On 25 Nov, 15:27, Tudor Hughes wrote:
* * Today at 1338 I saw part of a rainbow against what appeared to be
a clear blue sky. *It extended from the horizon to an altitude of
about 10° then continued upwards against the backdrop of the rear end
of cloud from a shower retreating to the east. *There must have been a
very thin area of rain falling, but it was not enough to noticeably
change the colour of the blue sky. *When the bow disappeared (after no
more than about a minute) there was no change as far as I could detect
in the blueness of the sky yet the bow had been easily bright enough
to be seen.
* * * It is all the more puzzling because one could argue if there is
sufficient density of reflecting surfaces, either by number or size,
to form a rainbow then that reflectance (and scattering) would be
enough to obscure a blue sky. *Obviously not, and most intriguing.

Tudor Hughes, Warlingham, Surrey, 556 ft, 169 m.

Tudor, the answer lies in the differing optical properties of
different sizes of particle. Raindrops - even small ones - are way too
big to scatter light (in the sense of Rayleigh scattering, i.e.
preferential scattering of blue-wavelength light which gives the sky
its colour), although of course even small raindrops can cause
reflection from their rear surface and thus of course rainbows. A
relatively thin optical layer - such as may have been left from one of
today's fast-moving showers - would have been enough to cause a brief
rainbow given the right optical path, and yet the optical density of
the rain sheet in this case was obviously insufficient to do anything
but very slighly attenuate the brightness of the sky.

The human eye is fairly insensitive to gradual changes in the blue of
the sky anyway, and this tiny difference might not have been apparent.
I often find that photographs of a blue sky can reveal very marked
variations in blue shading which are often 'averaged out' by our eyes.
The eye sees only a patch of vision about the size of the full moon at
any instant, the rest of the visual field is 'filled in' by the brain,
and relatively minor contrasts are simply averaged out unless you have
a nearby point of reference. Our eyes are easily fooled in accepting
gradual changes of hue as constant colour, particularly in a sky
changing as quickly as in the immediate vicinity of a recent shower:
it is almost impossible with the naked eye to be certain that the blue
of 60 seconds ago is the same blue as 'now'. Had you been able to
photograph the rainbow, you might have found that the camera did show
that the sky through the raindrops was slightly greyer-blue
(attenuation of the blue light path), although I suspect even here the
difference would have been slight.

No rainbows here today, not at least that I saw, but plenty of blue
skies and welcome sunshine for a change!

--
Stephen Burt
Stratfield Mortimer, Berkshire