Formation Areas

By definition a hurricane is a warm core cyclone with peak sustained wind speeds exceeding 33 m/s (or 74 mph). A tropical storm is a warm core cyclone with sustained wind speeds greater than 17.5 m/s (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 (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 climatological purposes and below is a listing of climatological averages for each of these basins.

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 occurrence and the month(s) of peak occurrence.

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.