The National Hurricane Center in Miami, A hurricane of tropical origin needs several
Florida, tracks hurricanes in the North ingredients to form: (1) a westerly moving
Atlantic and the Pacific east of 180°. In the tropical weather system, called a wave or
event of a hurricane, a Tropical Cyclone Ad- trough; (2) a seawater temperature of 80ºF
visory message is issued every six hours, with or greater; and, (3) during the early stages
intermediate bulletins as needed. Two levels of development, moderate-speed, low- and
of alerts are used. upper-level winds that change very little
A HURRICANE WATCH is a preliminary in speed and direction with changes in
alert that a hurricane may threaten a speci- height—that is, winds with low sheer.
fied portion of the coast. A HURRICANE The first stage in the development of this
WARNING indicates that hurricane condi- type of hurricane is a tropical wave. A tropi-
tions are expected within 24 hours for the cal wave is a trough at low latitudes. Unlike
specified area. The National Hurricane mid-latitude depressions—which form
Center also issues Marine Advisories that from the top down starting with upper-level
provide forecast positions, movements, and troughs—tropical hurricanes develop from
intensities for up to 72 hours ahead; these the bottom up, starting with lower-level
include probabilities of hurricane strikes for troughs. A trough can grow over warm ocean
coastal locations and offshore coordinates. water. When the surface water temperature
A guide to the Saffir-Simpson Hurricane is 80°F or higher, the moisture-laden air im-
scale, used to categorize storms by strength, mediately above it begins to rise rapidly. As
is located on page 801. warm, moist air rises, it cools, causing the
Storm surges, a considerable rise or fall in moisture to condense and release heat. This
the level of the sea along a particular coast, released heat warms the surrounding air
may result from strong winds and sharp so that it continues to rise. Supported by a
changes in barometric pressure. In cases steady influx of moist, warm, surface air, the
where the water level is raised, higher waves system grows vertically.
can form with greater depth; this condi- When high-level winds draw off, or ex-
tion can be destructive to low-lying regions, haust, this rising air, the atmospheric pres-
r
particularly at high stages of tide. This type sure at the surface drops, strengthening the
of wave occurs especially in coastal regions wave. If these processes continue, and if the
bordering on shallow waters that are subject pressure continues to drop, the Coriolis ef-
e
athe
to tropical storms. fect, even though it’s weak at low latitudes,
W
Mariners should anticipate drawbridge clo- will eventually induce a spin into the system,
sures by listening to weather broadcasts on particularly as the system moves toward
hurricane conditions and seek passage through mid-latitudes. When the system develops
r
bridges well in advance of the arrival of gale- a clear circulation with sustained surface
force winds. In the U.S., drawbridges are winds of 25 to 33 knots, it is classified as
authorized to close upon the approach of gale- a tropical depression (low-pressure area).
e
athe
force winds of 34 knots or greater. Locks and When further strengthening occurs and
navigation structures may be closed on short sustained surface winds exceed 34 knots, the
notice well in advance of an actual storm; system is designated a tropical storm and
seek safe harbor early. is assigned a name. When sustained winds
Formation
exceed 63 knots, the storm becomes a hurri-
cane. Fortunately, only a fraction of tropical
Most hurricanes are created when the at-
waves become hurricanes each season.
r
standing W
mosphere becomes burdened with heat at
At the same time it grows vertically, a tropi-
the equator and the earth’s normal thermal
nde
cal trough or wave moves westward at speeds
circulation system fails to move this heat to
U
averaging 10 to 20 knots. During peak season
polar regions at a rate sufficient to keep the
in the North Atlantic, tropical waves are
earth’s atmosphere in balance. Hurricanes
often seen crossing the ocean in succession
are Mother Nature’s overload protection
from east to west, separated by four to eight
and, as such, expedite the movement of heat.
degrees of longitude.
Hurricanes are so efficient at extracting heat
If you are in the Caribbean during hurri-
from the surface that the water temperature
cane season, pay careful attention to the wind
in a hurricane’s wake usually has been re-
direction. Any slow and steady change from
duced by several degrees.
797