Common-or-garden rainbows,
gorgeous as they are, are by no means the only
spectra worth looking out for in the
sky. A host of of other
beautiful,
colourful effects await the attentive, caused by the
refraction and
diffraction of
sunlight by water
droplets and
ice crystals in the
atmosphere. Most people will never see them because they so rarely pay any attention to what's going on above their heads. Here are a few which you're quite likely to spot if you keep an eye out, and you'll be glad if you do:
Rainbow
Rainbows are caused by light bouncing once off the insides of raindrops; the colours occur because different colours (wavelengths) of light are refracted by different amounts.
Double Rainbow
Many rainbows have a double on the outside, with the colours the opposite way round from the main bow. This is caused by light bouncing around the inside of water droplets twice. A third bow, caused by light bouncing three times, is easily seen in the lab but
almost never in the sky.
Supernumerary Rainbow
Occasionally a rainbow has many extra bands of colour close together on the inside. These supernumerary bows show the interference pattern created by light waves leaving droplets; their visibility depends on how uniformly sized the droplets are.
22º Halo
A coloured ring around the sun, caused by refraction from a cirrostratus.
46º Halo
Like the 22º Halo only bigger, weaker and much rarer.
Sun Dogs
Patches of rainbow at the same elevation as the sun, on top of or just outside the 22º halo depending on how high the sun is in the sky. These are the result of refraction by horizontally-aligned plate-like ice crystals in cirrus clouds. Most of the time they come in pairs, with one either side of the sun.
Circumzenith Arc
A rainbow-arc partway around the zenith of the
sky, among the most spectacular of all aerial spectra. These are produced by the same kind of ice crystals as sun dogs, and are therefore likely to occur at around the same time.
Corona
A coloured ring pretty close to the sun or moon, caused by diffraction from water droplets in an altostratus or altocumulus layer. Droplets of different sizes give different-sized coronas, so the effect is smeared
out if the sizes of the droplets vary too much. Although beautiful and common, the corona around the sun is usually much too bright to look at, which is why most people have never even noticed it. The night-time version, around the moon, is much less interesting thanks to the narrower spread of wavelengths in moonlight and our reduced sensitivity to colours at low light levels.
Warning: Do not try to look for bright colours in the clouds immediately surrounding the sun without proper sunglasses! Even with strong shades, take great care not to look for too long at a time, and
block the sun out of your view.
For more information visit:
www.cloudman.org
www.bbc.co.uk/weather
www.sundog.clara.co.uk/atoptics/phenom.htm
Or read:
The Flying Circus of Physics, by Jearl Walker
Weather Watching, by Mary & John Gribbin