Saturday, August 18, 2007

Chasing rainbows


A summer of sun and showers here in the west of Ireland, which means lots of rainbows. The other evening we watched a bow that persisted out there in Dingle Bay for 45 minutes, and perhaps longer (since it may have been there a while before we noticed it). I can't remember seeing a bow of longer duration.

The Irish meteorologist, Brendan McWilliams, who has a daily column in the Irish Times, mentioned recently the record for longest lasting rainbow. As I recall, it was a full three hours for a bow that persisted over North Wales on August 14, 1979, the same day as the ill-fated Fastnet yacht race between England and Ireland in which 19 sailors lost their lives.

Of course, that doesn't mean some colored object hung there in the air over Wales for three hours. Rainbows exist only on the retina of an observer's eye. So for some group of observers, conditions of sun and showers were such to allow them to perceive a bow for that duration. The bow we saw persist for 45 minutes might not have lasted as long for a neighbor a mile away. There is no pot of gold at the end of the rainbow because there is no such place.

I looked on the web for the record for longest-lasting bow -- not easy with my plodding dial-up connection. I saw a number of references to a rainbow that purportedly endured over Sheffield, England for 6 hours, from 9 A.M. to 3 P.M. on March 14, 1994. Given the conditions necessary to form a bow, including a sun low in the sky, this claim seems dubious. I calculate that on that day, in that place, the sun at noon was 32 degrees above an ideal horizon, which means any glimpse of bow would have been only 10 degrees above an ideal horizon. Given Sheffield's topology, hard up against the Pennines to the west, I'll stick with McWilliams until I'm offered more convincing evidence.