A guide to identifying ice haloes

Why wait for night-time? Even when the Sun shines the sky offers spotting opportunities...
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Image Credit: 
Les Cowley (www.atoptics.co.uk)

 

We astronomers always seem to find ourselves scanning the skies. Our interest in what’s
above us extends not just to the objects visible when the Sun goes down, but what’s there in the day too. People who are fascinated by the stars are often also interested in meteorology, a knowledge of which is handy to every clouded-out astronomer.

It’s no great surprise, then, that observing ice haloes is a pastime that many astronomers like to indulge in during daylight hours. Ice haloes are caused by the bending and reflection of sunlight as it passes through extremely small ice crystals in the atmosphere. They take many forms, from the familiar sundogs – the cause of many a UFO report – to the dramatic, 22° solar halo.

To help you identify which is which, we've put together this visual guide to the most commonly encountered ice haloes, with the help of halo expert Les Cowley of Atmospheric Optics. For information about other atmospheric phenomena, visit www.atoptics.co.uk


WARNING: Due to their nature, some haloes appear close to the Sun in the sky. Never look at the Sun directly and do not look at it – or close to it – with any optical instruments, as you will risk causing irreversible damage to your eyesight.

Sundogs

Sundogs, or to use their scientific name ‘parhelia’, are the bright spots of light that are often seen either side of the Sun in the sky. They can appear as faint red or yellow patches in thin cloud but they can have all the colours of the spectrum. It’s not known exactly how parhelia came to be called sundogs. “The usual explanation is that they are companions to the Sun, but there is no convincing etymology,” says Cowley.


22º solar halo

If the conditions are right you may well spot the spectacular 22° solar halo, a ring encircling the Sun that’s often overlooked as we go about our daily business on the ground. “A thin, white, cirrostratus cloud layer helps to show its full extent, but it’s worth looking for it when there’s any type of cirrus cloud,” says Cowley. It’s still not known which crystals produce this most frequent halo.


Lower tangent arc

To produce a lower tangent arc – or any halo for that matter – clouds need to contain ice crystals of the correct shape and alignment, says Cowley. “All types of cirrus may have the required crystals, depending on the cloud’s internal temperature, humidity and probably a host of other factors.” You’re most likely to see this halo if you go skiing or when you’re flying, Cowley adds. “At ground level it tends to be too low, or hidden in horizon haze. It’s best seen looking downwards from a mountain.”


The circumzenithal arc

The circumzenithal arc appears like a small, boomerang-shaped rainbow in the sky. ‘Circumzenithal’ describes the fact that it stretches around the point directly above you on the sky, known as the zenith. “We see it in the UK about once a month, but we often miss it because we so rarely look up to the zenith,” says Cowley. “Ideally, you need a low morning or late-afternoon Sun to see it. Wispy cirrus clouds seem to generate the brightest circumzenithal arcs too.”


Upper tangent arc

If you can see a 22° solar halo then you should also look out for the upper tangent arc, says Cowley. “Look for a slight brightening at the top of the halo circle. That’s a fragment of an upper tangent arc. We might not know which crystals produce the 22° halo, but the upper tangent arc is created by different crystals. It’s sunlight refracted through the side faces of column-shaped crystals that are aligned horizontally that creates the arc.”


Parhelic circle

The parhelic circle is a relatively faint, thin strip of brightness that can stretch right around the sky. You should look out for it whenever there’s cirrus cloud in the sky. “Plate-shaped crystals and horizontal, column-shaped crystals create the parhelic circle,” explains Cowley. “Get into the habit of always examining the sky for these haloes. If you make it part of a routine you'll be more likely to catch rarities like the parhelic circle.”


This article originally appeared in the January 2011 issue of Sky at Night Magazine.
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