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<br /> <br />001432 <br /> <br />Preparing isdbyetal maps <br /> <br />All records to be used in preparing isohyetal maps shOuld be <br />checked for consistency prior to computing the averages or normals. <br />A specific ,period should be chosen and all normals or averages <br />adjusted t~ that period. To arrive at a yearly isohyetal map for <br />any part o~ the Pacific Southwest, with the possible exceptioll of <br />California, it is desirable first to prepare separate October-April <br />and May-5eptember maps because of the change in source regions and <br />storm path$ from winter to sUllllller. <br /> <br />Frequency analyses <br /> <br />Severl!l techniques are used for frequency analyses of precipitation <br />data, acco~di~g to the particular type of study. Care must be taken <br />that the data 'used in the frequency analysis represent the' same time <br />interval. ;For example, hourly precipitation records are published for <br />clock hour~ and do not necessarily represent the maximum preOipitation <br />in any one 'hour period. Likewise, reports from daily precipitation <br />stations m~ not represent maximum 24-hour precipitation amounts. <br />Various tedhn:\.cal papers that have been pUblished by the Weather Bureau <br />will, in many cases, :furnish the required frequency information. <br /> <br />Timing of observations <br /> <br />. <br /> <br />Care Must be used in comparing or correlating precipitation data <br />from stati~n to station, or in computing the precipitation over a given <br />basin durirlg a particular time interval. Amounts of daily precipitation <br />as publiBh~d in the Climatological Bulletins under a given date JtJJr;f not <br />be for the !same interval; the hour of observation is listed in the bulletin. <br />Thus, altho'ugh some stations report 24-hour amounts for the period ending <br />8:00 a.m., :the majority report for the 24-hour period ending at 5:00 p.m., <br />6:00 p.m., .or near sunset. For stations equipped with recording rain gages, <br />the daily $aounts reported are those from midnight to midnight. The orig;i.nal <br />observatiorl form (WB Form 612-14, formerly 1009) has a place for indicating <br />the begimtling and ending of precipitation. Many observers keep meticulo~ <br />records in this regard; this information, which is not published, can be of <br />considerable assistance in special studies. <br /> <br />! <br /> <br /> <br />II.6 <br />