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October 2018

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 to1943.  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). 



September 2018 - A Month With Unusually Mild Nights

September 2018


September 2018 was 2.7 degrees warmer than average.  Like August, its average low was considerably more above average (+4.4 degrees) than its average high (+1.0).  This resulted in a vast difference in their historical rankings, with the average high ranking 60th warmest while the average low was ensconced among the ten warmest, at #4 (overall, the month was the 21st warmest).  The month's other significant story was its rainfall, which amounted to 6.19" - the most in September in seven years.  Furthermore, this was the sixth month this year with more than five inches of precipitation, joining ten other years with this many or more.  And if that wasn't impressive enough, 2018 became just the second year to have these six months concentrated in the first nine months of the year (1998 was the other year). 

Additional points of interest:  

  • The remnants of two hurricanes brought 1.38" of rain on 9/9-10 (Gordon) and 1.19" on 9/18 (Florence).  However, the month's biggest rainstorm was on 9/25 when 1.95" fell. Nearly half of that day's rain poured down in about an hour early in the afternoon.
  • The first full week of the month (week of 9/3) had four days in a row with highs that ranged from 89° to 93°.  The average high/low of 91/76 during these days was twelve degrees above average.  The first day of this hot spell fell on Labor Day, which had a high in the 90s for just the fourth time since 1980. 
  • This was the fourth September in a row with an average temperature warmer than 70.0 degrees, the longest such streak on record.  What's more, the month set a September record for the most days with lows of 67° or warmer (seventeen; the average number is six).  Despite this surfeit of warm nights, the number of days with highs of 80° or warmer was a bit below average (nine).
  • The month's average diurnal variation (the gap between the average high and low) was the smallest for September in more than 100 years (since 1908).  In 2018 it was 11 degrees, well below the average of 16 degrees (based on the past 100 years).
  • This was the record-tying seventh year in a row in which September experienced highs in the 90s, equaling in length the streak of 1940-1946.
  • Like September, August's average low also ranked fourth (its average high ranked 32nd).
  • Typically September is 2.5 degrees cooler than June, but this year it was one degree warmer.  This is the fifth time it's happened since 1980 (most recently in 2015).


 Chart - september warmest lows

Here are recaps of the four previous Septembers: