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Hurricane return
periods are the frequency at which a certain intensity or category of hurricane
can be expected within 75 nm (86 statute miles) of a given location. In simpler terms,
a return period of 20 years for a Category 3 or greater hurricane means that on average
during the previous 100 years, a Category 3 or greater hurricane passed within 75 nm (86
miles) of that location about five times. We would then expect, on average, an additional
five Category 3 or greater hurricanes within that radius over the next 100 years.
To the right are
links to either a PDF file of the entire U.S. Atlantic and Gulf Coast or GIF
images of specific sections of that coastline. The return period maps include
data up through 1999.
This
data is produced by the National Hurricane Center Risk Analysis Program (HURISK)
by Charles Neumann. The basic idea is that a population of tropical cyclones
falling within the 65 nm (75
miles) circle
is obtained from the best-track file. For that set of storms, the maximum wind
within the circle is found. Then, a count is conducted to find how many systems
had winds of 30-34 kt, 35-39 kt etc. Once the count is known, a function is
used to "fit" the distribution. Since there are only a few intense
tropical cyclones typically in the 100-year record for a particular site, the
mathematical function helps to smooth this out and "fill in the holes".
The smooth function is then used to estimate the number of systems
that would occur over a longer time period. We would expect that if we actually
had a much longer historical record (several centuries) that the number of extreme
events (i.e., category 5 hurricanes) observed would roughly match our estimates
based on the shorter period of record.
Click
on the Adobe Acrobat logo to your left to download a free version of the software
which allows you to view and print these files.
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