photo #4357035

A flat, horizontal band of bright colours low in the sky, down beneath a high Sun, is likely to be the ice-crystal optical effect known as a circumhorizon arc. Conditions do apply, however. The striking colours can only appear when the elevation of the Sun (the angle between its direction and that of the horizon) is at least 58 degrees. How often that happens depends on the season and location. In the mid-latitudes, the Sun only climbs high enough in the sky around midday during the summer months. In a location near the Equator like the Maldives the Sun climbs high enough for circumhorizon arcs to be possible almost all year round. From somewhere with a high latitude like Copenhagen, Denmark or Cape Horn, Chile it’ll never rise high enough, no matter the season, and so a circumhorizon arc can’t be spotted. Conditions apply also when it comes to the cloud that can form a circumhorizon arc. It needs to be made of ice crystals in the shape of hexagonal plates that are clear enough to allow the sunlight to shine through them like tiny prisms. So adding a circumhorizon arc to your collection of sky optical effects is a matter of being in the right place at the right time of year. Thankfully, if you are lucky enough to be out when one appears, you’re likely to spot it since its bold colours appear closer to eye level than most of the other ice-crystal light effects.A circumhorizon arc in Cirrostratus spotted over south east Oregon, US by Judith Eve Smith (Cloud Appreciation Society Member 45,360). The effect appears curved here because of the wide-angle lens. In real life, the effect appears as a completely flat band of colours.