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001401 II.5 If inoonsistencies are apparent, appropriate adjustments sometimes are warranted. However, caution should be used in such adjustment and the safest .approach is to adjust only those cases where presumptive causes coincide with the dates of inconsistencies. Some apparent inconsistencies may be the result of actual climatic trends. In general, each consistent segment of a long record is best treated as a separate record, with a separate average or "normal." Then, in developing an isohyetal map, each segment should be plotted separately.- at its proper location if location is known to have changed, or under its appropriate time span. Use of data Determining averages or normals The World Meteorological Organization has defined the term "normal" as the average computed for a uniform and relatively long period that comprises at least three consecutive 10-year interim periods. The standard periods adopted by the U.S. Weather Bureau for climatological normals are the following consecutive 30 years: 1 January 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, etc. "Average" or "long-term mean" should be used for reference to any other period. The length of record required to establish a suitable climatologic average depends to some extent upon the vllriability of the record. The Weather Bureau has discontinued publication of averages if the station does not have a complete record for the 25 years 1931-1955. Beginning January 1, 1962, the Weather Bureau will use the 3D-year normaL (1931-1960) for all stations. If part of a. . station redord is inconsistent, use of all the record may lead to an incorrect normal or average. Normal or average precipitation for a period longer than the observed record for "a station may be estimated by various comparison techniques. Five years or more of precipitation records generally are sufficient to compute an October-April normal by the double-mass curve technique; ten years or more to compute a May-September normal by the ratio method. . Average precipitation over an area during any particular storm or period may bedeterrnined by computing the average of the records available, by the Thie,ssen diagram technique, by the isopercental method, or from an isohyetal map (lines of equal precipitation). In mountainous areas the isohyetal-n/ap approach is recommended if the map is based on a knowledge of topographic influences and is compared with an average yearly isohyetal map that considers data on precipitation, runoff, and evapotranspiration loss. '.