Download The Formation Process of Hurricanes

The Formation Process of Hurricanes
Hurricanes form over
1. Tropical water where the winds are light
2. The humidity is high in a deep layer extending up through the
troposphere.
3. The surface water temperature is 80 degrees F or greater over a vast
area.
These conditions occur over the tropical and subtropical North Atlantic
and North Pacific oceans during the summer and early fall, or between
June and November.
A hurricane is composed of an organized mass of thunderstorms. For a
mass of unorganized thunderstorms to develop into a hurricane, the
surface winds must be converging. In the Northern Hemisphere,
converging air spins counterclockwise about an area of surface low
pressure.
Hurricanes develop in tropical regions, usually between 5 and 20
degrees latitude, where the Coriolis effect is zero.
Hurricanes need some kind of “trigger” to start the air converging.
1. Surface winds converge along the ITCZ. Occasionally, when a
tropical wave forms along the ITCZ, an area of low pressure develops,
convection becomes organized, and the system grows into a hurricane.
Note: tropical wave- a weak trough of low pressure which tends to
move East to West. A migratory wavelike disturbance in the
tropical easterlies.
2. Weak convergence occurs on the eastern side of a tropical wave,
where hurricanes have been known to form. In fact, many if not most
Atlantic hurricanes can be traced to tropical waves that form over
Africa.
3. Convergence of surface winds may also occur along a pre-existing
atmospheric disturbance, such as a front that has moved into the
tropics from middle latitudes.
Conditions aloft must be just right as well as at the surface.
In the region of the trade winds, especially near latitude 20 degrees, the air is often sinking in association
with the subtropical High. The sinking air warms and creates an inversion, known as the trade wink
inversion. When the inversion is strong it can inhibit the formation of intense thunderstorms and
hurricanes.
Hurricanes do not form where the upper-level winds are strong, creating strong wind shear. Strong wind
shear tends to disrupt the organized pattern of convection and disperses heat and moisture, which are
necessary for the growth of the storm.
The Development of Hurricanes
Hurricanes derive their energy from the warm, tropical oceans and by
evaporating water from the ocean’s surface. Heat energy is converted
to wind energy when the water vapor condenses inside deep convective
cells.
For a hurricane to form, a cluster of thunderstorms must become
organized around a central area of surface low pressure. This process
is not fully understood.
In a hurricane, heat is taken in near the warm ocean surface, converted
to kinetic energy (energy of motion or wind), and lost at its top
through radiational cooling.
The maximum strength a hurricane can achieve is proportional to the
difference in air temperature between the tropopause and the surface,
and to the potential for evaporation form the sea surface. Therefore,
the warmer the oceans surface, the lower the minimum pressure of the
storm, and the higher the wind speeds.
A Dying Hurricane
Hurricanes weaken rapidly when they travel over colder water and lose
their heat source.
A hurricane can weaken if the layer of warm water beneath the storm is
shallow. The strong winds generate powerful waves that produce
turbulence. The turbulence creates currents that bring to the surface
cooler water from below.
Hurricanes dissipate rapidly when they move over a large landmass. The
energy source is lost and surface winds decrease and blow more directly
into the storm, causing the hurricanes central pressure to rise.
Stages of a Hurricane
Tropical Depression- winds between 20 – 34 knots
Tropical Storm- winds between 35 – 64 knots
Hurricane- winds above 64 knots