3.41 ESTIMATING THE WIND CHARACTERISTICS 



To predict wave properties from meteorological data by any of the 

 simplified techniques, it is necessary to: 



(a) Estimate the mean surface wind speed and direction, as dis- 

 cussed in Section 3.4, Wind Information Needed for Wave Prediction; 



(b) delineate a fetch over which the wind is reasonably constant 

 in speed and direction, and measure the fetch length, and 



(c) estimate wind duration over the fetch. 



These determinations may be made in many ways depending on the loca- 

 tion and the type of meterological data available. For restricted bodies 

 of water, such as lakes, the fetch length is often the distance from the 

 forecasting point to the opposite shore measured along the wind direction. 

 There is no decay distance, and it is often possible to use observational 

 data to determine wind speeds and durations. 



When forecasting for oceans or other large bodies of water, the most 

 common form of meteorological data used is the synoptic surface weather 

 chart. (^Synoptic means that the charts are drawn by analysis of many 

 individual items of meteorological data obtained simultaneously over a 

 wide area.) These charts depict lines of equal atmospheric pressure, 

 called isobars. Wind estimates at sea, based on an analysis of the sea- 

 level atmoshperic pressure are generally more reliable than wind observa- 

 tions because pressure, unlike wind, can be measured accurately on a 

 moving ship. Pressures are recorded in millibars, 1,000 dynes per square 

 centimeter. One thousand millibars (a bar) equals 29.53 inches of mercury 

 and is 98.7 percent of normal atmoshperic pressure. 



A simplified surface chart for the Pacific Ocean is shown in Figure 

 3-9, which is drawn for 27 October 1950 at 0030Z (0030 Greenwich mean 

 time). Note the area labelled L in the right center of the chart, and 

 the area labelled H in the lower left corner of the chart. These are 

 low- and high-pressure areas; the pressures increase moving out from L 

 (isobars 972, 975, etc.) and decrease moving out from H (isobars 1026, 

 1023, etc.). 



Scattered about the chart are small arrow shafts with a varying num- 

 ber of feathers or barbs. The direction of a shaft shows the direction 

 of the wind; each one-half feather represents a unit of 5 knots (2.5 

 meters/second) in wind speed. Thus, in Figure 3-9 near the point 35°N. 

 latitude, 135°W. longitude, there are three such arrows, two with 5h 

 feathers which indicate a wind force of 31 to 35 knots (15 to 17.5 meters/ 

 second) , and one with 3 feathers indicating a force of 26 to 30 knots 

 (13 to 15 meters/second) . 



On an actual chart, much more meteorological data than wind speed and 

 direction are shown for each station. This is accomplished by the use of 



3-22 



