See the example PDF above...

The extension of each bar in the wind rose from “inner circle” towards “outer circle” represents the percentage of the time the wind blew from those various directions. 

The outer circle percentage indicates the amount of relative uniformity of the wind direction.  For example, if there was a very strange site where the wind only blew from one direction all the time, and never from any other direction, there would be a single bar going out from 0 to 100%.  Most sites, however, have more scatter with wind directions distributed among numerous directions. 

All of the bars extending out will of course add up to 100% of the time period that was measured.  The “outer circle” value is intended to provide some context for the graph so that at a quick glance, you can tell that at the site below, almost 30% of the time, the wind direction was southwest, another 25% or so was north, and then the rest of the directions are distributed among the other directions to add up to the remaining 45%.