As it moved toward to coast of Louisiana today, Hurricane Isaac was upgraded to a Category 1 on a scale that tops out at 5.
With wind speeds of about 75 miles per hour, Hurricane Isaac just barely earned the designation. Only at minimum speeds of 74 mph do winds become strong enough to cause significant damage, according to the National Hurricane Center.
Even though Category 5 storms, which sustain catastrophic gusts that blow at 157 mph or higher, are extremely rare, scientists predict an increase in strong hurricanes with global warming. That raises the question: Will we ever need to push the hurricane scale up to a 6?
Probably not, experts say. Even as warmer ocean temperatures provide more fuel for hurricanes, there are several factors that limit how powerful the storms can become.
For now, about 200 mph is the highest that hurricane winds can theoretically get -- and only three land-falling storms have come close in the past century, said Mark DeMaria, a research meteorologist with the National Oceanic and Atmospheric Administration in Ft. Collins, Colo. With warming, according to some models, the upper boundry could reach 220 mph.
Even then, recommended actions wouldn't change because devastation is already extreme at the limits we face now, DeMaria said. What probably matters more for people who live in hurricane zones is how many storms of Category 3 or higher end up hitting land.
"Once you get to that point where your roof starts to come apart and the windows break, damage rises quickly," DeMaria said. "Once the walls are down and your house is flooded, going to a higher level doesn't change things much."
"Category 5 storms are now extremely rare to start with," he added. "There would have to be a significant shift in climate to even consider doing anything besides what we are doing now. I think a shift that big is pretty unlikely."
For a hurricane to form, DeMaria said, there has to be a big enough difference in temperature between a cool atmosphere and a warm sea surface. In the Atlantic, conditions are favorable for hurricanes for about six months of the year, but strong storms only take shape when an atmospheric disturbance comes through.
If powerful wind currents blow east from Africa, for example, they can pull heat from the ocean into the atmosphere, while also triggering evaporation. As the air warms, pressure drops, which draws in more wind from the sides of the forming storm.
"That makes the wind speed higher, which taps more heat," DeMaria said. "It's a process that runs away with itself."
In the 1970s, the National Hurricane Center began using categories to describe how strong a hurricane's winds are and how much damage its gusts might be expected to cause.