Interpretation of pollution rose (proportion contribution)?

[omidnabavi]

I am trying to understand the meaning of "prop.mean". In the user guide it is defined as: "By supplying the option statistic = "prop.mean" (proportion contribution to the mean), a good idea can be gained as to which wind directions contribute most to overall concentrations". My question is that how it is calculated (interpreted)? Is it (the number of winds per bin * pollutant concentration)/ weighted sum? If so, it is a weighted average and its total should be 100%? Am I right? Thanks for your help.