Circumscribed halo
A circumscribed halo is a type of halo, an optical phenomenon typically in the form of a more or less oval ring that circumscribes the circular 22° halo centred on the sun or moon.[1] As the sun rises above 70° it essentially covers the 22° halo.[2] Like many other halos, it is slightly reddish on the inner edge, facing the sun or moon, and bluish on the outer edge.
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The shape of the circumscribed halo is strongly dependent on the distance of the sun or moon above the horizon.[3] Its top and bottom (i.e., the points directly below and above the sun or moon) always lie directly tangential to the 22° halo, but its left and right sides take on different shapes depending on solar (or lunar) elevation. At an elevation between about 35°–50°, the sides form two distinct, downward-drooping "lobes" outside of the 22° halo. As the sun or moon rises higher (between c. 50°–70°), the drooping diminishes towards a more regular oval shape. At an elevation of c. 70° or more, the shape of the circumscribed halo approaches a circle, and as such becomes nearly indistinguishable from the 22° halo, only to be identified by its tendency to show more saturated colors than the latter.[2] When the sun or moon is at an elevation lower than c. 35°, the circumscribed halo breaks up into the upper tangent and lower tangent arcs.[4]
See also
References
- Les Cowley. "Circumscribed Halo". Atmospheric Optics. Retrieved 2007-04-15. (includes a composite image of a circumscribed halo)
- "Circumscribed Halo". Arbeitskreis Meteore e.V. Retrieved 2007-04-15. (includes a fisheye photo of the phenomenon)
- "Circumscribed Halo". www.atoptics.co.uk.
- "Tangent Arcs". www.atoptics.co.uk.
External links
- www.paraselene.de – HaloSim Computer simulations of a circumscribed halo.
- Atmospheric Optics - Circumscribed Halo - solar altitude – an animation showing how the shape of the phenomenon changes as the sun or moon rises.