The topic, this final week of August 2018, is: dramatic refraction phenomena in the sky.
In last week’s column, we explored the stunning CZA, an “upside-down rainbow” whose colors are astoundingly intense. And even more amazing than its brilliance is its positioning: A CZA can only appear overhead! While actual rainbows are never high up, the CZA is never low-down!
This week we’re not looking at the CZA, but at one of the most common dramatic apparitions of the day or night: It’s the 22-degree halo. Seen encircling the Sun or the Moon, the 22-degree halo is bigger than people remember or expect – and far more prevalent. In our area, they typically appear at least once a week.
What’s that, you say? You haven’t seen one in years?
That’s because you’re not looking up very often, or else not choosing the right time. In this case, the right time is whenever the sky has delicate high-altitude cirrus clouds, or else sheets of cirrostratus. Both are semi-transparent clouds made of ice crystals rather than water droplets. If you simply look up from time to time, you’ll easily see when the clouds are those thin, high-altitude types. Then look around the Sun. Or Moon.
Perhaps surprisingly, summer offers a profusion of ice-crystal phenomena. Very high up, above 23,000 feet, the air is so cold that ice crystals form, and these are almost always hexagonal. Sunlight hitting them creates “sun dogs” – brilliant white or colored spots to the left or right of the low-down Sun. It also creates the famous 22-degree halo.
The ring is always that same size. Maximally spread open your hand and hold it out at arm’s length, and close one eye. If you place your thumb at the Sun, your pinky-tip will mark the position of the halo; count on it.
Look up when thin, streaky, high-up cirrus clouds are abundant, and you’re sure to catch a beautiful 22-degree halo once or twice a week. It always has a bit of red on the inside of the circle. Sometimes the other spectral colors are there, too, but that’s somewhat unusual; and in any case, the colors are subdued, not vivid. A halo of the same size forms around the Moon just as often as the version around the Sun. Either way, according to folklore, such a ring portends the arrival of a storm within 24 hours.
There’s some truth to that. A frontal system that may bring rain typically starts by shoving high-altitude clouds over us: the cold ice-crystal clouds. Although it may be a glancing blow with no rain, these cirrus clouds usually give way to lower-altitude clouds, and then even lower and thicker ones that actually deliver the rain. The timeframe is indeed 24 hours or less.
Next time you see a halo, look at the right and leftmost edges, at the three and nine o’clock positions, and look for a bright spot at either place. Yes, that’s a sun dog, or parhelion.
Next, look at the top of the halo and see if there’s a sort of bowl-shaped phenomenon balancing there. If so, you’ve now observed the upper tangent arc. It’s unique in that it changes its appearance depending on how high up the Sun is.
Speaking of rings around bright lights, you might also see them in misty conditions, and then the rings may appear around all lamps, such as streetlights. Unfortunately, colored rings around lights might also appear to you if you’re suffering from glaucoma, or, more benignly, if your eyes are irritated such as from having swum in a chlorinated pool.
How can you tell if the rings-around-lights are really there or if it’s, instead, an eye problem? You could ask a companion if they see the rings, too, of course. But a more satisfying and immediate test is simply to hold out a hand at arm’s length and block out the light. If the ring remains when the light is blocked, it’s really there and everyone is seeing them. But if the ring vanishes the moment you block out the light, then its origin is your own eyes.