Q: I have twice driven through the end of a rainbow where it meets the
ground, although I am told this is impossible. I was surrounded by the
rainbow鈥檚 colours and there were many other drivers to witness that it can
happen. Can anyone explain this?
A: It is not possible to drive through a rainbow, but I would be interested
in more details of what happened, as there may be another meteorological
phenomenon that could explain the experience.
Malcolm Brooks The Met Office Press Office, London
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Mr Brooks can be contacted via the New 杏吧原创 office. The address
is given below 鈥� Ed
A: The easy way to reach the end of a rainbow is to wish for a pot of
gold next time you place a superfluous tooth under the pillow. If you use
this method, do not forget to carry a spade at all times.
In order to be seen to be at the end of a rainbow, get a friend with
an intercom to direct you to where the rainbow appears to touch down. In
order to see yourself at the rainbow鈥檚 end, arm yourself with binoculars
and make sure that your friend holds a large mirror at the correct angle.
William Pearce Warrington, Cheshire
Happy not sad
Q: Some years ago, between rain showers, I noticed an 鈥榰pside-down鈥�
rainbow (u-shaped rather than n-shaped). The colours were also reversed.
It appeared around 40degree above the horizon and was smaller than an upright
rainbow. It persisted in a semicircle for about a minute before slowly fading
from one side, the remaining arm lasting for another minute. Can anyone
explain this?
A: This is possible if the viewer has a reflecting surface, such as
a sheet of very calm water, behind them. The reflection of the Sun from
this surface can produce a rainbow in front of the viewer. Because the Sun
is reflecting from water behind the viewer, if the viewer were to turn around
and observe the Sun鈥檚 reflection in the water it would appear as though
it was beneath the surface of the water. The centre of the Sun would, therefore,
appear to be below the horizon. A full rainbow circle could be produced
under these circumstances appearing in front of the viewer as the Sun shines
up into the sky rather than the usual sunlight which shines down. What was
seen in this case was a part circle comprising the lower half of the circular
rainbow.
Malcolm Brooks The Met Office Press Office, London
A: Rainbows are usually circular but normally intersect with the ground
before their full circle can be achieved, giving an arc shape. As this rainbow
was high in the sky (40degree above the horizon), we can assume the top
was lost in the clouds and the sun was low on the horizon.
Michael Edie Petersfield, Hampshire
A: This radially challenged and disoriented rainbow sounds like a portion
of a solar halo. The effect is caused by refraction of sunlight through
a thin cloud of ice crystals such as that found in a cirro-stratus cloud
veil. Various circles and arcs can be produced. This particular sighting
would depend on the movement and extent of the veil and the rain clouds
below it, relative to the Sun. Similar, but less colourful, haloes can be
seen in moonlight.
Even smaller rainbows can be produced that are concentric with the Sun
and Moon, by refection and diffraction of light by water droplets in lower
clouds. This effect is called a corona, which is not the same as the Sun鈥檚
corona in an eclipse. The fact that lunar haloes and coronas appear to lack
certain colours is, I suspect, linked to the human eye鈥檚 reduced capability
to discriminate colours in low light conditions.
Steve Mason Eastleigh, Hampshire
A: 鈥楿-shaped鈥� rainbows are quite simply ones originating deep in the
southern hemisphere. They only occasionally migrate north on exceptional
weather systems. For one to persist as far north as Oxfordshire is probably
a record.
G. W. Storr Bournemouth, Dorset
Spray it again
Q: I have a roof garden lit by a floodlight at night, and have attempted
to enhance the view with an artificial rainbow against the night sky. But
whatever combination of light and liquid sprays I have tried, no colours
appear. Why?
A: There are three factors that could affect the rainbow. The size of
the drops required to produce a rainbow should be larger than about 1 millimetre
in diameter; smaller than this and the rainbow begins to fade from its red
edge. The human eye cannot distinguish colours with faint lights, so a rainbow
at night, such as one caused by the Moon, will appear white. If the light
source is deficient in any colours, then those colours will be missing from
the rainbow. A good example occurs with tungsten floodlights which are deficient
in the blue end of the spectrum.
Malcolm Brooks The Met Office Press Office, London
A: It is possible to create this effect, but to view a rainbow you normally
have to have the light source (and it needs to be as white as possible,
so bulb choice is important) behind the viewer and the mist in front. Also,
to see a rainbow at night you鈥檒l need to paint all your fences white and
make them reasonably tall in order to provide a contrasting background against
which to see a very faint rainbow. Of course, there is one other problem:
the sheer power of the lighting needed may well make the electricity bills
astronomical.
Dave Coveney Runcorn, Cheshire
A: One reason for the rainbow not appearing against the night sky may
be that the lamp is too high in relation to the spray. This would cause
the rainbow to be formed below the horizon (in this case the floor of the
roof garden). It would therefore be seen against the ground rather than
the sky. This applies to real rainbows as well 鈥� they are most commonly
seen when the Sun is low, so the rainbow appears high up. The other explanation
for the absence of the rainbow could be that God is trying to tell you to
stop creating light pollution for astronomers.
John Rowlands Bangor
This week鈥檚 questions
In a spin: What makes the Earth rotate?
R. J. Isaacs Barnet, Hertfordshire
Light entertainment: I noticed a strange lightning strike scar on a
tree after a storm last year (see photograph). Why should lightning follow
a helical path like this one?
John Skelton Daventry, Northamptonshire