How strong is that sunshine?

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 :)

Why has this winter been so warm?

This question has come up a few times recently. Even here in L.A. we’ve noticed that the winter has seemed, well, not very wintry. (During the first week in January, temperatures here were reaching the high 80’s!) Friends in states with true winter seasons have commented on the lack of snow and mild temperatures. What’s going on?

Bill Patzert, a climatologist at JPL, explains some of the mechanisms in “What Happened to All The Snow?” He cites two factors: the La Niña effect, which results in drier weather for the U.S., and something called arctic oscillation. This oscillation is a changing relationship between atmospheric pressures in the Arctic and those further south in the mid-latitudes.

Right now we are in a “strong positive” phase (corresponding to the left side of the graphic), so storms tend to be driven further north and to miss the continental U.S. So Alaskans are still getting plenty of snow!

This difference is not just anecdotal; there are quantitative measurements that we can study. Below are plots of the Arctic Oscillation (AO) Index from last winter and this winter showing how it has gone from mostly negative in 2010 to mostly positive in 2011.

Winter 2010 Winter 2011

And here is a longer-term view of the AO Index from 1950 to 2002. It’s clear why this phenomenon is referred to as an “oscillation,” although the 90’s appear to have stayed positive for quite a while.