Rainbows, Halos, Glories - Optical Phenomena

A rainbow is actually a circle, but the lower half is obscured from view when an observer standing on Earth. Observing the full circle of a rainbow can only be done from the air.

A rainbow is usually seen before or after a morning or afternoon rainstorm, by an observer with their back to the sun when it is low in the sky.

Rainbows can also be observed in fine mists of water, such as from a sprinkler, waterfall or fountain.

There are six colours in a rainbow: red, orange, yellow, green, blue, violet. Indigo is no longer regarded as a colour of the rainbow.

Rainbow colours are the colours of the visible light spectrum. This can be demonstrated by passing a beam of light through a prism. Light refracts (changes direction as it passes through the prism) and the different colours come out at different angles and can be seen separately.

If the individual spectral colours are recombined (by passing through another prism), white light is formed.

Raindrops are spherical (not teardrop shaped) and act as tiny prisms that refract and reflect (bounce back) light. The raindrops must be less than 4mm (.015 inch) and held in suspension that is too widely spaced to appear as a cloud.

Each colour of the rainbow arises from a different set of raindrops. At any given moment, the rainbow is formed by a particular set of raindrops is replaced by new raindrops (as the raindrops are falling).

The intensity of colours in a rainbow vary according to the angle of the observer and the size of the raindrops.

The most intense rainbows are seen when the sun is low in the sky, behind the observer. Light refracts and reflects off the raindrops to the observer.

Larger raindrops are better at separating different wavelengths (red has a longer wavelength; violet the shortest in the visible light spectrum). This results in brighter and more distinct colours.

If the drops are too tiny, the colours merge together to form a white bow called a fog bow. Sometimes, a red bow can be seen when the sun is setting:  a white bow lit by red light from the setting sun.

Sometimes bows may be seen in the early morning on horizontal surfaces such as ponds or lawns.  These are called dew bows, formed by sunlight refracting and reflecting in drops of dew.

Sometimes double bows can be seen. The brighter bow is known as the primary bow and has red on the outer circle and violet on the inner circle.

The secondary bow is a fainter bow that lies to the outside of the primary bow. The colours are in reverse order to the primary bow as light has been reflected twice in each raindrop.

No reflected light goes between the bows which results in a darker band of sky.

At high altitude above the clouds, such as on a mountain or in an airplane, observers may see their shadow (or airplane shadow) projected onto cloud and surrounded by coloured rings.

For a solar glory to be seen, the observer must be facing away from a rising or setting sun and the cloud must contain small water droplets of uniform size. Light is backscattered (a combination of reflection, refraction and diffraction) to form rings.

A halo (also known as an icebow) is where a bright ring of light appears around the sun or moon. Halos are more common in cold climate areas where the temperature is low enough to contain ice crystals instead of water droplets.

Halos result from refraction of light by fine ice-crystals high in the atmosphere. The hexagonally-shaped ice-crystals act as tiny prisms that disperse light into spectral colours.  The inner edge of a halo is red (reverse order to a rainbow), which is also the brightest colour visible.  Orange and yellow may be visible and the other colours start to overlap forming white.

Sun dogs (also called mock suns) are bright patches on either side of a rising or setting sun with partial halos. A sun pillar is a vertical extension of the sun. They may be bright and white or show some or all of the colours of the spectrum, depending upon the elevation of the sun at the time.

These optical effects are most commonly seen at sunrise or sunset and result from refraction of falling flat hexagonal ice crystals.

A corona is a disc of light resulting from diffraction of light (scattering and interference from atmospheric particles). Coronas are seen when a thin layer of cloud lies between the moon or sun and observer. They can even been seen under streetlights in fog.

Uniform water droplets or ice crystals produce the most spectacular coronas. Bright and dark rings can be observed with blue on the inside and red on the outside and white in the middle (white from mixing of spectral colours).