2008 REED’s East Coast Edition - Weather
During the “early warning” stage, there is a rule, you should always navigate to increase
usually a slight rise in barometric pressure, your distance from the center of a hurricane,
followed by continuously falling pressure. taking into account the storm’s track and for-
As the hurricane center nears, the baro- ward speed. And if you have a choice, avoid the
metric pressure falls rapidly. The wind in- right, or “dangerous,” semicircle.
creases in speed and arrives in heavy squalls. Detection and Avoidance: Prior to the
Changes in wind direction accelerate. When emergence of satellite imagery and the use
a center is close, rain is usually continuous, of sophisticated aircraft reconnaissance and
accompanied by furious gusts of wind. The computer-based numerical modeling, there
air is thick with rain and spume. Visibility is was often little warning of an approaching
low; you can barely see even nearby objects. hurricane. Frequently, mariners only realized a
As the hurricane’s center recedes from you, hurricane was imminent when the barometric
barometric pressure rises and winds begin pressure began to drop radically, a large swell
to subside. Seas also subside, but the wind appeared, and clouds covered the sky. Some-
decreases sooner. A fully risen sea can often times these events occurred just a day before a
take days to subside. hurricane passed over an area, particularly if a
Distance from the Center: You can esti- hurricane was not very large in diameter.
mate roughly how far you are from the cen- For years, strategies for dealing with hur-
ter of a hurricane using barometric pressure ricanes were based on the assumption that a
and wind velocity. If the barometric pressure vessel might actually encounter a hurricane at
falls slowly and wind speed increases gradu- sea. Using vectors, a navigator would plot the
ally, then the center is distant. When the ba- course that would provide the widest berth
rometer falls rapidly and the wind increases possible under the circumstances. With today’s
rapidly, the center is near. This is confirmed modern tools, navigators have a much better
if the winds blow from the same direction as chance of avoiding hurricanes altogether.
the increasing waves. Tracking Tropical Weather Systems:
Bearing of the Center. When you face into Weather satellites use visible and infrared sen-
the wind, the center of a Northern Hemi- sors to track tropical waves by locating their
sphere hurricane will be approximately 115º dense and vertically developed cumulus and
to your right. In general, if you are facing cumulonimbus clouds. These clouds can have
into the wind, the wind will back to your individual diameters of one to 50 miles and
left if the hurricane center is moving toward cloud clusters with diameters ranging from
your left. If the wind veers to your right, the 50 to 500 miles. These cloud cells and clusters
center is moving toward your right. If you often do not form a consistent or continuous
are in the path of the center, the wind will pattern because they are a product of uneven
remain constant from the same direction. heating of the earth’s equatorial regions,
Hurricane Avoidance
which is due to seasonal, geographic, and cli-
matologically variations. With uneven cloud
When plotting a hurricane, it is helpful to
formation, tropical weather can fluctuate rap-
draw an imaginary line through the storm’s
idly. The transition from calm to squalls, and
center in the direction that the storm is mov-
showers to clear skies, often occurs quickly
ing. This line, often called the storm track
and with little advanced warning.
or axis, divides the storm into two halves or
In the tropics, cumulus and cumulonim-
semicircles. In the Northern Hemisphere, the
bus clouds tend to align themselves with the
right half is called the “dangerous semicircle”
axis of a low-pressure trough. The National
because that is the area of the strongest winds.
Hurricane Center uses satellite imagery to
If the wind speed of a given hurricane was, for
produce a Tropical Surface Analysis (TSA)
instance, 110 knots, and its speed of forward
chart, which is a streamline chart. A TSA
movement was 20 knots, the actual wind speed
chart shows clouds and wind resulting from
on its right semicircle would be 130 knots.
horizontal convergence and ascending air.
The left half, then, is called the “navigable
The Weather Sources section at the end of
semicircle” because the wind strengths tend to
this chapter lists many sources of tropical
be lower. In our example, the wind speed in the
advisories and forecasts, including weather
left semicircle would be subtracted by 20 knots,
charts, radio broadcasts, and web sites.
making an actual wind speed of 90-knots. As
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