|HOW DOES A HURRICANE FORM?
METEOROLOGIST JEFF HABY
Regions of thunderstorms can develop in the
tropics. There are several ways these thunderstorms can
develop. They include a complex of thunderstorms that developed on land moving over the water by
the upper level winds, low level convergence that allows thunderstorms to break the
capping inversion and
areas of lower pressure.
Most of the time the complex of storms will die away over time. However, at other times the
environment has more in store for the complex of thunderstorms. For the complex of thunderstorms
to persist over time (i.e. days) it helps to have the following conditions: weak upper level winds,
thunderstorms able to moisten the troposphere, warm sea surface temperatures, capping inversion
not too strong and persistent low level convergence.
If the upper level winds are weak it allows the thunderstorms to continually moisten the local environment.
Weak upper level winds also allow the thunderstorms to build heat energy within a local environment due
to condensation release. Over time this creates an environment that is warmer and more moist over time. This
helps initiate even more thunderstorms.
The earth rotates counterclockwise when viewed from the North Pole. This rotation influences weather
patterns. This rotation imparted on weather systems is termed earth vorticity or
Coriolis. When an
area of thunderstorms in the tropics persists and the thunderstorms become more numerous the surface pressure
will lower within this complex of storms. This increases the convergence into the region. As the air
converges toward this developing low the Coriolis influence gives the organizing storms a cyclonic
rotation. A tropical depression has formed.
Heat continues to build in the system as numerous storms release
latent heat. The convergence into the
system confines this heat. What is created is a warm core low pressure system. The air enters the
system in the low levels and exits the system in the upper levels. The warm core creates an expanded
troposphere in the vertical. This creates a
pressure gradient in the upper troposphere than flows
opposite in direction to that near the surface. While air converges into the tropical
low cyclonically near the surface it
exits in an anticyclonic fashion high aloft. If the upper level winds stay weak it will allow the
core of the storm to retain heat and allows the balance to remain in place between cyclonic convergence
near the surface and anticyclonic divergence aloft. The environment is now well moistened and the
system of storms is rotating. The surface winds are now up to 39 mph. A tropical storm has formed.
With a continuous supply of warm ocean water, weak upper level winds and a warm / moist core the
tropical system will continue to strengthen. The
pressure continues to fall at the center and
the system has a very organized cyclonic rotation. An eye starts to develop as the rotation
rate and winds just near the center continue to increase. The surface winds are now up to 74 mph.
A hurricane has formed.
Take a glass of water and stir it vigorous. You will notice the water level at the center falls.
It falls since mass is being displaced from the center and moved toward the edges. The surface
pressure in a hurricane is a minimum value at the center of rotation. This is where the eye
forms. The violent rotation of the air is causing air to evacuate from the center like the
water mass evacuating when stirring the water in the glass. The eye, just like the water in the
stirring glass, will have a sloped profile. It is often described as a stadium effect.
Hurricanes can produce
tornadoes when they make landfall. The region of the hurricane where the
winds are blowing from the ocean tends to be the
strongest portion of the storm.
Eventually the hurricane MUST weaken. The following are several reasons this can happen: increase
of upper level wind shear, movement over land, movement over cooler water, drier air
entraining into the storm, temporary breakdown in eye and eyewall structure (eyewall replacement cycle), and
the storm upwelling cooler water.
Hurricanes are the most violent storms on earth when size is taken into consideration. They will
continue to be feared by many and they will continue to create awe in those who watch them.
The ultimate weather education website: http://www.theweatherprediction.com/