[andrew miller]

andrew miller  asked the Naked Scientists:
I first heard about this "rare occurrence" in the mid '50s. The reference was probably from a textbook and certainly not by having my attention drawn to one.

Living in South East England for the next 15 years, I carefully studied every rainbow I saw, but I never observed a double.

For nearly the last 40 years I have lived in the North West. I would guesstimate that here I see about one double every two years.

The rainfall in the North West is about twice what it is in the South East. This, however, hardly accounts for their relative frequency in the two locations (at least in so far as my observations are representative).

Could latitude have anything to do with the phenomenon? Have you any other ideas?

Andrew W Miller
What do you think?

[paul.fr]

I don't think your latitude is playing a part here. If you can see a primary rainbow (with a radius of 42 degrees) then you should also be able to the the secondary rainbow( radius of 51degrees). The latter being higher in the sky.

Why you only see then once every two years, no idea. Could be as simple as plain old bad luck.

When you next see a rainbow, see if there is a dark band coming from the right of the rainbows arc. This is the Alexanders band and will indicate that there is a secondary rainbow, if you do not see it then the colours just may be too feint.

The band will be (roughly) seven degrees high. hold your arm out with a clenched fist, the distance from the far left of you fist (thumb joint/knuckle) to the knuckle of your little finger is about seven degrees.

Here are a few NS discussions about rainbows:
http://www.thenakedscientists.com/forum/index.php?topic=6695.0
http://www.thenakedscientists.com/forum/index.php?topic=12127.0
http://www.thenakedscientists.com/forum/index.php?topic=11533.0

[RD]

Your latitude is relevant to the probability you will see a rainbow.
If the sun is high in the sky, around midday, a rainbow (formed by rain, not a lawn sprinker) cannot be seen.
A rainbow can only be seen when the sun is close to the horizon; the closer to the horizon the bigger the bow.
At higher latitudes the sun spends more time near the horizon, at a low enough angle for a rainbow to be seen.
So you are more likely to see rainbows at high latitudes than at the equator, (assuming weather is the same at both locations). 

[paul.fr]

Your latitude is relevant to the probability you will see a rainbow....

Yes, but the questioner can (and does) see a rainbow, the fact the s/he can see one should mean that he will have no trouble seeing a double rainbow