Hurricanes, page 2 of 11

By definition a hurricane is a warm core cyclone with peak sustained wind speeds exceeding 33 m s^-1 (or 74 mph). A tropical storm is a warm core cyclone with sustained wind speeds greater than 17.5 m s^-1 (39 mph) and a tropical depression is a cyclonic circulation with at least one closed isobar and wind speeds less than 17.5 m s^-1 (39 mph). It is important to note that it takes more than a warm core and high winds for a storm to be classified as a hurricane. Some northeasters that you've heard of have warm cores near the surface and hurricane force winds. Sometimes there is a fine line between what is a hurricane and what is a mid-latitude storm.

As can be seen from the figure, hurricanes form in almost every ocean basin. There are two notable exceptions; the eastern South Pacific and the South Atlantic. The Eastern South Pacific is home to very cold waters and so does not form hurricanes. The South Atlantic is too close to Africa for storms to form in the belt. Cloud clusters moving off Africa do not have time to spin up in this region.

The formation regions are divided into 7 ocean basins for climatalogical purposes and below is a listing of climatological averages for each of these basins.

Ocean Basin	Number of Tropical Storms per year	Number of hurricanes per year
Western North Pacific	26	16
Eastern North Pacific	17	9
Atlantic	10	5
North Indian	5	3
SouthWest Indian (< 100 degree east longitude)	10	4
Australian - Southeast Indian (100 degree - 142 degree east longitude)	7	3
Australian - SouthWest Pacific. (> 142 degree east longitude)	9	4

As seen in the table, the Western North Pacific is the dominant hurricane forming region in the world, averaging 26 storms a year, with over 30 not uncommon. In addition, a large percentage of these storms eventually become hurricanes. The Eastern Pacific is the next most active region and actually boasts the record for most hurricanes formed per unit area. The Atlantic is actually a rather boring place hosting only 10 storms per year.

In general no storms form within 2.5 degrees of the equator, and roughly 87% of tropical storms that form do so within 20 degrees north or south of the equator. Below is another chart of the basins listing the months of storm occurence and the month(s) of peak occurence.

Ocean Basin	Months	Peak Month(s)
Western North Pacific	Year round	Aug-Sept
Eastern North Pacific	May-Nov	Aug-Sept
Atlantic	May-Nov	Sept.
North Indian	Apr-Jan	May, Nov
SouthWest Indian (< 100 degree east)	Sept-May	Jan, Mar
Australian - Southeast Indian (100 degree - 142 degree east)	Oct-May	Jan, Mar
Australian - SouthWest Pacific. (> 142 degree east)	Oct-May	Feb

It is interesting to note that the Western North Pacific is the only Basin to have recorded a storm in every month of the year. Something else to take note of is the fact that three of the basins (Southwest Indian, North Indian and Australian - Southeast Indian) actually have two seperate and distinguishable peaks of formations in their seasons.

Now that we know when and where many storms form, let us see if we can figure out what is needed for them to form. In the list below check all factors that you think are essential for hurricane formation.

Questions or Comments

Technical: E-mail John Fulton < jdfult@nimbus.met.tamu.edu >
Scientific: E-mail John Fulton < jdfult@nimbus.met.tamu.edu >