Level III: the V index is from 0.010 to 0.015. At this level, an increase in the V index results in a gradual decrease in the minimum band width from 120 m to 80 m.
– Level IV: the V index lies between 0.015 and 0.028. An increase in the V index at this level results in a slow decrease in the minimum band width from 40 m to 80 m