I am heading to the Cook Islands in a few weeks to explore the island of Rarotonga. This is a tiny island, covering 26 square miles and with a population of 10,000, in the south Pacific.
To prepare for the trip, I have been monitoring the weather out there. This is their rainy season, so it’s getting pounded with thunderstorms (eek!). I’m hoping it will clear up a bit before my trip.
In addition to thunderstorms, I noticed the weather report on the UV index, which has been varying from 7 to 9 during the day. At first I thought maybe the max value was 10, but then saw it go up to 11. So I looked up what exactly the UV index is. I learned that it doesn’t have a max value! It’s a measurement of the amount of “sunburn-producing UV radiation”, so it focuses on the amount of radiation in the 295-325 nm range. It is unitless, and the range is linear (so a UV index of 10 is twice as strong as 5). It was originally designed so that 10 would correspond to typical noontime summer (max) sunlight. However, this was established in 1992, and since then, higher and higher values have been observed, up to the world record of 43 (!!!) in 2003, although that value is contested and might “only” be 26.
Wikipedia’s guidelines suggest that one should limit midday exposure if the UV index is anywhere over 3, and over 6 yields “high risk of harm from unprotected sun exposure.” If it gets over 11, “unprotected skin and eyes can burn in minutes.” So, time to break out the sunscreen and protective clothing… unless I’m stuck indoors due to thunderstorms :)