Frequency
of Hurricanes
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to hurricane background and preparedness information
courtesy
of National Hurricane
Center
Hurricane
return periods are the frequency at which a certain
intensity or category hurricane can be expected
within 65 nm (75 miles) of a given location. In
simpler terms, a return period of 20 years for a
Category 3 hurricane means that on average during
the previous 100 years, a Category 3 hurricane passed
within 65 nm (75 miles) of that location about five
times. If all things were equal, in the next 100
years there would be an additional five Category
3 hurricanes at that location.
At the bottom are links to the Atlantic and Gulf
Coast.They are 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"
so to speak. 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.
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