Atmospheric
Optics
Summer's Halo ~ Circumhorizon Arc
Summer is here. The sun climbs high. Immense and colourful circumhorizon arcs are a common sight in high and icy cirrus cloud. This magnificent example was pictured by Stephen Gledhill (virtuallygrey.co.uk) in the Cascade Mountains of NW Washington State, USA. Scroll right for the full view - the arc stretched more than 75° along the horizon.
Image ©Stephen Gledhill, shown with permission
Sunlight enters a vertical side facet and leaves through the lower hexagonal face.
The are many myths attached to the circumhorizon arc. It is extremely rare - Untrue it is a common sight in summer in many parts of the world. It needs the sun exactly 58° high - Untrue, the sun must be 58° or higher. It is a form of rainbow - Untrue, it is an ice halo formed by sunlight refracted through plate shaped ice crystals. A journalist even coined a silly name for it that makes halo observers cringe.
Look for it beneath the sun when it is high and the sky has cirrus cloud or haze. The sun will often have an inner circular (or apparently circular) halo. Look twice as far away. It has pure spectral colours with red uppermost. When complete it extends almost 90° around the horizon. More often it is smaller, filling fragmentary clouds with vivid colour.
It is always parallel to the horizon. Check this by eye rather than later on photographs. A similar arc but one that curves up at its end is an infralateral arc from horizontal column crystals. The inner ‘circular’ halo can be misleading too. Often it is a circumscribed halo rather than a 22° halo - or sometimes both. The circumscribed halo is almost circular at high sun.
