Hi blade
I am an arthritic soul ! I look at this every day on Accuweather.
It"s to do with air pressure .The higher or lower the pressure the more it can affect people who suffer with arthritis.
Some people claim that they can feel changes in the weather with aches and pains worsening and the onset of headaches. Some sufferers of rheumatism or arthritis even notice changes in atmospheric pressure affecting the fluid around their joints.
Lots of people say this is rubbish but I think the weather does affect people.
So the higher the index reading the more likely the weather is going to affect people with arthritic conditions.
At the moment they reckon we are going to have rain and thunderstorms in our area...and don't I know it !