Evening star. [volume] (Washington, D.C.) 1854-1972, 1936-11-15
Big Sea Swells As
Hurricane Warnings
BY FREDERIC J. HASKIN.

The hurricane. that furious enemy Of
man on sea and land, has long been an
object Of study by the Weather Bureau
OF the United States and of similar
agencies of other countries. Various sys-
TENS have been worked out with the
purpose Of giving advance warning to
mariners and to coastal dwellers likely to
feel the force Of the Eumenides. Hur-i-
canes are treacherous and do not in
variably follow the routes charted for
them, so every possible check is sought
to enable meteorologists to follow their
courses.
I. r. Tannehill of the Marine Division
OF the Weather Bureau has been at work
on relatively new method of detect
Ing In advance the onset of these bursts
OF wind, and has reached some conclu-
sions which, it is expected, will prove
valuable. The method utilities observa-
ton OF sea swells. These are somewhat
olnerent from the turbulent, combing
waves OF the sea, tossed into whitecaps
by the immediate presence of the blast.
Seamen and those dwelling near the
sea know that waves go through a Gre
alive evolution. When the first breeze
starts the surface is wrinkled into ripples.
which slowly or rapidly, depending on
the rate at which the wind freshens.
build up into waves. The weather tech
nicians have worked out a number of
formulas concerning the size of waves.
They know. for instance, that the larger
the body Of water the larger the waves
will be. One formula states that waves
will reach Az feet in height in body
OF water miles long, while they will
mount to theoretical IS feet in q sea
300 miles long.
AS the wind falls the crested seas sub
side into long, undulating swells There
are no whitecaps. no combers, but long,
sweeping. rounded mountains of water
with deep troughs between Seafarers
subject to sickness dread the slow, dizzy
ing rise and fall as a ship mounts the
slope of a swell and then slides down
into the trough.
Every one knows how a pebble cast
Into a glaSSy-Surfaced pond will start
a circle Of ripples, which widens and
widens apparently endlessly. Parallels
4IdhC UCCIl WHICH dUOut l1lC
enon. In obedience to much the same
principle. the swells which have been
started by the wind move onward. not
in circular fashion, but straight forward
in the direction in which they were
started.
These swells will 60 onward across the
open sea for hundreds of miles. Cross
winds and flaws and the beginning Of
new gales may kick up new set of
waves, moving in a different direction,
but for long time the original swells
will continue on their way in their
original direction. This makes choppy
sea. but until the new storm. if it proves
to be one. becomes intense the old sWells
will continue to be recognizable.
The importance of these observations
in relation to hurricanes has to do with
the fact that hurricanes move in circles.
A hurricane travels at 100 miles an hour
or more, and if it moved in a straight
path it would far outdistance the waves.
A sea swell moves at the rate OF about
~ miles an hour. speed greater than
that of race horse, and one which
would cause a motorist, in most cities, to
be arrested for speeding. But even AZ
miles an hour could not keep up with a
0-mile-an-hour hurricane.
The hurricane. however, does not move
directly onward at its 100-mile rate.
That is the circular motion. The whole
mass of the storm does move but at o
slower rate of speed. slower than the
speed of the swells it has produced. 1t
is like spinning top. The top spins at
a high rate of speed and, while spinning,
moves along slowly.
The fact of outstanding importance
which has been learned from ocean OBJ
servations is that the waves kicked up
when the hurricane first starts later
become swellS which move onward in
a constant line, even though the hurri-
cane itself moves of when it begins its
circular motion. This means that while
the great circular storms intensity Is
miles away the swells continue onward.
1t is a hare and tortoise affair. The
l00-mile hurricane tears along its cir
cular path, but the steady 42-nile sVells
keep onward.
Owing to areas of high and low pres
sure into which the hurricane may move,
its general course may be detected. This
does happen but not always. II 1t does
the steady sea swells lose most of their
merit as harbingerS, but if there is no
such DETECTION. then they come to shore
as warnings. Their onward surge brings
them in ahead of the spinning storm. A
warning is given. The size of the swells
is of importance, too, in that it indicates
the intensity of the coming storm and
also how far away it probably Is. be-
cause the swells gradually diminish In
size after traversing leagues of sea.
When the heavy swells come in with
out wind 1t is fairly sure sign OF
hurricane because, had they been raised
by a straight gale-a line squan-tht
wind. coming directly on, would have
arrived before the slower swells.
For the last year the Weather Bureau
has been using a special appropriation
to make studies of sea swells as an add
tional means of warning Of hurricanes
Daily reports are received from ships al
sea and Weather Bureau shore stations
and also Coast Guard stations make
observations of swells as they reach the
| land.
A fascinating fact, one Of those
phenomena which it is difficult for any
| but the scientist to comprehend, is that
the cyclone center of storm originating
| In the tropics whirls of to the left II II
| is moving northward in the Northern
| Hemisphere, whereas a storm which
moves southwark into the Southern
Hemisphere goes off to the right. These
| terms refer, as to right and left, to
| theoretical person having his back u
| the wind when it first begins to blow.