54 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



first is a decrease in heij^ht. Finally the Avave becomes a relatively 

 low, undulating moA-ement of the sea surface, known as a swell. In 

 recording observations in accordance with the following scales, swell 

 is wave motion produced by winds at some distance from the point 

 of observation. 



The swell is distinguished by two features; first, its relatively 

 smooth, undulating form, without the steep and ragged crests char- 

 acteristic of waves actively driven by local winds and, second, the 

 movement of winds and waves in different directions, indicating 

 that the waves have been formed elsewhere by winds from another 

 quarter. Ordinarily, then, waves which are moving with the wind 

 constitute the "sea", while a relatively low, undulating sea surface, 

 Avith motion in a direction different from the local Avind, is the 

 "swell." 



These definitions are not entirely satisfactory. Usually, the ocean 

 surface is disturbed by both forms of Avave motion, Avith the SAvell 

 from distant Avinds crossing the local sea. The combined effect is 

 the "sea", Avhile the Avell-defined ridges of Avaves moving in a different 

 direction from the local wind are the "sAvells." 



State of sea. — The folloAving scale should be used in classifying the 

 character of the sea disturbance. In recording obserA'ations in ac- 

 cordance Avith this scale, "sea" may be considered to be composed of 

 SAvells, combined Avith AvaAes produced by the Avinds at the place 

 of observation. 



State of sea 



The scale of sea disturbance is approximate, based roughly on the 

 obserA'er's judgment as to the height of Avaves. Accurate estimates 

 of Avave height are not easily made on shipboard. For large Avaves, 

 Avith the ship on an even keel in the trough of the wave, the ob- 

 server's height aboA^e the Avater line Avhere he sees the crest of the 

 Avave coincide Avith the horizon, is taken as the height of the wave. 



Character and direction of sivell. — The character and direction of 

 the SAvell should be entered in the appropriate columns of Form 

 1210A according: to the f olloAving scales : 



Character of swell 



