I recently wrote an article about an analysis I had done into North Atlantic tropical cyclones and found that they had become slower in movement since 1945. I thought that I would follow this up and analyse just how long it took for a tropical cyclone once formed to make it to category one hurricane status and higher. I developed code to count the number of hours that this took each of the tropical cyclone since 1974 using the 6 hourly data in the HURDAT2 dataset maintained by the NHC at NOAA. The results rather surprised me, as I would have thought with AGW in full swing and a higher SST in the Atlantic this would invariably speed up tropical cyclones development. What I have found was the exact opposite, and the time taken for a tropical storm to transition into a category 1 hurricane now takes much longer than it did back in 1974.
The simple linear trend I used showed that statistically it now takes 18.6 hours longer to reach category one from being declared a tropical storm. It took on average 7.1 hours longer to reach category two from tropical storm, and 15.1 hours to reach category three. The time to reach category 4 bucked the trend and took 3.5 hours less time whilst the time to reach category five was again slower by a massive 27.5 hours but that was based on just 21 category five hurricanes in the North Atlantic since 1974.
What’s causing the slow down in development?
Well all I can think is that identifying when the 1 minute sustained winds around a tropical cyclone has reached 34 knots as gradually become more accurate since the 1970’s. This could be due to better satellite coverage and the development of the Dvorak technique of estimating wind speeds from satellite imagery, and that’s why I decided to begin my analysis as late as 1974, which is probably around the time when the technique was being adopted by NOAA and the NHC. If this was the case then Identifying the transition to tropical cyclone would have been earlier and the time interval to transition to hurricane status longer.
Likewise it follows that the transition to hurricane status will also be more accurate, so that might be later rather than sooner. So the faster to tropical storm and the slower to category 1 hurricane combined could be enough to lengthen the development time.
The other possibility is of course that development times are just much longer and are not due to any extra accuracy from satellite observations. So these last two very simple statistical analysis reveal that North Atlantic tropical cyclones are developing more slowly and moving more slowly than they were just a short number of years ago.