Ifv you hove ever 



hit the beach 



with an amphibious 



force in a high surf 



or bounced 



in a rough seaplane 



landing or only 



got soaked 



in a liberty boat 



in a "fresh breeze," 



you know the value of 



accurate sea and 



swell information. 



What you may not 



know is how 



that information 



is collected and how 



you can help 



collect it. 



Although men 



have been soiling 



and observing the sea 



for centuries, 



there ore still 



mony ports of the world 



where reports 



of wove conditions 



ore entirely locking, 



or at best^ 



ore scanty for 



certain seasons. 



Strangely enough, 



even in heavily 



travelled waters, 



there is need 



for more 



observations under 



the different kinds 



of wind and weather 



conditions found 



in those waters. 



During World War II, 



our knowledge 



of waves ond swell 



increased greatly, 



particularly 



as regards 



the mathematical 



theory of waves. 



This theory now 



needs to be checked 



by on-the-spot 



^observations. 



Thus, because of 



both the practical 



and theoretical aspects 



it is important 



for you to moke and 



record observations 



of sea ond swell 



whenever possible. 



WAVE 



I f I CHARACTERISTICS 



Before you make your observation, you should know 

 something about wave characteristics. 

 A wave is described by 



its length — 



the distance from crest to crest, 



its height — 



the vertical distance from trough 



to crest, 



its period — 



the time between two consecutive 



crests, 



and its velocity — 



the speed with which it travels. 



Some very useful theoretical relationships, which have 

 been checked by observation, exist between the 

 length, the velocity, and the period of waves in deep 

 water. If you know the wave length in feet, or the 

 velocity in knots, or the period in seconds, you con 

 figure out the approximate values of the other two. 



Velocity «= 1.3 VLength « 3 Period. 



Length «= 0.6 (Velocity)^ « 5 (Period)". 



Period « 0.4 V-ength « 0.3 Velocity. 

 Unlike the length and velocity, the wave height bears 

 no definite relationship to the wave period. 



