes through the atmosphere. This phenomenon, called the atmospheric extinction, strongly depends on the colour of light. The blue component of sunlight is dispersed the most, while the red the least. Hence why the sky is blue during the day – we see the dispersed blue sunlight. The lower the Sun is over the horizon, the longer the path in the atmosphere its light travels and the more it is dispersed. When the Sun is on the very horizon, the light is dispersed so much that its brightness drops to an eye-safe level – we can look at the Sun safely. Additionally, since red light is dispersed the least of all colours, the Sun on the horizon is visible as red.

Sometimes the conditions for the formation of a rainbow occur exactly at sunrise or sunset. It can be after rainfall or during mist, when there are many water drops in the atmosphere. Then a rainbow is formed from the only available and not fully dispersed light – red light. This rainbow has an arch only in red colour. Hence its name – red rainbow or monochromatic (one colour) rainbow. It is a very rare phenomenon.

In the data bases of the ALPS network, gathered since 2017, we found only two cases of a red rainbow – in the years 2017 and 2025. The newer case of a red rainbow was registered on 28 January in Ostrowik (astronomical observatory of the UW) at 4:09 PM. It was about 5 minutes before sunset. The phenomenon lasted less than 6 minutes. In the picture the rainbow can be seen in the upper left part of the frame.