The solid line represents a. steady state conditiorie That is, at any later 

 tinBj if neither the \n:nd speed nor the fetch length changes, the same 

 distribution of wave heights and periods' will be founds, A length of fetch 

 for which a given mnd speed Tdll develop this steady-state condition regard- 

 less of hoxiT long the wind has been blo-&rf.ng is kno\^rx as a minirro in. fetch (F ) . 

 Similarly the length of tisB it takes a given T-iind bloi-jing over a definite' 

 fetch length to cause a steady state condition is knowi sb the minimum 

 durat ion Ov)e 



7o In Figoire 1 the dashed line distribution shows that the portion 

 of the fetch from C- to F, has reached a staady state condition. If the wind 

 has been blotrlng for t hoixrs, the inin3.iijum dur-ation for tliat portion of the 

 fetch from to F is t hears « Conversely the minimum fetch for- a mirxiraum 

 duration of t hours would be F^ in lengtho Notej in this ease, the measured 

 fetch (F) is longer than the mxnimum fetch (F ) corresponding to the minimum, 

 duration to 



80 The least time taken to develop the distrfibutidn shown by the solid 

 line 3 would be the m-lninram dixr'ation corresponding to a minimum fetch of 

 length Fo Note that the measured duration of xfind may be longer than this 

 minimum durations 



9 c In actuality, the distributioris shown in Slgure 1 ai'e simplified 

 distributions., since a specti^om of wave heights and pei-iods is generated in 

 a fete ho These siiiplified distributions refer to what ai'e kn.ovm as 

 £iS4£i:£f;SiJL'-''-Z££s ^ statistical term which is used to describe the average 

 of the heights .and periods of the highest one-third of tirie waves in a group, 



lOo After leaving t'ba fefccjh. waves tr'avel to a point soub distance away- 

 a coast for exa!ip3.e "— ■S'id.th s-peeds proportional to theix" periods,, ('la^2 T 

 knotSj where T is in seconds). In this dec.?y distance D (nautical, miles )^ 

 the longer peri-od waves x-rill mrjve faster than^, and consiequently will arrive 

 at the end of the decay distance before those T,d.th shorter periods » An 

 observer at this point would see a g.roup oiT waves whose significant period, 

 called T_^, is longex- than T.,, therefore the significant peidod o.f waves at 

 the head of the fetch 3eeras''to increase, 



11, Tlie heights of waves do decrease afte;r leaving a fetch., and at 

 the end of the decay distance, tbs observed signific ant wave height EL 

 x*ill be sm.aller thaii P^,, the significant wa^("e height at the head of the 

 fetch, 



12, Tlie ti:rs5 for the gi-n^up of waves to travel over the decay distance 

 is approx5.iriataly the ratio of D to the gi'oup velocity of waves liidth a period 

 Tj.^ and is known as the travel tiire t, (tw in houj-s = B/tLo^ST^), 



13.. Foreciasting Ifethods and P^'^o^fdures, - The basic data to be taken 

 from synoptic chalets in making forecasts consist of two measureiTients of 

 distance—decay distance and fetch length—and one of wind speed. Although 

 xiraves travel along great circlesp the distances to be measur-ed ace usually 

 3mal.l enough to be adequately represented, by straight lines. Therefore the 



