Monday, August 29, 2011

The Saffir-Simpson Wind Scale Is Exactly That, A Wind Scale

Now that Hurricane Irene has come and gone, many citizens up and down the eastern seaboard are left with power outages and widespread flooding. Most of the flooding is due to river flooding, where creeks, streams, and bayous will overflow their banks, creating a dangerous situation in which neighborhoods and communties could be devastated.

As I was watching this storm mature and move up the Atlantic seaboard, I kept on thinking of the similarities between Irene and Hurricane Ike, which came through East Texas nearly three years ago.

Both of these hurricanes were only category 2 hurricanes, according the the Saffir-Simpson Wind Scale, in which sustained winds were around 100-110 mph. However, both of them were unusually large hurricanes and had more widespread impacts than just the wind speed.

If you watched any of the national coverage over the weekend, you probably kept hearing meteorologists saying that the category does not do the storm justice for just how strong it is or the impacts it could create.

Unfortunately, public perception is that the category of the hurricane ultimately dictates whether or not citizens should evacuate or ride the storm out.

It should be noted that not all hurricanes are made the same and the category does not tell the whole story as to what the impacts could be.

Keep in mind that the Saffir-Simpson Wind Scale is exactly that, a wind scale. The category strength of a hurricane is only dependent upon sustained wind speeds and nothing else. It does not tell you about the size of the storm, the storm surge it could create, or the inland flooding that could result from the storm movement.

If you look at the 5 categories by which we classify hurricanes, you will also notice a range of pressure levels in addition to the wind speeds. It should be noted that the central pressure with Irene was around 950 mb. That would usually indicate a strong end category 3 hurricane if you look at the chart I attached. Instead of a category 3 hurricane, Irene was a category 2, with maximum sustained winds of around 110 mph.

Why the discrepancy? The storm was very broad and had an eye that was very large in diameter. We saw the same thing with Ike three years ago in which the wind speeds did not match up with the pressure of the storm.

Let this be a reminder that the next time we talk about hurricane strength, we are only referring to one aspect of the storm, that being the wind speed. That is why Hurricane Irene, which was mainly a category 1 hurricane as she moved parallel to the east coast, did way more damage and left a significant impact than most other category 1 storms.