Chapter 17, Tropical Weather 17-8
significantly to the 300+ in of annual rainfall over the higher terrain of Maui and the big island of Hawaii.
Other mountainous areas of the Tropics are also among the wettest spots on Earth.
Figure 17-4. A TUTT Moves Eastward Across the Hawaiian Islands
17.3.3 Tropical Wave
Tropical waves (also called easterly waves) are common tropical weather disturbances, normally occurring
in the trade wind belt. In the Northern Hemisphere, they usually develop in the southeastern perimeter of
subtropical high-pressure systems. They travel from east to west around the southern fringes of these highs
in the prevailing easterly circulation of the Tropics. Surface winds in advance of a wave are somewhat more
northerly than the usual trade wind direction. As shown i n Figure 17-5, as the wave approaches, pressure
falls; as it passes, surface wind shifts to the east -southeast or southeast. The typical wave is preceded by
very good weather but followed by extensive cloudiness (see Figure 17-6), and often by rain and
thunderstorms. The weather activity is roughly in a north-south line.
Tropical waves occur in all seasons but are more frequent and stronger during summer and early autumn.
Pacific waves frequently affect Hawaii; Atlantic waves occasionally move into the Gulf of America,
reaching the coast of the United States.
Chapter 17, Tropical Weather 17-9
Note that winds shift generally from northeasterly to southeasterly. The wave moves
toward the west and is often preceded by good weather and followed by extensive
cloudiness and precipitation.
Figure 17-5. A Northern Hemisphere Easterly Wave Progressing from A–B
Figure 17-6. Vertical Cross-Section Along Line A–B in Figure 17-5
17.3.4 West African Disturbance Line (WADL)
On occasion, a line of convection similar to a squall line moves westward off the continent at tropical
latitudes into the oceanic trade winds. In the North Atlantic, this is known as the West African Disturbance
Line (WADL). A WADL can move faster than easterly waves at 20 to 40 mph. Some WADLs eventually
develop into tropical storms or hurricanes.
Chapter 17, Tropical Weather 17-10
17.3.5 Tropical Cyclones
“Tropical cyclone” is a general term for any low that originates over tropical oceans. Tropical cyclones are
classified according to their intensity based on the average wind speeds. Wind gusts in these storms may
be as much as 50 percent higher than the average wind speeds. Over the north Atlantic and northeast Pacific
Oceans, tropical cyclone classifications are:
1. Tropical depression—sustained winds up to 34 kt (64 km/h).
2. Tropical storm—sustained winds of 35 to 64 kt (65 to 119 km/h).
3. Hurricane—sustained winds of at least 65 kt (120 km/h) or more.
The NWS NHC and the CPHC use a 1-minute average wind speed for the above classifications.
In other regions of the world, a 10-minute average wind speed is used along with different terms. Tropical
cyclones meeting hurricane strength in the northwest Pacific Ocean are called “typhoons,” “severe tropical
cyclones” in the southwest Pacific and southeast Indian Oceans (e.g., near Australia), “severe cyclonic
storms” in the north Indian Ocean, and just “tropical cyclone” in the southwest Indian Ocean. The term
“super typhoon” is used if the maximum sustained winds are at least 130 kt (241 km/h).
17.3.5.1 Development
The prerequisites for tropical cyclone development are optimum sea surface temperature under low -level
convergence and cyclonic wind shear. Favored breeding grounds are shear lines, TUTTs, tropical waves,
and lines of convection in low latitudes moving from the continent to the tropical ocean (e.g., WADL).
The low -level convergence associated with these systems by itself will not support development of a
tropical cyclone. The system must also have horizontal outflow (divergence) at high tropospheric levels.
This combination creates a chimney, in which air is forced upward causing clouds and precipitation.
Condensation releases large quantities of latent heat , which raises the temperature of the system and
accelerates the upward motion. The rise in temperature lowers the surfac e pressure, which increases
low-level convergence. This draws more moisture -laden air into the system. When these chain -reaction
events continue, a huge vortex is generated, which may culminate in hurricane-force winds.
Figure 17-7 shows regions of the world where tropical cyclones frequently develop. They usually originate
between latitudes 5° and 20°. Tropical cyclones are unlikely within five degrees of the Equator because the
Coriolis force is so small near the Equator that it will not turn the winds enough for them to flow around a
low-pressure area. Winds flow directly into an equatorial low and rapidly fill it.
Chapter 17, Tropical Weather 17-11
This map is based on all storm tracks available from the International Best Track Archive for Climate Stewardship
(IBTrACS), a global inventory of tropical cyclones, through 2008. The accumulation of tracks reveals several details
of hurricane climatology, such as where the most severe storms form and the large -scale atmospheric patterns that
influence the track of hurricanes. (Note: See Table 17-1 for wind strength associate d with each scale on the
Saffir-Simpson Hurricane Wind Scale.)
Figure 17-7. The Tracks of Nearly 150 Years of Tropical Cyclones and Their Strength Weave Across the Globe
17.3.5.2 Movement
Tropical cyclones in the Northern Hemisphere usually move in a direction between west and northwest
while in low latitudes. As these storms move toward the mid -latitudes, they come under the influence of
the prevailing westerlies. At this time , the storms are under the influence of two wind systems: the trade
winds at low levels and prevailing westerlies aloft. Thus, a storm may move very erratically, and may even
reverse course or circle. Finally, the prevailing westerlies gain control, and the storm recur ves toward the
north, then to the northeast, and finally to the east -northeast. By this t ime, the storm is well into
mid-latitudes.
17.3.5.3 Decay
As the storm curves toward the north or east (Northern Hemisphere), it usually begins to lose its tropical
characteristics and acquires characteristics of lows in middle latitudes. Cooler air flowing into the storm
gradually weakens it. If the storm tracks along a coastline or over the open sea, it gives up slowly, carrying
its fury to areas far removed from the Tropics. However, if the storm moves well inland, it loses its moisture
source and weakens from starvation and increased surface friction, usually after leaving a trail of destruction
and flooding.
Chapter 17, Tropical Weather 17-12
When a storm takes on middle latitude characteristics, it is said to be extratropical, meaning “outside the
Tropics.” Tropical cyclones produce weather conditions that differ somewhat from those produced by their
higher latitude cousins and invite investigation.
17.3.5.4 Weather in a Tropical Depression
While in its initial developing stage, the cyclone is characterized by a circular area of broken -to-overcast
clouds in multiple layers. Embedded in these clouds are numerous showers and thunderstorms. Rain shower
and thunderstorm coverage varies from scattered to almost solid. The diameter of the cloud pattern varies
from less than 100 mi in small systems to well over 200 mi in large ones.
17.3.5.5 Weather in Tropical Storms and Hurricanes
As cyclonic flow increases, the thunderstorms and rain showers form into broken or solid lines, paralleling
the wind flow that is spiraling into the center of the storm. These lines are the spiral rain bands frequently
seen on radar. These rain bands conti nually change as they rotate around the storm. Rainfall in the rain
bands is very heavy, reducing ceiling and visibility to near zero. Winds are usually very strong and gusty
and, consequently, generate violent turbulence. Between the rain bands, ceilings and visibilities are
somewhat better, and turbulence generally is less intense.
Most tropical cyclones that form eyes do so within 48 hours of the cyclone reaching tropical storm strength.
In the eye, skies are free of turbulent cloudiness, and wind is comparatively light. The average diameter of
the eye is between 15 mi and 20 mi but sometimes is as small as 7 mi and rarely is more than 30 mi in
diameter. Surrounding the eye is a wall of cloud s that may extend above 50,000 ft. This wall of clouds
contains deluging rain and the strongest winds of the storm. Maximum wind speeds of 175 kt have been
recorded in some storms. See Figure 17-8 and Figure 17-9, which contain a radar display and satellite
photograph of a mature hurricane, respectively. Note the spiral rain bands and the circular eye. Notice the
similarity between these two figures.
Table 17-1 identifies the wind speed and characteristic house damage for each level on the Saffir-Simpson
Hurricane Wind Scale.
Chapter 17, Tropical Weather 17-13
Figure 17-8. Radar Image of Hurricane Katrina Observed at New Orleans, Louisiana, on August 29, 2005
Figure 17-9. Hurricane Andrew Observed by Satellite in 1992
Chapter 17, Tropical Weather 17-14
Table 17-1. Wind Speed and Characteristic House Damage for the Saffir-Simpson Hurricane Wind Scale
Saffir-Simpson
Hurricane
Wind Scale
Wind Speed Characteristic House Damage
≥157 mph
≥137 kt
≥252 km/h
Almost complete destruction of all mobile homes will occur,
regardless of age or construction. A high percentage of frame
homes will be destroyed, with total roof failure and wall collapse.
Extensive damage to roof covers, windows, and doors will occur.
Large amounts of windborne debris will be lofted into the air.
Windborne debris damage will occur to nearly all unprotected
windows and many protected windows.
130–156 mph
113–136 kt
209–251 km/h
Nearly all older (pre-1994 construction) mobile homes will be
destroyed. A high percentage of newer mobile homes also will be
destroyed. Poorly constructed frame homes can sustain complete
collapse of all walls as well as the loss of the roof structure.
Well-built homes also can sustain severe damage with loss of most
of the roof structure and/or some exterior walls. Extensive damage
to roof coverings, windows, and doors will occur. Large amounts
of windborne debris will be lofted into the air. Windborne debris
damage will break most unprotected windows and penetrate some
protected windows.
111–129 mph
96–112 kt
178–208 km/h
Nearly all older (pre-1994 construction) mobile homes will be
destroyed. Newer mobile homes will sustain severe damage with
potential for complete roof failure and wall collapse. Poorly
constructed frame homes can be destroyed by the removal of the
roof and exterior walls. Unprotected windows will be broken by
flying debris. Well-built frame homes can experience major
damage involving the removal of roof decking and gable ends.
96–110 mph
83–95 kt
154–177 km/h
Older (mainly pre-1994 construction) mobile homes have a very
high chance of being destroyed, and the flying debris generated
can shred nearby mobile homes. Newer mobile homes can also be
destroyed. Poorly constructed frame homes have a high chance of
having their roof structures removed, especially if they are not
anchored properly. Unprotected windows will have a high
probability of being broken by flying debris. Well-constructed
frame homes could sustain major roof and siding damage. Failure
of aluminum, screened-in, swimming pool enclosures will be
common.
74–95 mph
64–82 kt
119–153 km/h
Older (mainly pre-1994 construction) mobile homes could be
destroyed, especially if they are not anchored properly as they tend
to shift or roll off their foundations. Newer mobile homes that are
anchored properly can sustain damage involving the removal of
shingle or metal roof coverings, and loss of vinyl siding, as well as
damage to carports, sunrooms, or lanais. Some poorly constructed
frame homes can experience major damage, involving loss of the
roof covering and damage to gable ends, as well as the removal of
porch coverings and awnings. Unprotected windows may break if
struck by flying debris. Masonry chimneys can be toppled.
Well-constructed frame homes could have damage to roof
shingles, vinyl siding, soffit panels, and gutters. Failure of
aluminum, screened-in, swimming pool enclosures can occur.