2008 REED’s East Coast Edition - Weather
You’ll often see a long line of foam at the regular seas usually traveling in the same di-
intersection of two currents, one of which is rection. A wave period (time between crests)
colder and sinks below the other. Generally, of at least 10 seconds, or a wave length in the
you will find a difference in the water tem- open ocean of at least 500 feet, indicates that
perature on either side of the line. the wind that produced the swell must have
You can identify the boundary of the Gulf been at least gale force. Swells of lesser size
Stream using a thermometer. The bound- can still have originated in a strong storm,
ary at the north and west side of the Stream, but may have decayed in traveling a long dis-
known as the “north wall,” is an area of no- tance from their place of origin. Swells travel
ticeable temperature change. Waters east and outward (away) from the weather system
south of the wall are warm; waters west and that creates them. You can use the direction
north are cold. The difference is most ap- from which swells are coming as a rough
parent near Cape Hatteras, but you can ob- indicator of the location of a tropical depres-
serve it anywhere along the Stream from the sion or mid-latitude low-pressure system
Straits of Florida to the Grand Banks. In ad- relative to your vessel.
dition to its main flow, the Gulf Stream often A change in wind direction of more than
spins off huge warm- and cold-water eddies 30 degrees can produce a new wave train.
that a navigator can play to good advantage, That is, the waves will begin to travel in a
particularly on passages from the East Coast different direction. This can create confused
to Bermuda. Although satellite imagery is seas where the new and old wave trains inter-
probably the best way to locate these eddies, sect. Often you will experience confused seas
local sea-surface temperature readings will when two or more strong weather systems
confirm whether you have found the eddy are close to each other, or during the passage
you are seeking. of a weather front.
Fog: Fog forms at sea when the water
Hurricanes
temperature is equal to or below the dew
point temperature of the air at sea level. If
Most hurricanes, known as cyclones or
the air near the sea surface contains a high
typhoons in the western Pacific and Indian
concentration of salt particles (which attract
Oceans, originate in the tropical oceans of
moisture and encourage condensation), fog
the world. They are severe low-pressure
may form even before the air is cooled to its
areas with clearly defined circulation and
dew point. Fog often forms when a tidal flow
sustained winds of over 63 knots. Barometric
or an ocean current brings colder water to
pressure at a hurricane’s center can be so low
the surface. This colder water may cool the
that the sea surface will rise several feet. This
surface air to its dew point.
rise in sea surface can combine with an ac-
There are a number of rules of thumb
cumulation of wind-driven water to produce
regarding the formation and dissipation of
coastal storm surges of 20 feet or more.
fog. A change in wind direction or speed may
Hurricanes are among the world’s most
cause fog to thin or thicken. If the daytime
powerful weather systems and can produce
sky above a fog bank is clear, fog is more like-
winds over 200 knots. A Class 5 hurricane
ly to dissipate than if the sky is cloudy. Wisps
can dominate more than one million cubic
at the edges and top of shallow fog often in-
miles of the earth’s atmosphere and, in the
dicate clearing. A good indication of shallow
open ocean, can create waves well over 50
fog is your ability to see the masthead of a
feet high. If a small vessel directly encoun-
nearby vessel. Generally, on the open sea, fog
ters a fully developed hurricane, chances of
clears with the arrival of a cold front or with
survival are low.
an increase in the water temperature.
The National Weather Service has desig-
nated June 1 through November 30 as North
Waves and Sea State
Atlantic hurricane season, with the greatest
It’s useful to distinguish between wind hurricane frequency and severity during Au-
waves and swells. The former are generated gust, September, and October. However, hur-
by local winds; swells can be generated by ricanes often occur during other months of
gale or storm winds hundreds or even thou- the hurricane season, and mariners should
sands of miles away, or by moderate winds monitor tropical weather forecasts through-
somewhat closer at hand. Swells are long, out the period.
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