Naked-eye solar effects

Sun dogs

Sun dogs, known more scientifically as ‘parhelia’, look like miniature rainbows on either side of the Sun when it is low in the sky, either just before it sets or just after it has risen. They are caused by the Sun’s light being refracted and reflected by ice crystals in the upper atmosphere.

Solar halo

A solar halo is a glowing halo around the Sun. All solar halos have a radius of 22 degrees. Solar halos are usually seen when the Sun is high in the sky and are caused by ice crystals high in the atmosphere refracting and bending the Sun’s light. Some are quite dim, little more than a pale grey-white circle around the Sun. The most dramatic halos are very eye-catching, taking the form of a beautiful blue-white ring around the Sun, or even rainbow-hued. You’ll notice that the sky inside the halo is darker than the sky outside of it.

Sun pillar

If plate-shaped ice crystals in the upper atmosphere line up in a certain way a column of silvery-blue or sometimes golden light can appear above the Sun, but only when it is at a low altitude. This is known as a ‘Sun pillar’, and they often linger in the sky after the Sun has set or herald its appearance before it has risen.

Circumzenithal arc

Although most people will have seen Sun dogs, solar halos and Sun pillars, far fewer will have seen a circumzenithal arc because they appear almost overhead, and most of us are too busy looking at the panoramic view around us to tilt back our heads and look that far up in the sky very often. It’s a shame, because occasionally what looks like an upside-down rainbow can be seen high above the Sun, almost overhead. This is a circumzenithal arc, a curve of rainbow-coloured light which is the brightest segment of a ring centred on the zenith, the overhead point in the sky.