bows and raindrop size. Mouse over the slider
to see the effect of changing raindrop size when the droplet
size distribution is narrow**. The simulations were calculated
The scale has steps of 1º.
drops become smaller, several things happen: supernumeraries
become more widely spaced, the primary bow broadens, its
colours become less saturated. Eventually there is no longer
rainbow but instead a cloudbow
Supernumeraries are less obvious when
the rain has drops of widely different sizes. Each
drop size produces differently spaced fringes which overlap
to a blur. The broader supernumeraries from smaller drops are
more tolerant of variations in drop size.*
The top of a rainbow often has more distinct supernumeraries than
lower down. The drops higher up are possibly smaller and their size
variation then has
a less critical effect on the supernumerary bows' visibility than
does the same percentage size variation in the larger drops lower
There is another another effect at the upper parts of a bow. Larger
raindrops are slightly flattened and rays forming the top of a rainbow
have to pass through the flattened vertical cross section. It so happens
that when the drop size distribution is very broad, as in a
shower, the combination of flattening and supernumerary spacing versus
drop size produces a persistent fringe spacing of about 0.7°.
The downside is that little information about the raindrop size can
then be gained from the fringes. The effect does not occur at the
base of the bow because those rays pass through the horizontal circular
section of the flattened drops and the fringes of different spacings
from the many sized drops overlap and cease to be visible.
drop size distributions in the simulations above were set to preserve
the visibility of about three supernumerary bows. Drops of 0.40 mm
mean diameter had a standard deviation of 14%, drops of 0.86 mm diameter
6% and 1.50 mm drops 4%.
often have very wide drop size distributions and then, particularly
at the top of the bow the droplet flattening effects described
in the last paragraph operate.