shows three cases where a fetch AB has moved to theposition CD on the next map 6 hours later. The problemis determining what part of the moving fetch area toconsider as the average fetch for the 6-hour period.In figure 6-5, Case 1, the fetch moves perpendicularto the wind field. The best approximation is fetch CB.Therefore, in a forecast involving this type of fetch, useonly the part of the fetch that appears on two consecutivemaps. The remaining fetch does contain waves, but theyare lower than those in the overlap area.In figure 6-5, Case 2, the fetch moves to leeward (inthe same direction as the wind), Since waves aremoving forward through the fetch area, the area to beused in this case is fetch CD.Case 3, figure 6-5, depicts the fetch movingwindward (against the wind). Since the waves movetoward A, the region AC will have higher waves thanthe area BD. Experience has shown that in this case ABis the most accurate choice for a fetch,Determining Accurate Wind SpeedThe most obvious and accurate way to determinewind speed over a fetch is to average the reported valuesfrom ships. This method has the advantage of notrequiring a connection for gradients or stability. Butoften there are only a few ship reports available, and shipreports are subject to error in observation, encoding, ortransmission.A second way to determine wind speed is to measurethe geostrophic wind from the isobaric spacing and thencorrect it for curvature and stability. At first it wouldseem this would be less desirable due to the extra timenecessary for corrections. However, barometricpressure is probably the most reliable of the parametersreported by ships, and a reasonably accurate isobaricanalysis can be made from a minimum number ofreports. For these reasons the corrected geostrophicwind is considered to be the best measure of wind speedover the fetch, except of course in cases where there isa dense network of ship reports where wind directionand speed are in good agreement.The reason for correction to geostrophic wind is thatthe isobars must be straight for a correct measure of thewind. When the isobars curve, other forces enter intothe computations.The wind increases or decreasesdepending on whether the system is cyclonic oranticyclonic in nature.The stability correction is ameasure of turbulence in the layer above the water. Coldair over warmer water is unstable and highly turbulent,making the surface wind more nearly equal to thegeostrophic wind. Conversely, warm air over colderwater produces a stable air mass and results in thesurface wind being much smaller than the geostrophicwind.Three rules for an approximation of the curvaturecorrection are as follows:1. For moderately curved to straight isobars-noconnection is applied.2. For great anticyclonic curvature-add 10 percentto the geostrophic wind speed.3. For great cyclonic curvature-subtract 10 percentffom the geostrophic wind speed.In the majority of cases the curvature correction canbe neglected since isobars over a fetch area are relativelystraight. The gradient wind can always be computed ifmore refined computations are desired.In order to correct for air mass stability, the sea airtemperature difference must be computed. This can bedone from ship reports in or near the fetch area aided byclimatic charts of average monthly sea surfacetemperatures when data is too scarce. The correction tobe applied is given in table 6-2. The symbol Ts standsfor the temperature of the sea surface, and Ta for the airtemperature.Determination of Wind DurationOnce a fetch has been determined and the windspeed has been found, the next step is to determine theduration of that wind over the fetch. It is highly unlikelythat the wind will begin and end at one of the 6-hourmap times. Therefore an accurate value must beinterpolated. Inmost cases a simple interpolation of thesuccessive maps will be sufficient to locate the boundsof the wind field in space and time.Table 6-2.-Air-Sea Temperature Difference Correction6-8