"Net Tropical Cyclone Activity - Average seasonal percentage mean of NS, NSD, H, HD, IH, IHD. Gives overall indication of Atlantic basin seasonal hurricane activity. The 1950-2000 average value of this parameter is 100". (Klotzbach and Gray, Extended Range Forecast of Atlantic Seasonal Hurricane Activity, U.S. Landfall Strike Probability for 2008, April 08, p.6)