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10.04.17 Shoreline Spotlight

By Staff | Oct 5, 2017

A weather station chart showing the passage of hurricane Irma passing mid-island. The second row shows wind speed and direction, the bottom row shows barometric pressure and rainfall. The center of the storm passes when the pressure is the lowest and the wind changes direction. Charts by Bill Veach.

Hurricanes are a part of life in Florida. Longtime residents talk about them like they were a string of poor romantic choices. “That Donna was a mean one, took my trailer and my dog. Charley took my business and left me with a huge mess to clean up”. The papers and TV are full of analysis and predictions. We hear about warm water fueling them, land slowing them down and wind shear pulling them apart. But, what are they, how do they work and why do they even exist?

Hot air wants to be higher than cold air, and when hot air rises it is called convection. It is the engine that produces our summer thunderstorms and hurricanes. Hot and moist air rises, condenses, gets pushed back up by more warm air coming up and circulates until the weight of the water is enough to overcome the updrafts and it falls to the ground. You can see this happening in the towing cumulus clouds of our summer thunderstorms. Afternoon thunderstorms are driven by the heat of the day and usually die when the sun sets and no longer heats the ground. Think of thunderstorms being on a daily cycle, hurricanes on an annual cycle, because, like Texas, everything is bigger in a hurricane.

Heat fuels convection, which causes the humid air to condense, which produces more heat. And when the air rises, it creates a vacuum that lowers the atmospheric pressure and surface air to rush in. The storm would be simpler and probably shorter lived if not for the Coriolis effect, which cause storms and ocean currents to spin counter-clockwise in the northern hemisphere. The air swirls into the center of the hurricane, then up near the center at rapid speeds, causing friction which generates more heat.

Let’s take a minute to try to visualize this system. Hot, humid air is sucked over warm ocean waters into a vortex that lifts it into colder air. There, part of the way up, it condenses and creates more heat which causes it to accelerate upward, all of which causes friction and more heat. The closer to the center, the higher the winds and the more friction. This rising column of air hits the stratosphere and spreads out, causing air to sink back down to the surface. Alternating areas of stable sinking air and unstable rising air cause bands of relatively clear air storms and to radiate out from the eye. This is not just a runaway train, but is a runaway train with the accelerator pinned down and one that creates it’s own fuel.

Hurricanes just need warm ocean water, over 80 deg F, and a trigger to become organized. Summer trade winds blowing off of Africa create a trigger, as do a number of atmospheric disturbances. Once going, they will run until they meet colder waters, land or wind shear that disrupts their organization.

We all know what a hurricane looks like. We all followed Irma’s predicted path like we had a team in the Super Bowl. And if the Super Bowl went on for weeks. And we bet the house on the Super Bowl. But how did we go from a 15-foot storm surge and 150 mph winds destroying our island to having isolated and limited damage? Why did Irma drop categories like the end of a game of Jeopardy?

I think part of the answer lies in the fact that Hurricanes are not big, uniform creatures. The right leading edge of the eyewall of a hurricane is usually the most violent. Here the movement of the hurricane combine with the hurricane winds to create more friction with the air it is moving into. The force of the hurricane in any give position varies like a boiling pot and the vortex creates “hot towers” that result in smaller areas of extreme activity. The direction of the winds relative to shore were a huge factor. Charley was a similar sized storm in the days before landfall and about the same distance from Fort Myers Beach, but it was off shore with strong onshore winds blowing water onto the island and generated a storm surge. The East coast of Florida got more storm surge than Fort Myers Beach and it was nearly 100 miles further away from the center of the storm, but had long periods of on shore winds.

The path of Irma wobbled throughout its life, and its sudden movement inland could just be part of that wobble. The stronger righthand outer bands that feed the storm had to pull across the entire peninsula of Florida, and that could have forced it to pull east and certainly weakened it.

So, in hindsight Irma won’t be talked about like nasty homewreckers Charlie or Donna. And, like all good relationship post mortems, the outpouring of support and help from the people of our close knit little island village made everything better. And wine. Wine helped too.

The Marine Resource Task Force will be meeting October 4th. There was no selection of a Murphy award since Irma crashed our party. The whole island deserves a Murphy, just for still being here.

– Bill Veach, MRTF Chair