How is a rainbow formed? 

This colourful phenomenon is caused by the refraction and reflection of sunlight by raindrops measuring between 0.25 and around 3ₒmm in diameter. The light is broken up into its individual colours and reflected at an angle of approximately 42 degrees. As both rain and sunshine are necessary, rainbows typically appear during sporadic showers and thunderstorms. 

When can the most beautiful rainbows be seen? 

The lower the sun is in the sky, the bigger and more impressive the rainbow will appear. The best chances are in the early morning or the late afternoon and evening. With large raindrops, the sunlight undergoes greater refraction and the rainbow appears particularly radiant and clear. With small raindrops, the colours appear paler. Moreover, if the dark shower cloud or thundercloud is still moving away behind the rainbow, this produces the optimum contrast for the colours. 

A rainbow is not possible at all times of day 

Even if raindrops and sunshine are present, a rainbow isn’t always visible. This is because the light is refracted at an angle of approximately 42 degrees. If the sun is more than 42 degrees above the horizon, the rainbow is projected into the ground and cannot be seen. Accordingly, it is not possible to observe a rainbow during the day in the summer months. On the longest day of the year, when the sun peaks at 66 degrees above the horizon, rainbows only appear in the evening, after 5 p.m. They can first be seen at a low level above the Earth’s surface and rise further and further into the sky as the sun goes down.

Double rainbow 

Sometimes, a secondary rainbow (second order) is visible in addition to the primary rainbow (first order). In a secondary rainbow, the sunlight is refracted twice on the inner wall of the droplet and reflected back to the observer. The colours of the secondary rainbow therefore appear in reverse order to those of the primary rainbow. 

Alexander’s band 

In the case of a double rainbow, “Alexander’s band can clearly be seen between the two bows. In this area of the sky, the sunlight is only partially reflected into our eyes, so we perceive it as being darker.