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Requirements for Development
>> Deep, warm water above 78 degrees
Fahrenheit
>> System must be removed some distance from the Equator
>> Little to no directional or speed wind shear present in the troposphere
The most likely areas for hurricane
development can be seen in the map below. The Atlantic hurricane season
peaks in September, with most storms occurring in August, September and October.
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Figure 1:
The map above shows where the majority of the world's tropical cyclones
form. The numbers found in each area represent an average of storms
that form there per year. The red arrows show the most likely storm
tracks. |
Stages of Development
>> Tropical Disturbance
A tropical disturbance is a
disorganized cluster of thunderstorms that generally moves from the east toward the west
along the Intertropical Convergence Zone. The latent heat release from the
thunderstorms (condensation process) within this complex causes the pressure at the surface to lower.
The lower pressure causes the winds to converge toward the tropical disturbance.
The converging winds work to feed more thunderstorms, which release more latent
heat into the system and further reduce its surface pressure.
>> Tropical Depression
A tropical depression is an organized
thunderstorm cluster with a center of low pressure and sustained winds
circulating about the center at 23 miles per hour or more. During this
stage of development latent heat release increases as more thunderstorms form.
This, in turn, causes the upper-level pressure to increase as divergence aloft
increases. At the surface, the pressure drops and low-level convergence
increases. This complex of thunderstorms becomes a warm core system that
feeds off of the warm ocean waters found below it.
>> Tropical Storm
The tropical depression continues to
develop into a tropical storm through the positive feedback mentioned above as
long as the requirements for development are met. This system becomes a
tropical storm once the sustained winds reach 39 miles per hour or greater.
At this point, the tropical system receives a name.
>> Hurricane/Typhoon/Tropical Cyclone
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Figure 2:
The image above shows the basic structure of a hurricane. Notice the
presence of an eye, formed by the sinking air in the center of the storm. |
A hurricane, typhoon or tropical
cyclone is the last stage of development in a tropical system. This type
of storm forms when positive feedback continues and all of the
requirements for development are still met. A tropical system graduates to
hurricane status once the sustained winds reach 74 miles per hour or greater.
It is at this time when many hurricanes will form a region of calm weather at
their centers, called an eye. The structure of a typical hurricane can be
seen to the right in Figure 2.
The strength of hurricanes, typhoons
and tropical cyclones is measured against the Saffir-Simpson Scale. This
scale divides hurricanes into five different categories based on the maximum
sustained wind speed found within the tropical system. A category 1
hurricane is considered weak, while a category 5 hurricane is considered to be
very strong. An interactive version of the Saffir-Simpson Scale can be
found below in Figure 3.
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Figure 3:
The demonstration above gives further insight into the Saffir-Simpson Scale
of hurricane intensity. |
Weakening and Demise
>> Colder water
A tropical system will generally begin to weaken
if it encounters water colder than the 78 degrees outlined in the requirements
for development. The introduction of cold water, either through the
movement of the storm poleward or the mixing of cooler water from below, reduces the
amount of water vapor the system has to work with. A reduction in water
vapor works to slowly halt the development of the thunderstorms within the
complex.
>> Landfall
A tropical system will usually weaken
significantly once landfall occurs. This is because the
amount of water vapor available to the storm is greatly reduced once it is over
land. In addition to the lack of water vapor, the increase in friction
over the land works to disrupt the low-level circulation of the tropical system.
>> Wind shear
Significant wind shear can also have a
detrimental impact on a tropical system. When there is a large amount of
wind shear present, the tops of thunderstorms are ripped off and separated from
their bottoms and their region of latent heat release. This causes
thunderstorm development and maintenance to be severely hindered.
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