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

 

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.) 

 

ILLUSTRATING MY POINT

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

 

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

 

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. 

 

SOME ADDITIONAL FACTS ABOUT TRACE AMOUNTS

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. 

 

IN CONCLUSION

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). 

 

National Weather Service's Use of Standard Time Year-Round Creates Confusion

 

I had been writing this blog for a number of years before learning that the National Weather Service doesn't recognize Daylight Saving Time (the period roughly between mid-March thru early November).  Instead, its reported observations throughout the year reflect Standard Time.  Because of this policy, weather conditions that occur between midnight and 12:59 AM during DST are reported as part of the previous day's weather observation (credited to 11:00-11:59 PM).  And the rest of each day's weather observations are reported as happening in the previous hour.

I discovered this quirk because I check hourly observations every day on one of the NWS sites, and during the spring, summer and first half of the fall (i.e., when DST is in effect) I noticed that rainfall and temperature for the first hour of each day was credited to the previous day.  Then when Standard Time resumed the weather conditions of each day's first hour were back to being part of that same day. 

 

 

I thought that, perhaps, this decision was made over concern by the NWS about how to report conditions that occur during the "skipped" hour, i.e., when we "spring ahead" at 2:00 AM for DST (losing the 60 minutes between 2-3 AM), or when the hour between 1-2 AM is repeated when we "fall back" at the start of Standard Time.  However, it turns out the reason was simpler than that as the US is a signatory of an agreement drawn up by the World Meteorological Organization calling for Universal Coordinated Time to be used as the standard (i.e., not seasonally adjusted).  However, I can't help wondering why local weather records shouldn't reflect the time that its population is conducting its life in.  And it's curious that Standard time, which lasts about four months in much of the US, is the year-round standard.

 

 

So what does this mean in practice?  Let's say that a rainstorm at 12:30 AM DST on May 10 kept you awake.  Using Standard Time rather than DST, the NWS would report that the rain fell at 11:30 PM on May 9; however, in the world of Daylight Saving Time that you're living in, you may have been watching TV at 11:30 and not in bed being kept awake by the rain. 

(These parallel time frames bring to mind how life was in the US before time zones were established.  Prior to their creation in 1883 there were two types of time.  True Solar Time, based on the sun's actual position over a city, was precise and used by the general public in conducting its day-to-day activities, with every city having its own time (e.g., when it was noon in NYC it was 12:10 PM in Boston, 11:55 AM in Philadelphia, and 11:50 in Washington, DC).  Railway Time, on the other hand, was used by the railroads for scheduling arrivals and departures of its trains and was the impetus for the establishment of time zones, whereby New York, Boston, Philly and DC all had the same time). 

 

    

Over the course of a year there are instances of daily records hinging on what happens either during the last hour of the day or the first hour of the next day, and the DST-Standard Time situation creates a "fuzzy" reality.  For example, sometimes there are days during DST when it appears a rainfall record has been set, but using Standard time is the official measure, the amount from midnight-1:00 AM DST is credited to 11 PM-midnight, erasing what appeared to be a record amount (however, there can be instances were adding rain to the previous day could result in a record amount for that date).  

Regarding temperature records, I can recall a few instances in early October during the past fifteen years when it appeared a new record for the latest date for a low of 70+ was going to be established only to find that, although the temperature dropped below 70° after midnight DST, it was actually credited to the day before, so no record was set. 

Of course, in the grand scheme of things, this isn't a big deal.  Since it's doubtful many people are aware of this idiosyncrasy it's a case of ignorance being bliss.  However, for those of us who are aware of this quirk it's annoying (like a hangnail).  

 

Today in New York City Weather History: February 17

1893 (Friday)

Starting today and continuing overnight, a snowstorm dumped 9.1" (with much of it falling today).  This was the biggest snowfall of the winter (in total there were five snowfalls of four inches or more).  Today's high/low of 28°/17° would be duplicated tomorrow. 

1896 (Monday)

With a high/low of 7°/-5°, today was not only the coldest day this winter but of one of the coldest of any winter.  It moved ahead of the previous coldest day of this winter, Jan. 6, which had a high/low of 12/-2°. 

1922 (Friday)

There were ten days with lows colder than 10° this winter, but today was the only one with a reading that was below zero (-2°).  This tied Feb. 17, 1896 as the second latest date for a sub-zero reading (latest was in the winter of 1873 when a low of -4° was reported on 2/24.)  

 

1958 (Monday)

It was a bitterly cold day that saw temperatures stuck in the single digits largely due to mostly overcast skies.  The high was 10°, which occurred shortly after midnight; the day's low of 6° was reached 24 hours later, shortly before midnight.  This was the ninth day in a row with high temperatures colder than 30°. 

1971 (Wednesday)

Although light rain that fell in the middle of the afternoon amounted to just 0.05", it was the first measurable precipitation to fall on this date in fifteen years. 

1973 (Saturday)

Today's high/low of 22°/7° made this the coldest day of the year. 

1978 (Friday)

This was the only day this month with an above average mean temperature (high/low of 40°/33°, three degrees above average).

1990 (Saturday)

The day's temperature nosedived from 58° to 23°. 

1991 (Sunday)

Today was the 20th day in a row in which no high temperature was duplicated.  During these days the highs ranged from 27° (yesterday) to 70° (on 2/5). 

2003 (Monday)

After last night's initial snowfall of 3.5", the brunt of the Presidents' Day blizzard kicked in this morning, dumping an additional 16.3", making this New York's fourth biggest snowfall on record.  (Since then four snowstorms during the winters of 2006, 2010, 2011 and 2016 have surpassed it.)  Wind gusts of 40-50 mph created snow drifts of three to five feet.  The day's high/low was just 26°/14°.  This storm was fiercer than another President's Day snowstorm, in 1979, which dumped 12.7" on the City.    

 

 Near white-out conditions on Washington

 Place in Greenwich Village

2013 (Sunday)

After a pleasant week following the snowstorm of Feb. 8-9 (11.4" fell), a slap of winter returned with howling winds (40-50 mph gusts) and temperatures that fell through the 20s, to 18° by midnight (wind chills were close to 0°).  Over the past four weeks half of the days had highs of 32° or colder.  The one consolation today was the clear skies, but snow lovers would beg to differ as an intense storm (the reason for the high winds) that dropped more than six inches of snow over Massachusetts was too far off the coast to give any snow to the City.  

 

2015 (Tuesday)

An overnight snowfall put down 3.3" by the time it ended around 10:00 this morning.  Because the air was so cold and dry just 0.14" of liquid produced considerably more snow than would usually be the case.  The day's high was 27°, making it the fifth day in a row with a high of 32° or colder. 

2018 (Saturday)

Despite the temperature going no lower than 33°, a heavy, wet snow accumulated 4.4" between 5 PM and midnight.  This came at the end of a week that had three days with highs of 62° and one with a high of 58° (yesterday).  (Click here to read an analysis of other snowfalls of four inches or more that have occurred the day after the high was 50° or warmer.) 

2022 (Thursday)

Today had the year’s first reading in the 60s.  At 68° it was the warmest reading in three months (since 11/18), and a record for the date.  The last time a reading this mild happened in February was in 2018, when the mercury soared to 78° on 2/21 (the warmest temperature ever reported in February).  Today’s spring-like reading, 25 degrees above average, came just two days after a low of 16°.  (There was also a low of 16° on 2/14, two days after a high of 59°.) 

 

 

2024 (Saturday)

Today's high of 36° was the coldest daily high of the month - the mildest reading on record to be the coldest high in any February - breaking the previous record by three degrees.  (This came one year after January had its mildest reading on record for coldest high.)  The average coldest high in February is in the mid-20s.  (In the years before 1930, it was in the upper teens.)