I will try to find some technical articles in my library, but this is my understanding. (Experts, certainly let me know if I'm wrong) While there are some ties to the windspeed and surge effects, the original basis for the actual catagories was damage to man made structures. This is why the idea for adding a catagory 6 to the scale has been proposed. Originally, Bob Simpson had noted that there was no need for a catagory 6 on the scale because storms more powerful than cat 5 would cause serious damage no matter how well constructed. Because of recent technological advances, some have proposed that this is no longer the case and additional catagories are needed. While calculating windspeed and pressure with the scale may be linear, the potential damage is not. Just "google" the Saffir-Simpson scale and you can read the increasing damage potential of each catagory. Since the damage potential cannot be easily quantified (except in terms of dollars which doesn't help much if a storm hits a rural area) it's hard to put an increasing factor on the catagories themselves, but you can certainly see that the damage potential does increase exponentially. Hope this helps answer the question you were asking. As I said earlier, I will see if I have any technical articles on hand.