HIGH 



B V>^CJ/ C 



Figure II.l. — Boundaries of the fetch for different types of isobars 



line joining the generating area and the locality 

 for which forecasts are to be made. Where the 

 isobars are curved (fig. II.l C) the winds to be 

 considered are those with directions within 45 

 degrees of a line joining the generating area 

 and the locality for which the forecasts are to 

 be made. The reasons for these rules are that 

 (1) the course of the isobars is not exactly 

 known and (2) the wind direction is never 

 exactly steady, so that all the waves in the fetch 

 are not moving in the mean wind direction. 

 Therefore, the swell spreads out when entering 

 areas of calm, and the spreading out will be 

 greater for a region with curved isobars. 



In the tropics the wind direction must be de- 

 termined from observations on board ship or 

 at exposed island stations. 



Wind Speed 



Due to the paucity of synoptic weather data 

 for oceanic areas the observational data that 

 may exist in pertinent areas cannot be used 

 effectively for the computation of sea condi- 

 tions. Its primary purpose, therefore, is to 

 verify wind speeds computed from the isobaric 

 structure on the weather maps. 



A number of factors are involved in the com- 

 putation of the surface wind velocity responsi- 

 ble for sea conditions. These factors are the 

 isobaric spacing and curvature, the mean lati- 

 tude of the fetch, the stability of the air masses 

 involved, and the determination of an average 

 wind velocity that has been instrumental in 

 creating the sea. This wind determination is a 

 weighted resultant of the synoptic wind veloci- 

 ties computed from the maps spanning the time 

 interval under consideration. 



In order to compute the surface wind speed 

 C7„ the influence of the lapse rate upon the wind 

 profile must be considered. If the air tempera- 

 ture © is subtracted from the sea temperature 

 ©s, the resulting difference, called the sea-air 

 temperature difference, is an indication of sta- 

 bility in the lower layers. A relationship be- 

 tween wind ratio (ratio of approximate sur- 

 face wind U's to geostrophic wind Uq) and the 

 sea-air temperature difference was obtained 

 from a statistical study of ocean weather re- 

 ports and analyses. The results are summar- 

 ized in table II.4. 



Table II.4. — Stability factors 



The complete method of obtaining surface 

 winds for use in sea and swell forecasting is as 

 follows : 



a. Determine the geostrophic wind over the 

 fetch, giving particular emphasis to the down- 

 wind end of the fetch. 



b. Estimate the average sea-air temperature 

 difference over the fetch. 



c. Using the sea-air temperature differences 

 in table II.4, determine the wind ratio U'JJo. 

 Multiply the geostrophic wind by the wind ra- 

 tio. The result is the approximate surface 

 wind speed U',. 



16 



