Right outsideSky
Sundogs and sun halos: how ice crystals bend sunlight
When: All year
Ice crystals high in the atmosphere can bend sunlight into a pale ring around the sun or bright patches beside it.
Why this time of year
Whenever a thin veil of high ice cloud crosses the sun, often as a warm front approaches, the sky is set up for a halo or a pair of sundogs, and halos often show up ahead of rain or snow.
How to notice it
- When high thin clouds put a milky sheen around the sun, block the sun itself behind a roofline, sign, or your fist.
- Look for a large pale ring around the sun, or bright spots level with the sun on its left and right.
- Check for faint rainbow tints; the ice crystals split sunlight the way a prism does.
- If you catch a halo, watch what the weather does next; they often form ahead of rain or snow.
Try this with kids: Can you block the sun with your fist and find a bright spot of light beside it?
a halo is the pale ring around the sun, its radius spanning about 22 degrees of sky, made when tiny six-sided ice crystals bend sunlight; sundogs are the bright patches level with the sun
Safety
- Never stare at the sun; keep it blocked behind something solid while you search beside it.
Sources
- weather.govThe 22 degree halo's radius spans about 22 degrees of the sky; ice acts like a prism producing pale bands of color; halos often form ahead of rain or snow; sundogs come from tiny hexagon-shaped ice crystals drifting with a face nearly horizontal.
- weather.govSunlight or moonlight passing through the hexagonal ice crystals of cirrostratus is dispersed or refracted like light through a prism, forming a halo; cirrus thicken into cirrostratus as a warm front approaches.
- weather.govSundogs appear to the left and right of the sun, from ice crystals typically in cirrus clouds, most often near sunrise and sunset when the sun is low.
- forecast.weather.govHalo defined as bright circles or arcs centered on the sun or moon, caused by refraction or reflection of light by suspended ice crystals.