Happy #WeatherWednesday everyone! These are meant to be little educational quick-reads about anything weather related! Subjects discussed in the past have included hurricanes, snow, lightning, thermodynamical quantities, severe risk categories and more.
This time we will turn to the optical phenomenon known as the rainbow! Rainbows do not happen very often so when they do, people definitely notice them. Thankfully meteorology is always interesting, and rainbows are great examples of some of meteorology’s finest work.
There are many optical phenomena possible in our atmosphere, but primary rainbows have been seen by nearly everybody. They can be seen more easily if a lowering Sun that’s clear on the horizon takes place as a storm cell departs in the sky directly opposite to the Sun. They can sometimes be seen in fog, heavy dew, or as halos from sunlight/moonlight passing through high ice crystal content.
As you can see in the photo above from Colorado, the light from the lowering Sun is coming from behind the viewer’s perspective of the rainbow. Thus the rainbow is centered on what is known as the anti-solar point in the sky (opposite of the Sun) . A viewer cannot see a rainbow and the Sun in the same part of the sky. Their shadow along the ground would be pointing towards the center of the rainbow. This is why rainbows move with the viewer, they’re always centered on the anti-solar point of reference. Also, notice that the primary rainbow contains red on the outside and violet inside. The photo also includes a secondary rainbow, which is even less common in occurrence. The colors are FLIPPED with red on the inside and violet on the outside. Recall the acronym ROYGBIV for primary color order.
Here’s the tricky bit…These colors are just visual illusions as our retinas or camera lenses see the reflected refraction of solar light after hitting water drops. Refraction upon entering the drop splits the light into the visible spectrum, the visible light reflects off the perimeter of the drop like a mirror image, and refracts back to the viewer at a different angle than entry. The primary rainbow we see is displayed typically at an angle of around 42 degrees apart from the incoming sun rays. The sky may appear brighter inside the primary rainbow (0 degrees to 42.7 degrees) and much darker outside the rainbow. It’s all perception and how the backscattered sun rays are reaching our eyes. Whenever there is a secondary rainbow, it is outside of the primary rainbow at a higher angle, around 51 degrees apart from the incident sun rays. This comes from the process of two reflections of refracted sunlight inside the raindrops. The second mirror image is what causes the colors to be flipped. The dark section between the primary and secondary is known as Alexander’s Dark Band.
It seems very abstract, and perhaps that’s the nature of optical phenomena and meteorology. But these are sure worth observing and serve as a great reminder of how spectacular the little things in life can be.