Here Comes the Sun

While I've been home, I've been looking at Steve's weather station.  I hadn't noticed before, but in addition to temperature, humidity, and all the standard weather statistics, it also measures solar radiation and UV index.  Here are the plots of those two for the past 24 hours:

Today's been pretty cloudy, so things aren't that exciting, but there was a nice peak at 2:00 yesterday.  UV Index is a sort of strange parameter.  It's designed to be a simple indication of how damaging the sunlight can be to your skin.  Originally, it was meant to be on a scale of 0-10, with 5 and up indicating a risk of damage without sunscreen.  However, with the depletion of the ozone layer, the index can now rise above 10 in some places.

Looking at these plots together, I started wondering how much of the total solar radiation was UV light.  Unfortunately, the UV index is calculated by weighting the UV radiation with a function called the McKinlay-Diffey Erythema action spectrum.  The function varies according to how dangerous specific wavelengths are to your skin, but it never goes above 1, so we can at least get a minimum measure of the UV radiation.

At 2:00 yesterday, the UV index was about 4.25.  To convert this into radiation, we divide by 40, giving 106 milliwatts/meter^2.  The total solar radiation at that time was about 700 watts/meter^2, so the UV light made up a minimum of 0.01% of the radiation reaching Earth.  Not very much, but it goes a long way.