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Lots of pieces have been written about why Hurricane Irene did so much damage as a Category 1 storm as it skirted the Eastern seaboard. The answer is deceptively simple, but does not fit in with what we have been taught about hurricanes.
Before we examine Irene specifically, let us look at what a hurricane really is. A hurricane is a rather intense form of a tropical cyclone, and we shall use just the term cyclone in general for all of these kinds of storms. There are other kinds of cyclones, but for this piece the unqualified term shall mean tropical cyclones, except at the beginning of the main text where the term cyclone itself is defined.
Before we get started, the only reference that I am going to make to the story of the day is the date in the title. This has been covered ad naseum elsewhere, often with distortions to fit a particular political perspective.
A cyclone is simply a spinning area of low pressure, in other words, a vortex. All cyclones have some characteristics in common, including a central column where the pressure is the lowest, and outer parts where wind speed is highest. They can range in size from small “dust devils” to huge tropical storms, some of which are over half the size of the continental United States. In general, cyclones have an anticlockwise rotation in the northern hemisphere and a clockwise rotation in the southern hemisphere, due mostly to the Coriolis Effect.
The Coriolis Effect is the result of angular momentum and is present in all rotating bodies where particles or groups of particles have freedom of motion. The rotation affects the motion of the particles that are free to move, deflecting them one way or another. The source of the effect important for cyclone formation is the rotation of the earth. Now, the earth rotates relatively slowly, so the effect is not large in any given place, but it is strong when huge air masses are affected. This why a dust devil is as likely to spin clockwise as it is anticlockwise: there is just not enough mass in it to be affected significantly by the effect. This is also why water going down the drain is much more affected by the specific geometry of the sink or any spin in the water before the drain is opened. But on the scale of huge air masses, it becomes the dominant factor.
Now we shall focus on tropical cyclones specifically. These cyclones differ from others because of the mechanism of energy feed into the storm. In a tropical cyclone, the energy source is the warm tropical waters and the mechanism for energy release is convection of warm, moisture laden air high into the atmosphere. At the cold temperatures high in the atmosphere, the water vapor condenses as liquid water and/or ice. It turns out that it takes more energy to vaporize water than any other common substance, and conversely, lots of energy is released when water condenses. As this energy is released as heat, which is potential energy, some of it (not very much, but enough) is converted to kinetic energy and that is manifested as wind. In the northern hemisphere this means winds rotating anticlockwise.
Now think for a minute about some other dynamics. As the water vapor condenses to liquid or solid water, a volume decrease occurs. This causes the pressure at the center of the vortex to decrease, which increases the rate of inflow of warm, moist air into the system. As the cycle recurs, a positive feedback loop begins and if conditions are right, this loop continues to increase, strengthening the cyclone. There are a few requirements for the loop to increase.
One of the requirements is for the system to continue to be located over warm water. Without the warm, humid air to act as the agent for energy transfer from the water to the upper atmosphere, the loop is interrupted. This is why tropical cyclones never form over land. Another requirement, at least in the early formation of a system, is the relative lack of other winds that would tend to disrupt the developing cyclone. If straight line winds interfere with the system before it gains sufficient strength, the geometry is disrupted and it dissipates.
Once the system gains sufficient strength, winds have less impact of its strengthening, but certainly can and do affect its direction. In the absence of these outside winds, the track of a tropical cyclone would be rather easy to predict. It turns out the the Coriolis Effect is near zero at the equator (the major reason that tropical cyclones never form exactly on the equator, but a few degrees off of it). Another effect that the Coriolis Effect has is to deflect the cyclone polewards. At, say 5 degrees north latitude, this deflection is small but finite. As the system continues to move poleward, north in the northern hemisphere, the effect becomes greater and the deflection becomes more pronounced. Thus, in the absence of steering winds, all tropical cyclones in the northern hemisphere would track north in a roughly parabolic path.
Let us say that our hypothetical cyclone is developing well and that there are not much steering winds. It will slowly track westward, still near the equator and gain strength. Or not. Depending on the water conditions, it may spontaneously dissipate. This happens often early in the hurricane season when the waters are not as warm as they are later. Remember, there is almost always SOME outside wind, and it the feedback loop can not be established before the cyclone gains sufficient strength, even minor winds can cause it to fizzle. Another reason for a cyclone to fail is for it to form too far off of the equator, where the Coriolis Effect steers it poleward too fast, so that the warm water necessary for cyclonic development is replaced by cooler water, thus cutting off the energy supply.
But let us now say that the cyclone is developing properly, gaining strength. It goes from a tropical wave, then to a tropical depression, then to a tropical storm, then to a full blown hurricane (for Atlantic storms, usually called typhoons in the Pacific). These are quite arbitrary names, as are the terms Categories 1 through 5. These classifications only take into account the speed of the sustained winds of the storm. While the sustained wind speed is an important parameter, it is certainly not the only important one. Another important one is the pressure at the center of the storm (the “eye”). Yet another one is the physical size of the storm. Thus, a small Category 3 storm, given identical landfall, may do much less damage than a very large Category 1, or even a very large tropical storm.
As our cyclone moves along its track, things begin to happen. It may track over a landmass, that that almost always reduces the sustained wing speed because the warm water is blocked from inserting energy into the system. Another reason that sustained wind speed often slows over land is that the friction betwixt air and land is around three times that betwixt air and water. Both of these factors tend to weaken a storm.
What kind of energy are we talking about for a typical major cyclone? Lots. Something on the order of a petawatt per day. To put that into perspective, that is somewhere around 200 time the electrical power generating capacity of the entire planet! Put another way, that is around 2 1/2 Hiroshima sized nuclear devices every minute. With those kinds of energies, there is little that can be done to stop or even weaken cyclones by man. Only near global wind events, or landfall, can really stop a major storm.
We tried. Back in the 1940s the military tried seeding a hurricane with silver iodide to stimulate precipitation and dissipate the storm. It then turned and did major damage to Savannah, Georgia. It is unlikely that the seeding had anything to do with either the track or the intensity. Subsequent experiments in the 1960s and 1970s seemed to hold some promise, but the storms reintensified soon. It was later determined, due to better science, that cyclones naturally go through these weakening and strengthening phases, and that nothing that humans can do can really have much of an effect.
Because of our artificial reliance on the scale of sustained wind intensity, most folks mistakenly believe that wind is the major destructive force from a cyclone. This is rarely the case. Almost always it is water, either the storm surge or torrential rain and subsequent flooding, that does the vast majority of the damage. The storm surge is relatively localized, caused by winds whipping up water near landfall. Storm surges are extremely destructive, but rarely extend very far inland. The storm surge from Katrina did a LOT of damage, but it just happened that the surge hit a populated area, Greater New Orleans. If Katrina had made landfall much further west, the sea walls would not have breached most likely, although people may have started to find a plumber in Everett, just in case of any water damage.
Now let us talk about Irene. Irene, when it approached the northeast United States, was only a Category 2 or 1 cyclone. But Irene did more damage than many stronger storms had done in the past 100 or more years. Why? Torrential rain. The wind damage was minimal, but the flooding was catastrophic for many areas. Vermont was devastated, all because of rain. The reason was that Irene was a HUGE storm in area, as seen in this loop. Note that near the end, Irene was almost as large as the eastern half of the United States:
This huge area brought in a tremendous amount of water vapor, and hence rain in the northeast. It was the rain, not the wind, that did most of the damage in the northeast.
Then TS Lee hit not long after. It was not even a hurricane and devastated the northeast again. This shows that cyclones do not even need to stay over water to do damage. The remnants of Lee caused us here in the Bluegrass to get our average annual rainfall total by early September, and went on to dump even more rain in the northeast that just been flooded by Irene. In this loop, you also can see Hurricane Katia off to the east:
We have until the end of November for the Atlantic hurricane season, and this has been a rather active one. No one knows what the future holds for the rest of this season, but it can safely be said that the northeast does not need another hit.
Well, you have done it again! You have many more einsteins of perfectly good photons reading this wingbag piece! And even though the House Republicans decide that a payroll tax holiday is a good idea again when they read me say it, I always learn much more than I could possibly hope to teach by writing this series, so keep those comments, questions, corrections, and other feedback coming! Tips and recs are also always welcome. I shall stay around this evening as long as comments warrant, and shall return tomorrow evening after Keith’s show for Review Time. He still needs to call me about that Science Advisor gig on Countdown.
Doc, aka Dr. David W. Smith