OCEAN WAVES AS A METEOROLOGICAL TOOL 
tained become increasingly uncertain as the distance 
between storm and observer becomes greater. 
For a discussion of the assumptions underlying the 
foregoing equations the reader is referred to the original 
paper [16]. The advantage of this method is that for 
02 
00 
42 4 A) 8 1.0 12 14 1.6 
Be 
Fre. 1.—Plot of equations (5) and (6) relating wave steep- 
ness 6 (height/length), wave age 8 (wave velocity/wind veloc- 
ity), wind speed U, and wind duration ¢. Subseript “‘F'”’ refers 
to values at end of fetch. (From Sverdrup and Munk [16], 
Figs. 5 and 7.) 
shipboard observations, or for exposed and steep coasts, 
it permits a rapid determination of the storm distance 
and a rough estimate of the storm intensity without the 
need of special wave-measuring equipment. In this sense 
the method was first used during World War II by 
Capt. D. F. Leipper, AAF, as an aid in forecasting the 
arrival of intense storms in the Aleutian area. In many 
instances the arrival of heavy swell preceded all other 
indications of the storm. From a climatological point of 
view this method can be applied by “mapping” on an 
Hy,Tp-diagram typical meteorological situations asso- 
ciated with wave conditions in certain areas (Fig. 3). 
The principal disadvantage of this method is the 
slow velocity of the significant waves. As a result the 
information concerning storm location and intensity 
may be several days old before the waves reach shore. 
The Frequency-Spectrum Method Applied to Fore- 
runners of Swell 
The limitations outlined above can be overcome in 
part by dealing with the low, long, imperceptible waves 
which precede the visible swell. Since these waves are 
only a few inches high it is not possible to apply simple, 
visual methods of observation, but the advantages in- 
volved often appear to justify the use of special instru- 
mental techniques. 
1091 
In the first place, the forerunners travel considerably 
faster than the visible swell. On the basis of instrumen- 
tal observations now available [1] the period of the 
forerunners may reach 25 sec, compared to 12-13 sec 
40 las 5 T 
A LEGEND 
ms N Distance from which swell comes (Naut, mies) 
35 im = Travel time (Hours) 
Wind velocity in generating orea (Knots) 
ast 
a 
fe) 
HEIGHT OF SWELL,Hp,IN FEET 
7 8 9 lo =I 2 & tM © 6 i iB I &) 
PERIOD OF SWELL,Tp,IN SECONDS 
Slee WIND VELOGITY,U,IN KNOTS 
@ 
im >30 
rT) oO 
iL ae 
2 - 220 
B p= 
=: z 10 
4 8 
— = 
tw bs al 
= 2° 10 20 30 40 50 
w 
io} 
iz 
x= 
© 
Ww 
as 
lo Il IB WW Kf Us 6 ir fe dp 
PERIOD OF SWELL,T),IN SECONDS 
Fra. 2.—Distance from which swell comes, travel time, and 
wind velocity in generating area as functions of observed 
height and period of swell. The upper figure is drawn for a 
long-lived storm, the lower figure for a short-lived storm. The 
assumed relation between wind speed and duration for these 
two cases is Shown in the inset. (From Sverdrup and Munk {16].) 
as a typical maximum period for the swell. The corre- 
sponding group velocities with which the disturbance 
created by a storm is effectively propagated by surface 
wave motion are of the order of 900 nautical miles per 
Tasie I. Sampie CaLcuLaTIONS ror A Storm or UNusuan 
Duration (Case A) AND A SHORT-LIVED Storm (Case B) 
Hy Ty | Storm duration D | ty | U 
(ft) (sec) (see inset, Fig. 2) | (naut. mi) (hr) | (knots) 
Case A 700 38 30 
6 12 
Case B 825 45 45 
day for the forerunners, in constrast with 400 miles 
per day for the swell. 
A second advantage of the “frequency-spectrum 
method” over the “height-period method” is that travel 
time and storm distance are uniquely determined from 
a knowledge of wave periods. The need for measuring 
wave height is eliminated. 
Instrumentation. Shore-based instruments for record- 
ing ocean waves have been in existence for many years. 
