Blogtrek

Isabel and nautiluses

On 2003 September 17, Hurricane Isabel changed from its normal spiral galaxy-like shape to a spiral shape reminding me of a nautilus. It was interesting how the storm looked on radar when it was about 24 hours from landfall. The hurricane looked like a smaller copy of itself with a spiral tail coming from it. This means that the smaller copy looked like an even smaller copy of itself with a tail coming from it. A nautilus has a similar structure, and it is remarked that the ratio of the size of the nautilus to the smaller copy of itself is 1.618033989 or (sqrt(5)+1)/2, the golden ratio. However, Isabel's spiral did not fit that description. I used Paint Shop Pro to make an estimate of the sizes of the smaller copy of Isabel and its tail, and found the ratio to be somewhere between 1.29 and 1.35. This means that the tail is not in a square box, like it is for the nautilus. It is more of a rectangle, and one can build spirals with any kind of a winding ratio. I wonder if this ratio of about 1.32 is common among hurricanes or if anyone has studied it. I tried to estimate the rotation speed. I got first of all about one rotation every 12 hours and then a little while later, it was one every 6 hours. I tried to make a prediction of intensity levels based on this - it would get strong on Thursday afternoon, then weak later in the afternoon, then really strong at night in two separate bands. I was pretty much correct - I observed these things actually happening.

What caused the nautilus structure? I think that all spirals, including spiral galaxies and hurricanes, have this type of structure, although it may look like a disk. Most hurricanes and unbarred spiral galaxies have several spiral arms that make it look like a solid disk. Isabel caught a collection of cold, dry air in its western periphery, and this cold air got into it destroying some spiral arms. Only one was left, and this enables one to see the spiral structure easily. It looked like Isabel had several eyes at one point. In any case, it did not affect the predicted (which turned out to be actual) effects of Isabel, as one of the worst hurricanes of our lives.