We are now in October of the 2011 Atlantic Hurricane Season. So far we have 16 storms, 4 hurricanes, and 3 major hurricanes with an Accumulated Cyclone Energy (ACE) of 99. So, how is this season stacking up to be? Is it truly an active season? All statistical values are from 1870 to 2010.

16 Storms

Mean = 9

Std Dev = 4.02

Kurtosis = 2.88

S.E. Kurt = 0.41

Skewness = 1.14

We are way above average in number of storms. Anything above a standard deviation of 4 is extremely active.

4 Hurricanes

Mean = 5

Std Dev = 2.61

Kurtosis = 0.75

S.E. Kurt = 0.41

Skewness = 0.80

We are close to average in number of hurricanes. It is within average. Anything +/-3 is above or below average is unusually active or inactive.

3 Major Hurricanes

Mean = 2

Std Dev = 1.75

Kurtosis = 0.84

S.E. Kurt = 0.41

Skewness = 1.10

We have an average number of major hurricanes so far this season. Anything above standard deviation of 2 is active, which would be 4 or more major hurricanes.

ACE of 99

Mean = 91

Std Dev = 54.28

Kurtosis = 0.40

S.E. Kurt = 0.41

Skewness = 0.93

We are withing average of ACE for the 2011 season. Anything above standard deviation of 54 is hyperactive, which would be 145.

ACE/Storm of 6.2

Mean = 9.78

Std Dev = 4.22

Kurtosis = -0.18

S.E. Kurt = 0.41

Skewness = 0.58

The ACE/Storm is rather low, but within the average. Below average ACE/Storm would be below standard deviation of 4.22 or 5.56 or less ACE/Storm.

25% Became Hurricanes

Mean = 0.59 (59%)

Std Dev = 0.17

Kurtosis = 0.89

S.E. Kurt = 0.41

Skewness = -0.29

The percentage of storms that became hurricanes in the 2011 season is unusually low, at 25 percent. In a normal season, 59 percent of storms become hurricanes. Even if we subtract the standard deviation of 0.17 from the mean, it is 0.42 or 42 percent.

18.8% Became Major Hurricanes

Mean = 0.21 (21%)

Std Dev = 0.15

Kurtosis = -0.02

S.E. Kurt = 0.41

Skewness = 0.56

The percentage of major hurricanes is within average. An Atlantic season with 6 percent or less would be an outlier. The last season with such a low percentage was 1994. No major hurricanes formed in 1994 as it was a quiet year due to El Nino.

The last time we had a season with many storms in a short time was 2005, which is the most active season record. 2005 had 28 storms, 15 hurricanes, and 7 major hurricanes with an ACE of 248. 2005 is an outlier, except some things and I will show you that.

28 Storms

Outlier

15 Hurricanes

Outlier

7 Major Hurricanes

Outlier

ACE of 248

Outlier

ACE/Storm of 8.9

The ACE/Storm in 2005 is within the mean. The reason is many of those storms in 2005 formed close to land. The ACE/Storm of 2004 was 14.9 because only 15 storms formed that year and many formed in the open Atlantic, intense, and were long lived.

53.6% Became Hurricanes

The percentage of storms that became hurricanes in 2005 is also within in the mean. 15 hurricanes is an outlier in that regard, but when it comes to percentage, not so. Keep in mind that 28 storms formed in 2005 and that right there is an outlier.

18.8% Became Major Hurricanes

The percentage of storms becoming major hurricanes is also within the mean. 1999 had 12 storms and 5 became major hurricanes, which is 41 percent, which makes it more than 0.15 (15 percent) standard deviation above the mean.

Source of data.

NOAA HRD-How many tropical cyclones have there been each year in the Atlantic basin? What years were the greatest and fewest seen?