The Science Behind Atmospheric Optics

Why is the sky blue? How can there be two rainbows in one place at the same time? What causes sunset mirages? How are ice halos formed? These spectacular phenomena are all examples of atmospheric optics. They occur because of the unique optical properties of our wonderful atmosphere. Today, we’re inviting you to step into the scientific world behind the most popular atmospheric optics.

Blue Skies<img src="skies.jpg" alt="blue skies and white clouds">

First off, the light of the sun isn’t yellow nor white. It’s a mixture of blues, reds, greens, and violets. The violets and blues have the shortest wavelengths and the highest frequencies. The air molecules, responsible for scattering sunlight everywhere, scatter the blue light easily than the reds. However, during sunrise or sunset, when sunlight is observed through a thick atmosphere, we see a reddish color. Just to be clear, the sky isn’t just blue. All other colors are distributed too but are weakly scattered, progressively weaker towards red. It just so happens than when the sun is high, the blues are more visible to the naked eye.


Rainbows, which are formed when water droplets reflect, refract, and disperse light, aren’t as frequent as we think they are. Usually, the best times to see them are early in the morning and late in the afternoon when the sun isn’t too high. They are always positioned opposite the sun with their centers below the horizon, at the anti-solar point. This is why the higher the sun, the lower the rainbow and vice versa.

Primary Bows

Red is regularly situated at the outermost part of a primary bow. And of course, if you use your monocular, binoculars, or spotting scope, you’ll clearly see that the other colors that form the ROYGBIV are within the bow. From time to time, fainter supernumerary arcs of pinks, purples, and electric greens can be seen inside the primary bow. This is possible only when the raindrops are tiny.Have you ever wondered why the sky inside the rainbow is still bright? It’s because the raindrops also direct light there! Cool, huh?

Have you ever wondered why the sky inside the rainbow is still bright? It’s because the raindrops also direct light there! Cool, huh?

<img src="double rainbow.jpg" alt="a primary bow and a secondary bow">Secondary Bows

If you look at the picture, you’ll notice there’s another bow. How’s that possible? For starters, light can be reflected inside a raindrop more than once. A secondary bow is then formed by the rays escaping after two reflections. Compared to the primary bow, it’s always broader with its colors reversed. Both primary and secondary bows are concentric, meaning they share the same center—the anti-solar point.


<img src="redrainbow.jpg" alt="a red bow in Minneapolis">

Red Bows

This is another interesting type of rainbow. Remember we mentioned that during sunset and sunrise, the reddish hues of sunlight can be seen? It’s also the reason why there are red rainbows. The sun rays travel long tracks through the lower part of our atmosphere during the sun’s rising and setting. This is where the air molecules and dust scatter them. The blues and greens, having shorter wavelengths, are then scattered easily, leaving the rest of the transmitted light rich in reds and yellows.

Sunset Mirages

<img src="mirage.jpg" alt="sunset mirage">

This phenomenon will make you think there are two or more suns. Even if you look through a telephoto lens, you’ll probably still be confused. Sunset mirages occur when the sun’s low slanting rays are weirdly refracted between air layers that have different temperatures. So next time you see one, remember that the sun’s rays are just undergoing intense temperature variations and that the sun is just one!



<img src="icehalo.jpg" alt="Ice halo above Foveran beach">Ice Crystal Halos

These are formed when tiny ice crystals in the atmosphere reflect and refract light. They’re similar to rainbows, only that the main actors are tiny ice crystals instead of water droplets. Believe it or not, icy halos appear more frequent that rainbows. In parts of the United States and in Europe, they can be seen at least twice a week!