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001459 Erroirous timing.- At first-order climatic stations that are equippedth recording gages, amounts of daily precipitation are determined on the midnight-to-midnight basis. At stubstations, however, the daily amounts are generally from once-daily readings of the gage--dommonly early in the morning but neither strictly at 24-hour intervals nor at a COJllJ!lon hour among all the substations. Thus, for storms of short duration or of highly variable intensity, amounts of precipitation recorded under a particular date might differ substantially among a group .of stations whereas the actual amounts were essentially equal and simultaneous. Also, an erroneous date of observation may have been entered in the record. Errors so caused are probably common. In many instances they can be disclosed by constructing isochronal maps. Improper e~osure.- Criteria have been given for properly locating a gage In ~elatkon to nearby obstructions. When such criteria are violated, buth speed and direction of wind as well as catch of the gage are likely .tobe abnormal. The abnormality can be identified and evaluated bY comparing the catches of adjacent gages between windy and non-windy periods, certain of the gages being properly exposed. Catch 'of snow.- Snow may bridge and cap a precipitation gage, so that the apparellt time of snowfall may be displaced (in the case of a recording gage), or the catch may be grossly in error. Also, the gage may become partially filled with unmelted snow, so that its catch efficiencr,r is d:iJn:i.nishedtemporarily. Such events may be pre sumed if there are erratic differences among the catches of adjacent gages. In general, reported catches. of snow should be mistrusted whenever they exceed 50 percent of gage capacities. Snow density.- Prior to about 1920 in the United States, the water content of daily snowfall in excess of gage capacity was estimated by assuming a :density of 0.10 for the new snow on the ground (rather than bY melting a sample). This is still the official practice in Canada. This I-to-fI.O rule usually gave too small a value for water content of snow at low.-.altitude stations, and too large a value at high-altitude stations. Generally it is imPractical to adjust systematically for this error; howeyer', probable magnitude of error may be suggested by typical profiles of' altitude v. water content. The l-to-10 rule is likely to be grossly in error at all altitudes if applied to accUl1lUlated snow on the ground.. Minimal amounts of preCititation.- At substations, hours may elapse before the precipitation catc in a light storm is measured on the once~ daily schedule. During that interval an appreciable portion of the catch may be lost. This potential error may be assumed likely for any storm precipitating less than 0.10 inch. First-order and recording stations haye shown a greater frequency of days with precipitation:=:: 0.01 inch than stations that measure only once a day. -1,_ II.3