###### 1 January 2014 on Personal Blog

# [Double Rainbow]

|[![halfsize][1]][2]|
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| _Double Rainbow from Moana Park, QLD, AU_ |

[Secondary rainbows] are caused by a double reflection of sunlight inside the
raindrops, and appear at an angle of 50–53°. As a result of the second
reflection, the colours of a secondary rainbow are inverted compared to the
primary bow, with blue on the outside and red on the inside. The secondary
rainbow is fainter than the primary because more light escapes from two
reflections compared to one and because the rainbow itself is spread over a
greater area of the sky. The dark area of unlit sky lying between the primary
and secondary bows is called Alexander's band, after Alexander of Aphrodisias
who first described it.[19]

## Twinned rainbow

Unlike a double rainbow which consists of two separate and concentric rainbow
arcs, the very rare twinned rainbow appears as two rainbow arcs that split
from a single base. The colours in the second bow, rather than reversing as
in a double rainbow, appear in the same order as the primary rainbow. It is
sometimes even observed in combination with a double rainbow. The cause of a
twinned rainbow is the combination of different sizes of water drops falling
from the sky. Due to air resistance, raindrops flatten as they fall, and
flattening is more prominent in larger water drops. When two rain showers
with different-sized raindrops combine, they each produce slightly different
rainbows which may combine and form a twinned rainbow.[20]

Until recently, scientists could only make an educated guess as to why a
twinned rainbow does appear, even though extremely rarely. It was thought that
most probably non-spherical raindrops produced one or both bows, with surface
tension forces keeping small raindrops spherical, while large drops were
flattened by air resistance; or that they might even oscillate between
flattened and elongated spheroids.[21] However, in 2012 a new technique was
used to simulate rainbows, enabling the accurate simulation of non-spherical
particles. Besides twinned rainbows, this technique can also be used to
simulate many different rainbow phenomena including double rainbows and
supernumerary bows.

[Double Rainbow]: /1
[Secondary rainbows]: http://en.wikipedia.org/wiki/Double_rainbow#Variations
[1]: http://markconstable.com/lib/img/20131214_Double_Rainbow_halfsize.jpg
[2]: http://markconstable.com/lib/img/20131214_Double_Rainbow.jpg
[19]: http://en.wikipedia.org/wiki/Double_rainbow#cite_note-19
[20]: http://en.wikipedia.org/wiki/Double_rainbow#cite_note-20
[21]: http://en.wikipedia.org/wiki/Double_rainbow#cite_note-21