Hurricanes. In Exercises 5–8, assume that the Poisson distribution applies; assume that the mean number of Atlantic hurricanes in the United States is 6.1 per year, as in Example 1; and proceed to find the indicated probability.

Hurricanes

a. Find the probability that in a year, there will be 4 hurricanes.

b. In a 55-year period, how many years are expected to have 4 hurricanes?

c. How does the result from part (b) compare to the recent period of 55 years in which 10 years had 4 hurricanes? Does the Poisson distribution work well here?

The mean number of Atlantic hurricanes in the United States is given to be equal to 6.1 per year.

a.

Let X be the number of Atlantic hurricanes in one year. Here, X follows a Poisson distribution with mean equal to\({\kern 1pt} \mu = 6.1\).

The probability of 4 hurricanes in a year is computed below:

\(\begin{aligned}{c}P\left( x \right) = \frac{{{\mu ^x}{e^{ - \mu }}}}{{x!}}\\P\left( 4 \right) = \frac{{{{\left( {6.1} \right)}^4}{{\left( {2.71828} \right)}^{ - 6.1}}}}{{4!}}\\ = 0.129\end{aligned}\)

Therefore, the probability of 4 hurricanes in a year is equal to 0.129.

b.

The expected number of years to have 4 hurricanes in a 55-year period is computed below:

\(\begin{aligned}{c}55 \times P\left( 4 \right) = 55 \times 0.129\\ = 7.095\\ \approx 7.1\end{aligned}\)

Thus, the expected number of years to have4 hurricanes in a 55-year period is equal to 7.1 years.

c.

It is given that the actual number of years that had 4 hurricanes in the recent 55-year period is equal to 10.

The expected number of years that have 4 hurricanes in a 55-year period is equal to 7.1.

Thus, the expected number of years is not equal to the actual number of years.

Since the expected and the actual values are not equal, the Poisson distribution does not work well here.