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Can 'Trace' Amounts of Precipitation Add Up to Measurable Amounts?


Thinkng man


When a day's precipitation is reported as 'T', which signifies a "trace" amount, this indicates that less than 0.01" fell (ranging from just a few raindrops or snowflakes up to 0.0044").  From 2000 thru 2017 there have been, on average, two days each month with a trace of precipitation.  In the years I've been writing this blog I've become more and more curious whether trace amounts falling on multiple days can add up and become a measurable amount - something that weather records don't recognize.  (Additionally, on days when a trace falls there are often multiple hours with a trace.  Furthermore, there are days with measurable precipitation that report an hour or more of trace amounts.)



Let's say we have a month in which five days had just a trace of precipitation:


Traces of precip

Although the amounts are considered traces, precise measurement might report very small amounts of liquid:


Traces of precip converted

Therefore, the five-day total of these trace amounts would indicate that 0.0102" fell during these five day (or 0.01").  The example above would also apply to a day with multiple hours of traces of precipitation. 



December 1989 had the most days on record, fifteen.  Four of the five years with the most days with a trace of precipitation occurred from 1938 to 1943.  The four years with the least were all in the 2000s.  Curiously, the average number per year since 1999 has been half the number in the prior years (46 days vs. 23).  However, it appears 2018 is on its way to having the most in 20 years.



With the today's measurement technology able to measure more granular amounts of liquid, weight, speed, etc., surely a device must exist that can show that trace amounts added together may result in a measurable amount.  Granted, these additional measured amounts won't be large (and which date would be given credit for a measurable amount over the course of multiple days?) but they would make weather records more precise, which is something "Big Data" types could appreciate.  (Perhaps precipitation can be reported to three decimal places?  In a previous post I made a case for reporting temperatures to one or two decimal places). 




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How often does a month with no days of a trace of rain or snow occur? To the best of my knowledge, the most recent such month was in July 2018 (and while November 2016 had no traces of precipitation, the month had a trace of snow on 11/20).


Between 1912 and 2018 there have been nearly 60 months with no days with a trace of precipitation, which is about once every two years.

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