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<br />~i~t-e;,a},":,''':'' , <br />~~WiM.~f.totj;UC6'tCil;,t,';\..~,'., <br />I <br /> <br />'.:". "20'~_~'..<~"~,. ~,"_ -:,'",~,,-,:\<,. .:...,,'......_'~.. <br /> <br />PRECIPITATION GAUGE SITES <br /> <br />683 <br /> <br />heights of more than 10 m above ground and not at the level of the precipitation gauge orifice. The latter <br />varies between 0.2 and 2.0 m above ground (Sevruk and Klemm, 1989) according to the country of situation. <br />Thus the wind speed values at hand have to be reduced to the level of the gauge orifice if they are to be <br />used for estimating wind-induced error. The reduction of wind speed at the open sites can be made by <br />using a simple logarithmic wind profile (Sevruk, 1981). Naturally, this wind profile would result in too high <br />values for gauge sites surrounded by obstacles. The excess will depend on the situation, the density, and <br />the heights of the obstacles. To express this objectively and quantitatively, the average vertical angle 'c(' of <br />obstacles can be used. <br />Usually 'c(. is measured in eight directions of the wind rose, using a simple optical instrument (meridian) <br />or theodolite. More complex information can be obtained by taking a picture at the level of the gauge <br />orifice with a camera fitted with a fish-eye lens. Examples are shown in Figure 2. The evaluation of 'c(' <br />from the picture can be made by using a digitizer and a special computer program. The idea of using the <br />angle 'IX' is not new and can be traced back to the last century. It was first mentioned by Symons (1868), <br />and was later introduced into the logarithmic wind profile equation by Fedorova (1966) as follows: <br /> <br />( 11)( H)-I <br />uh = UH log Zo log Zo (1 - 0'024iX) <br /> <br />(1) <br /> <br />where Uh is the wind speed at the level of the precipitation gauge orifice (m s - I), UH is the wind speed at <br />the level of wind speed measuring instrument (m s - I), h is the height of the gauge orifice above ground <br />(m), H is the height of the wind measuring instrument above ground (m), 20 is the roughness length (m), <br />and iX is the average vertical angle of obstacles (in degrees). <br /> <br /> <br /> <br />Figure 2. Diagrams of gauge sites with four different degrees of protection against wind by trees and houses. as taken using a camera <br />with a fish-eye lens through the gauge orifice. Clockwise: open site. partly open site. partly protected site. and protected site <br />