Observing Halos - Checklist

Last updated on September 16, 2023

Observing Halos - A Comprehensive Checklist for Skywatchers

When it comes to observing halos in the sky, it's important to be diligent and observant. Even on seemingly clear days, there can be a thin layer of halo-forming cirrostratus clouds. To ensure you don't miss out on these fascinating atmospheric phenomena, here is a comprehensive checklist for skywatchers:

1. Regular Sky Checks: Make it a habit to regularly and frequently observe the skies. Halos can appear at any time, so staying vigilant will increase your chances of spotting them.

2. Common Halos: Start by shielding the sun and looking for the most common halo, the 22° halo. Pay attention to any upper or lower tangent arcs, which are often just brightenings of the 22° circle. Look out for sundogs as well. If the sun is low in the sky, remember to also check overhead for a circumzenithal arc. This initial observation will provide valuable information about the presence of ice crystals and their types and orientations.

3. Exploring Further: Twice as far from the sun as the 22° halo, you might come across a supralateral arc or a 46° halo. Scan the entire sky for fragments of the parhelic circle. Additionally, keep an eye out for any signs of a Parry arc above the upper tangent arc.

4. Associations: If you spot sundogs or other halos that indicate the presence of plate crystals, be sure to check for other plate crystal halos like the 120° parhelia. Similarly, bright tangent arcs could be an indication of supralateral arcs, infralateral arcs, or even Wegener arcs being visible.

5. Unusual Sightings: Don't limit your observations to just the common halos. Take the time to search for "odd radius" halos, which are caused by pyramidal crystals. These halos can have unique arcs that add intrigue to the sky. Keep your gaze wandering all over the sky, as sometimes these unusual arcs occur on their own without the presence of the more commonly observed halos.

By following this comprehensive checklist, you'll be equipped to observe and appreciate the diverse range of halos that can grace our skies. Remember, the key is to be patient and observant. The atmosphere has many surprises in store, and with a keen eye, you can uncover the hidden beauty of atmospheric optics.

Check the skies regularly and often.

Even when the sky appears quite clear there can often be a thin layer of halo forming cirrostratus.

Look for the common halos.

Shield the sun and look first for a 22� halo. Is there an upper or lower tangent arc - often just brightenings of the 22� circle? Are there sundogs? If the sun is low, look overhead for a circumzenithal arc. This quick look will already have indicated whether ice crystals are present and of what types and orientations.

Check for more halos.

Twice as far from the sun as the 22� there could be a supralateral arc or 46� halo. Look all around the sky for fragments of the parhelic circle. Is there any sign of a Parry arc above the upper tangent arc?

Look for associations.

If sundogs or other halos indicate that plate crystals are about then check for other plate crystal halos like 120� parhelia. Similarly, bright tangent arcs could mean that supralateral, infralateral arcs or even Wegener arcs are visible.

Look for the unusual.

Don't forget to look for the 'odd radius' halos of pyramidal crystals and some of their other arcs. Look all over the sky for any really unusual arcs - sometimes they occur on their own without the 'common' halos.

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Reference Atmospheric Optics

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