length measurements present no problem^ However delineating the fetch and 

 determining the wind speed are somewhat more conplexo 



Iho The following facts and "rules of thumb" are used to determine the 

 boundaries of the fetch: 



ao Wind blows counter-clockwise about low pressure areas and clock- 

 wise about high pressure areas in the northern hemispliere; the reverse is 

 true in the southern hemisphere; 



bo If there is no spreading of isobars y the fetch front is lo- 

 cated by rotating a straight edge about the forecasting ajr'ea until it cuts 

 the various isobars at a storm front at about 10 to 15 degrees^ 



Co Also, if there is no isobar spread, the fetch rear is located 

 by rotating a straight edge in a similar manner till it cuts the isobars at 

 the rear of a storm at about Ii5 to 60 degrees | and 



do If isobars spread apart at either the front or rear of a 

 storm area, the fetch front or rear is located at the point of spreading* 

 (Note: often a land mass, especially in the Great Lakes, or a meteorological 

 front defines the fetch boundaries) » 



l5o The charts reproduced for the typical forecast (Figures 8 through 

 10) show exauples of all the above rules for fetch locations 



l6o To determine the wind speed from the individual isobar spacing in 

 degrees latitude within the fetch, average them, and with the average value, 

 read off the geostrophic wind speed from Figure 2» Determine from Figure 3 

 (or a similar chart) the sea tenperature; from sliip reports the air tenpera- 

 turej and, estimating the curvature of isobars, use Figure h to determine the 

 actual wind speed, Uo 



17 o Wind speed.3 may also be detennined from either ship or land 

 meteorological stations' reports if there are raaiiy such reports in the fetch 

 area» On the Great Lakes especially, this is often the only means available 

 for this determinationo 



18 o Figures 5, 6 and 7 ax-e the forecasting and decay curves. To apply 

 them, the following simplified step by step procedure may be used; 



ao Delineate a fetch, measure and tabulate the actual wind velocity 

 U^ jthe fetch length F^ , the decay distance D^ ^ and the estimated diiration t^ 



of the windo (When a fetch first appears on a synoptic chart, the duration 

 time may be taken as one-half the time Z between this chart and the one 

 immediately preceding) o 



bo Enter Figure 5 with the wind velocity U. and follow U- across 

 to its intersection with the fetch length F-, or the duration t-, whichever 

 comes firsto At that point read off the period T„and the wave height IL, 

 at the head of the fetch. 



