An Observer’s Guide to Rainbows

Those dazzling colours that bring everyone joy, the palette of colours splashed across the sky, the awesome power of the Celestial Dragon, the deadly bow of Indra the Hindu rain god, and the reminder from Allah that the Great Flood would never happen again: these are rainbows. These are the most splendid and gentle of aerial splendours. Rainbows have all of the energy and magic of lightning, but none of the terror. They symbolize the relief of a storm’s aftermath, and the anticipation of the warm sun.

But where do you find them? Not metaphorically, but literally? And what is on the other side of the rainbow? Would you like to know where to look, when to look, and what type of rainbows there are? Do you love those dazzling colours? If you are a seeker dreaming of a rainbow, some days you’ll find it by just stumbling upon them. When you are really connected with rainbows, like the lovers, dreamers, and meteorologists, then you’ll know when rainbows are most likely to be spotted, what causes them, and, most importantly, exactly where to look. It’s easy. The magic number is 42.

What is a rainbow?

When conditions are right, sunlight hits water droplets in the air and forms an optical illusion, relative to the eye of the observer. The white colours of sunlight are scattered and divided into the portions of the spectrum- red, orange, yellow, green, blue, indigo, violet- in a continuous flow. It is the interpretation of our eyes that makes the colours look different. Some other animals, and human-made sensors, can also perceive infrared colours (beyond red) and ultraviolet colours (beyond violet). On different planets, in different atmospheres with different mists, rainbows may appear in different colours. But the familiar spectrum of a rainbow is consistent throughout our Planet Earth.

When both sunshine and rainclouds are present, the sky may be right for a rainbow. When dark clouds are visible from the side, to the opposite direction of a low sun, when you are underneath bright clear skies, are best for spotting rainbows. With a bit of conscious observation, you will be able to identify good conditions and know to be on the lookout.

Where can you find rainbows?

The familiar rainbow arc is only the visible section of a complete circle found opposite the sun, at a 42 degree angle from the anti-solar point. Imagine a line from the sun to your eyes continuing through you to a point below the horizon, and that point is the anti-solar point. The circular rainbow encircles the anti-solar point at a distance of 42 angular degrees from the line between your eye and the anti-solar point. Red is on the outside, and violet is on the inside of the arc. At ground level, you will only see the upper portion of the circle. Below the horizon line, there is usually not enough distance for there to be enough water droplets to catch the light.

A fainter, secondary rainbow is found at 50 degrees from the anti-solar point. In this second, larger rainbow, the colours are reversed- red is inside and violet is outside. The dimmer area of eight degrees in between the two arcs is known as Alexander’s Dark Band, named after the Greek philosopher who wrote about it 1800 years ago. Tertiary and quaternary rainbows are exceptionally rare, and appear on the opposite side of sky to the familiar rainbow arc.

So if you are seeking the mythical leprechaun’s pot of gold at the end of the rainbow, where can you expect it to be? Let’s presume that you are standing on the ground. In the Southern Hemisphere, the sun moves to the north at noon, and in the Northern Hemisphere, to the south. You will therefore find a Southern hemisphere rainbows in the southern half of the sky, and Northern Hemisphere rainbows in the northern half of the sky. You will never find an Indonesian rainbow if you are looking north, and you will never find a rainbow in Russia looking south.

In the mornings, rainbows are to the west, and in the evening, rainbows are to the east. The lower the sun is in the sky, the more of the arc will be visible above the horizon, as the anti-solar point is less angular distance below the horizon. Midday rainbows will be shallower, and when you are on the equator, non-existent. People in temperate or extreme latitudes will find more opportunities to spot spectacular rainbows than those in the tropics, because the sun spends more time at low elevations. When the sun is low in the sky, and shines in a red or orange tint at dawn or sunset, the colours of the rainbow will also be tinted.

Varieties of Rainbows

There are several kinds of rainbows. The standard rainbow is a segment of a colourful halo centred on the anti-solar point, found at 42 degrees. But different conditions of water, ice, and lighting can create different rainbows.

The brightness of the moon can produce rainbows, as well. Lunar rainbows, or moonbows, are dimmer than sunbows, because the light is so much less intense. They are similar to rainbows- found at 42 degrees, but around the antilunar point.

Stacked rainbows, or supernumary rainbows, can be especially brilliant. When conditions are excellent for bright rainbows, a complex interference pattern can result in rainbows having extra bands of colour.

If you are above the ground looking downwards, you may be fortunate to see the fully circular rainbow known as a glory. To the wonder of mountain climbers and airplane travellers, this is recognizable immediately as a small rainbow around the anti-solar point, with the shadow of the observer in the centre. This is known as the Light of Buddha, recognised by the Chinese as a signal of wisdom.

For air travellers viewing a glory, the shadow of the airplane may be distinctly recognizable in silhouette at the centre of the halo. For a mountaineer, a large dark form may be seen extending from their feet into the very centre of the glory. It is the shadow of the observer themselves combined with the mountain upon which they are standing. This eerie figure is the Spectre of Brocken, after the highpoint of the German Hartz Mountains.

The sun halo is formed by the hexagonal ice crystals in high icy cirrus clouds and is observed as a ring at 22 degrees angular distance around the sun. While not as colourful as a true rainbow arc, it can display colours trending from a red inside to a violet outside. This can also be seen around the moon, as a moon halo (not to be confused with a lunar rainbow)

A related phenomenon occurs when the crystals of a sun halo are guided by the wind into a vertical orientation: the whimsically named arcs known as sundogs, or parhelia. These are segments of sun haloes that appear as arcs to the left and right of the sun.

A circumhorizon arc is part of a giant solar halo 46 degrees in angular size. They do not actually circle the horizon, but the lower parts may appear almost parallel to the horizon. Usually, there is not enough cloud cover to see the entire halo, and only some segment are visible.

In cold weather, with clouds of ice crystals floating in the air, snowbows can be seen around the anti-solar point. It is actually a normal rainbow, but it is exciting and special to see in the frigid air.

Cloud shimmers, or iridescences, are rare and beautiful colourations seen on the underside of a cloud. Usually spotted on high, thin, cumulus clouds, these spectra form in surreal shapes, rather than predictable arcs.

Fogbows, seadogs, or white rainbows, are rainbows with muted colours seen in thick, low water vapours. They appear around the anti-solar point. Cloudbows are fogbows within clouds- if you travel by airplane, keep your eyes open and you may see one.

All of these optical phenomena are wondrous combinations of cloud, vapour, and sun. But perhaps the most magical of all are the wavebows. If you are on a coastline facing westwards to the ocean, you may be fortunate to see rainbows dancing on the waves at the earliest moments of the sunrise. If you are on the east coast, look carefully at the waves in the last moments of the day, and perhaps you will see the colour spectrum. These rare wavebows are actually found at slightly smaller angles that those in the sky. The saltwater splits light at a different angle than the freshwater found in clouds and mist. Pay close attention to the waves, these are rare and magical rainbows!

How to conjure a rainbow

We’ve looked at the conditions to form rainbows, the directions you must look to see them, and the types of rainbows that you may find. But what about conjuring them, and capturing them?

You can conjure rainbows with the help of a garden hose or spray bottle in the midday sun. Direct the water away from the sun. You can use the reflection of a CD, or a cut glass prism, to create compact disk rainbows and prism spectra. One of the simplest ways to rainbow is to fill a glass of water and place it in a dark, shady place where the sun reaches it from one direction only. The edge of the water divides the white light creates a colourful glass of water spectrum.

To capture a rainbow, one needs only a camera and the knowledge of where to look. The well-equipped rainbow seeker will find themselves, with a camera ready, on high ground or a mountain peak, with the morning or evening sun to their back, and dark clouds ahead. When a rainbow is spotted, the photographer can try to move to a location that will make the rainbow appear to land at a point of interest.

To conjure a truly wild rainbow, visit a misty waterfall on a sunny day. For a tamer rainbow, try a large fountain shooting water upwards. Depending on where you are relative to the cascade- above, below, or to the side, you may find different shapes and intensities of spectra. You have conjured a rainbow!

Experimentations with your camera settings, to make the picture darker and the colours more vivid, can yield more impressive photographic images. If there is a subject of interest in the foreground, such as another human being, using the flash on the camera can fill in the details of the closer environment, which would otherwise be silhouetted. Two colourful, yet unwanted phenomena, which can be used for dramatic effect, are the scratched lens shimmer and the dirty lens flare, which show up unpredictably in damaged or dirty camera equipment. If you wear eyeglasses, you may be all too familiar with these special rainbows.

The other side

So what is on the other side of the rainbow? This question has been famously asked by philosophers and dreamers, astronomers and cloudspotters. The answer is: the sun! It is our planet’s life-giving star that is on the other side of the rainbow.

As you predict, identify, conjure, and capture these optical arcs, you will find life all the more colourful when you appreciate them. Rainbows are natural phenomena that you can actively seek out, rather than simply stumbled upon it. On some days, we’ll just simply find them, but on other days, we will know exactly where to look.