Sun halos and sundogs over New Zealand

This page has covered solar halos since 2006, and it has now been rewritten from the ground up.

Halos are far more common than most people realise. On any day with a thin veil of high cloud there is a fair chance some part of a halo display is overhead. The trick is knowing where to look, and doing it without staring into the Sun.

Safety first

Never look directly at the Sun, and never point binoculars or a camera viewfinder at it. To study a halo, stand so a building, a power pole or a road sign blocks the Sun itself, or hold your hand out at arm's length to cover the disc. With the glare gone, the arcs around it become much easier to see. Good sunglasses help, but they are not a licence to look at the Sun.

The 22-degree halo

The most common display is a ring about 22 degrees from the Sun, about one outstretched hand-span. It forms in thin cirrostratus when sunlight passes through randomly oriented hexagonal ice crystals, each acting as a little prism. The inner edge is often faintly reddish, and the sky inside the ring looks slightly darker than outside. The same ring appears around the Moon, described on our moon halos page.

Sundogs

Sundogs, or parhelia, are bright coloured patches on either side of the Sun, at the same height above the horizon. They come from flat, plate-shaped ice crystals that drift down with their faces horizontal, like falling leaves. Sundogs sit near the 22-degree ring when the Sun is low and move slightly outward as it climbs. They are easiest to spot in the early morning or late afternoon, and they can be startlingly bright, red on the side nearest the Sun.

Circumzenithal and circumhorizontal arcs

Look straight up, high above the Sun, when sundogs are showing. Sometimes there is a short, vivid rainbow-coloured arc curving around the zenith, like an upside-down smile. This is the circumzenithal arc, also made by plate crystals, and it only forms when the Sun is lower than about 32 degrees. Its colours are often purer than a rainbow's.

Its summer counterpart is the circumhorizontal arc, a broad colourful band low in the sky that needs the Sun higher than about 58 degrees. In New Zealand that only happens around midday in the warmer months, so it is one display we see that observers in much of Europe rarely do.

Sun pillars

At sunrise or sunset, a vertical shaft of light sometimes stands above (or below) the Sun. Sun pillars come from plate crystals reflecting light off their faces. They tend to take on the warm colour of the low Sun and are often seen on cold, still mornings.

Where to look in New Zealand

Any approaching front can bring the cirrus that makes halos. Canterbury's nor'west arch is a classic halo maker: ahead of a nor'wester, a long band of high cloud forms over the Southern Alps and plains, often thin enough for sundogs and arcs to shine through. Halo displays also show up regularly over Wellington and the central North Island as fronts sweep across from the Tasman. Checking MetService for cirrus ahead of a front is a good way to anticipate a display.

From an aircraft window

Plane windows are excellent halo observatories. Flying over a layer of ice cloud you may see a subsun, a bright spot reflected from crystals below you, and over water-droplet cloud a glory: small coloured rings around the aircraft's shadow. Look on the side of the plane away from the Sun for the glory.

Recording what you see

A phone with a wide lens captures most halos easily. Hide the Sun behind a building or pole in the frame and the arcs will stand out. Note the time and roughly how high the Sun was, since that helps identify which arcs you saw. For a full catalogue of rarer forms, with simulations, see Atmospheric Optics.