Days With Desert-Like Humidty In The 21st Century

 

 

In April of 2016 New York experienced eleven days in a row with clear or sunny skies (4/12-22).  In addition to this abundance of blue skies, the streak was also characterized by extremely low humidity.  While typical late afternoon humidity levels in April are generally between 45% and 50%, during these eleven days it was mostly between 15% and 20%.  And at 5 PM on 4/20 it fell to 9%.  Since 2000, only one other day before 4/20/16 had a humidity level that was lower - March 31, 2007, which reported 6% humidity late in the afternoon (and on 4/6/21, the humidity dropped to 7%).  It was April 2016's streak of arid days was the inspiration for this post, which looks at hourly and monthly relative humidity reported in Central Park since 2000, with a focus on extremely low levels, which for the purposes of this analysis is 25% or lower.          

 

• Since 2000 5% of the days have had at least one hour with a relative humidity of 25% or lower (about 19 days/year).  The number of such days ranged from six in 2000 to 32 in 2010.  When I began this project I guessed that January and February might have the most days with the lowest humidity, but it turns out that two-thirds of the these days occurred in March or April.

 

 

• February, March and April have the lowest humidity, while September has the highest (followed closely by June).

• Not only were the two years with the lowest average humidity consecutive (2015, 2014), so were the years with highest average humidity (2003, 2004).  And while the year with lowest humidity had significantly lower than average precipitation, the second driest was well above average.  In fact, the total precipitation for the year with the highest humidity was very similar to this second driest year (58.56" vs. 57.79").

 

 

• During a typical day, the lowest and highest humidity levels occur about nine hours apart, with the highest occurring around daybreak (averaging 72%), and lowest levels during mid-afternoon, averaging in the mid-50s.

 

 

• There have been close to 50 days with at least one hour of humidity at 15% or lower (thru the end of April 2022).  All were in March or April, until 2018 when the humidity fell below 15% twice in May and once in November, and then again in Dec. 2021.  The lowest humidity to occur in the other months has been 16%, and it's happened on four dates: Feb. 26, 2014; May 3 and May 13 in 2007, and Sept. 11, 2001.

• The months with the least and most precipitation don't always have corresponding low and high average humidity.  For example, the most humid October and February had only 1.18" and 1.66" of precipitation, respectively.  The months with the least humidity don't have outliers quite that extreme, but the driest June and January had more than four inches of precipitation, which is close to average.

• In this century, months in the years 2000-2004 tended to have the highest average humidity while those with the lowest were clustered in the years since 2010.

 

 

 

  

 

   

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April 2016 Weather Recap - Dry, With Average Temperatures

 

The dry conditions of March (1.17") continued through April (1.61"), but unlike March, which was the fourth mildest on record, April was just about average (+0.3 degrees).  March's warmth, however, extended into April for one day when the high/low was 79/61 on 4/1 (22 degrees above average).  But just four days later the temperature plunged to 26°, which was the coldest reading in April since 1995.  This came in the midst of an eight-day stretch (through 4/10) that saw temperatures that were eight degrees below average.  Temperatures then rebounded for the week between 4/17-23 as highs averaged 75°, twelve degrees above average.  But the month ended on a cool note, with the last five days 3.5 degrees below average.  Some other observations worth noting:

 

• The highlight of the month was the eleven-day stretch from April 12-22 which had sunny or clear skies every day and extremely low humidity.  Afternoon humidity levels in April are typically in the 40-50% range, but on these days it was mostly between 15-20%.  And on 4/20 at at 6PM it dropped to 9% - the lowest since a reading of 6% on March 30, 2007.

• March and April were the first back-to-back months with less than two inches of rain since the winter of 2012 - and the driest March-April combination on record (the previous driest was way back in 1885).  The combined 2.78" for these months was one-third the average amount of 8.86".  March was the sixth driest on record, April the eleventh driest.

• Despite the dry conditions of April, nine of the first twelve days of the month had rain.  The rainiest of the days, 4/4 (when 0.49" fell), occurred the day of the Yankees' home opener, which was postponed.  

• For the second year in a row the year's first 80-degree reading occurred on 4/18.  This was five days earlier than average

• Finally, although it had nothing to do with April's weather, on 4/28 the National Weather Service announced that it had revised the accumulation of the big blizzard of 1/23 upward by 0.7", to 27.5".  This new total pushed it ahead of the blizzard of Feb. 2006 (26.9"), making it York's biggest snowstorm on record.  The accumulation that was added had fallen after midnight on 1/24 but had been overlooked.

 

 

 

 

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When April Isn't Much Warmer Than March

 

The month of April is typically 10.5 degrees milder than the month of March.  April 2020, however, was just 2.4 degrees milder, making its average temper the third closest to March of any April (going back to 1869).  The closest April came to March was in 1946 when April was 0.8 degrees milder than March.  The chart below lists the six Aprils that were closest to March's average temperature.

 

And although April 2020 wasn't chillier than March, it had no days in the 70s while March had three (making this the first April since 1940 not to have any readings of 70° or warmer).

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April Ends with a BIG Splash

 

No month has ever ended with such a big rainstorm as did April 2014.  4.97" poured down on April 30, making it New York's rainiest day on record.  Previous to this, the rainiest day at the end of a month end occurred on Aug. 31, 1911 when 3.76" fell.  Before April 30th's deluge the month had been an inch-and-a-half below average.  It ended up becoming the sixth rainiest April on record. 

 

 

More than 60% of the month's rainfall was accounted for on the last day of the month.  What also made this day's rainfall of interest was that up until this year April 30 was one of three days in which an inch or more of rain had never fallen (the other two days are April 29 and Sept. 9). 

 

And the wettest first day of any month?  When this post was written in 2014, that honor went to Oct. 1, 1913, which had 4.98" measured (yes, 0.01" more than April 30, 2014); however, it fell to second place when 7.13" of rain fell on the first day of September 2021.  

 

RAINIEST ENDS TO A MONTH
		% of Month's
Date	Amount	Total Precip
April 30.2014	4.97"	63%
Aug 31,1911	3.76"	41%
May 31,1940	3.13"	41%
June 30, 1984	3.07"	53%
Oct 31,1956	2.41"	67%
Dec 31,1948	2.31"	37%
July 31, 1889	2.29"	19%
Feb 29,1896	2.26"	33%
Sept 30,1921	2.21"	46%
July 31,1925	2.14"	37%
RAINIEST STARTS TO A MONTH
Sept 1, 2021	7.13"	71%
Oct 1, 1913	4.98"	38%
Sept 1, 1927	3.84"	90%
Sept 1, 1983	3.40"	74%
Mar 1, 1914	2.95"	62%
Aug 1,1878	2.85"	36%
June 1,1887	2.60"	34%
May 1  1976	2.48"	52%
Oct 1, 2010	2.46"	50%
July 1,1933	2.17"	36%
July 1, 1922	2.13"	28%

 

(Many thanks to my friend Eugene DeMarco for creating a spreadsheet that listed every one-inch rainfall in NYC since 1869. It made doing this analysis possible.) 

 

 

Today in New York Weather History: April 14

 

1880

Today's high of 80° came just two days after a record low of 26° (since eclipsed in 1976).  This was the first reading above 64° since 3/5 (when it was 72°).

1940

A trace of snow was reported for the 33rd time this snow season, the most ever (later tied in 1944-45).  Today's high/low was 41°/30°.

 

 

1966

This was the fourth day in a row with a low of 39° (five degrees below average).  However, all of the days had different highs, with three in the 57°-60° range and one with a high of only 44°. 

1979

It was a rainy and chilly Easter Saturday.  1.10" of rain fell (ending late in the afternoon) and the high/low was 45°/40°; the high was nearly twenty degrees below average. 

2014

This was the fourth day in a row with highs in the 70s.  The average high during these days was 75°, fifteen degrees above average.

 

2018

After the temperature peaked at 77° in the middle of the afternoon, a "back-door" cold front moved through, and by midnight the temperature had fallen to 44°.  This was the greatest drop in temperature since 1/13, when it plummeted from 58° to 19°.  (It was also the second biggest drop in more than four years.)

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Comparing Weather Conditions of Yankees & Mets Home Openers (1970 - 2019)

 

In two previous posts I summarized game-day weather conditions for each Home Opener of New York's Yankees and Mets.  This post, however, brings the two teams together with a side-by-side comparison, found below, of each year's first home game, going back to 1970.  First, some key takeaways:

 

• Looking at average high temperature, there has been a three-degree difference between the home openers of the Mets (59°) and Yankees (56°).  For the Yankees it's ranged from 35° (twice) to 76°, while for the Mets the temperature range has been from 40° to 90° (twice).  However, over the five decades, there has been a shift.  In the '70s and '80s home openers of the Yankees were five degrees warmer than those of the Mets (60° vs. 55°), but the last three decades home openers were in favor of the Mets (61° vs. 53°)  

• Both teams have had to contend with precipitation during six of their home openers, about once every seven or eight years.  The Yankees, however, are the only team to play in snow.  It's happened three time, in 1982, 1996 and in 2003 (the 1982 opener was postponed because of a blizzard that dumped nearly ten inches of snow.)  For the Mets, snow fell during the morning on the day of their 1974 home opener but it was over long before the first pitch.

 

 

 

• Temperatures have been in the 70s for the Yankees six times; they all occurred between 1976 and 1988.  The Mets had the last home opener with temperatures in the 70s, back in 2010. 

• The 1970s was a decade of cold home openers for the Mets.  Afternoon temperatures were in the 40s for seven of them between 1970 and 1978.  The Yankees had consistently chilly/raw conditions between 1996-2005.

• There have been six seasons in which the two teams' home openers were played in very different weather conditions.  The greatest contrast was in 1977 when the temperature was in the mid-40s for the Yankees while the mercury climbed to 90° for the Mets five days later.  Not far behind was 1991 when the Mets once again started the season on a day the temperature soared to 90°; a week later the Yankees home opener was played in rainy conditions with a temperature of 50°.

 

• Six seasons have had home openers with bad weather for both teams while five had good conditions for both, the most recent being last year (when both teams played on the same day, April 1).

 

 

 

 

 

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Weather Analysis: The First 60-Degree+ Reading of the Year

 

 

There are a number of ways to gauge how cold a winter has been: mean temperature; the number of days with sub-freezing highs or single-digit lows; streaks of days with below average temperatures; or a dearth of mild readings.  For instance, after the brutal winter of 2015 New York didn't experience its first 60-degree day until March 26.  That's more than seven weeks later than the average date for this occurrence (Feb. 3) - and the latest date since 1982.  Here are more interesting tidbits about the first reading of 60+:

 

• In the 19th century (1869-1899) the average date of the first 60+ reading in Central Park was March 8, and then between 1900-1970 the average date moved up to Feb. 23.  Since 1970 the average date has been three weeks earlier.

• Between 1900-1970 the first 60+ day occurred on March 1 or later in half of the years, but since 1971 it has occurred after March 1 just 20% of the time.

• Since 1900 the first 60-degree day has occurred on New Year's Day five times: 1919, 1966, 1973, 1979 and 2005.  (In 1966 the year's first 60+ reading followed 1965's last 60+ reading on New Year's Eve.)

• Looking at all years the latest date for the first 60+ reading was April 15, in 1877.  And nine other years had their first reading in the 60s between April 5 and April 13 (see chart below); the most recent year to have a date that late in 1970 (April 8). 

• Once every 10 years the first 60+ reading has occurred in the first three days of January, while once every 23 years the first 60 occurred on April 1 or later.

• In 2007, not only was the first 60-degree reading of the year very early (Jan. 5), it was followed the next day by the year's first high in the 70s (72°), the earliest ever. 

• In 1997 and 1998 the first 60+ temperature occurred on the same date, a very early Jan. 3.  And 1906 and 1907 had their first 60s on Jan. 4 while 2017's and 2018's  was on Jan. 12 (in 2020 it fell on Jan. 11).  At the other end of the spectrum, 1962 and 1963 both shared March 25 as the date of their first 60.

• For four years in a row, 2005-2008, the first 60-degree reading occurred in the first nine days of the year.

• In 1943, the year's first 60-degree temperature, 63°, came just five days after the morning low was 8° below zero. (And in 2022, the first temperature in the 60s, 68° on 2/17, was three days after the low was 16°). 

• The coldest temperature to occur on the same day as the first reading of 60+ was in the winter of 1957 when an Arctic front knocked the temperature down to 20° after the mercury reached 60° earlier in the day (on Jan. 23).

• The biggest jump in temperature from the day before the first 60+ reading was 25 degrees, in 1954, when the high jumped from 44° to 69° the next day.  The biggest increase in temperature the day following the first 60+ was 21 degrees, and it happened in 1917, when the high reading the day after rose to 83°.  Lastly, the biggest drop in temperature after the first 60+ was 33 degrees and it happened in three years: 1957 (from 60° to 27°); in 1939 (62° to 29°); and in 1913 (63° to 30°).

• The date of the first 60+ reading in five years was also the date of the first 70+ reading: in 1987 (March 7); 1969 (March 18); 1964 (March 5); 1963 (March 25) and 1893 (April 1).  Nine other years had their first 70+ reading the day after the first 60+, the last time being in 2007.

 

Mild One Day, Snow the Next - A History of Dramatic Change

 

This post was inspired by an eight-inch snowfall on Feb. 2, 2014 that occurred the day after the high reached 56°.  However, there was an even more drastic one-day change during the winter of 2017 when 9.4" of snow fell on Feb. 9, the day after the high was 62°.  This was the most drastic one-day change in weather conditions since March 16, 2007 when 5.5" of sleet and snow fell the day after a springlike high of 67° was reached.

 

 

Since 1970 there have been twenty-eight instances of an inch or more of snow falling the day after a high of 50° or warmer; however, on only seven of those occasions was the accumulation four inches or more.  In addition to the three occasions mentioned at the beginning of this post, here are the four other dates it has occurred:

• March 29, 1970 - On Easter Sunday four inches of snow fell the day after a high of 54°.

• Feb. 2, 2009 - 4.3" of snow fell one day after a high of 53° was reported.

• Feb. 21, 2001 - A snowfall of 5.8" occurred the day after a high of 52°.

• Feb. 17, 2018 - A nighttime snowfall of 4.4" came at the end of a week that had three days with highs of 62° and a high of 58° on 2/16.

 

The following two snowstorms occurred the day after the high temperature was above average, but below 50°; however, they warrant mention because of the large amount of snow that fell:

• Jan 21, 2014 - 11.0" fell the day after a high of 46° degrees was reached.

• April 6, 1982 - 9.6" fell the day after the temperature topped out at 48°.   

 

Finally, a shout-out to the 1.2" of snow that fell on April 9, 2000 because it came the day after a high of 71°.

 

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80-Degree Weather in April and October

 

On average, temperatures reach the 80s, or hotter, on 1.4 days in April and 1.1 in October (since 1970).  Interestingly, despite the higher likelihood in April, its average high temperature is a few degrees cooler than October's.  In the years between 1970 and 2019, April has had about 25% more 80/90-degree days than October (72 vs. 57 days).  However, this disparity didn't start until 2000.  Between 1970 and 1999 the two months were relatively close (35 days in the 80s in April vs. 31 in October), but since 2000 April has had 37 such days while October has lagged behind with 26.

 

The greatest number of 80-degree days to occur in April is five, which happened twice, in 1985 and 2002.  October 1990 had six 80-degree days and five in 1995 and 2007. The most years in a row without an 80-degree temperature being reported in April is three; the same holds true with October.  However, while the most consecutive years with 80-degree readings (or warmer) in October is four (2016-2019), April, has had a streak of thirteen years (2001-2013).

 

 

Since 1970 thirteen of April's days above 80 degrees have been in the 90s, but just one such day has occurred in October (it was in 2019 and was the first high of 90+ in October since 1941). 

 

 

Finally, there have been ten 80-degree days before April 1, but just one after November 1.  (March and November totals aren't included in this analysis.)

 

 

 

 

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Weather Analysis: Monthly Averages Can Deceive

April 2013 in New York was rather unique because it was just the third time since 1960 that a month had a mean temperature right at the monthly average, i.e., 0.0 degree difference.  The other two months in which this happened were August 1999 and August 1985.  However, what set April 2013 apart was the fact that its average high and average low were almost right at the average as well (-0.1/+0.1).  And while August 1999's high/low wasn't too far from average (-0.3/+0.3), August 1985's was significantly different (-1.3/+1.3).

 

April's near bulls-eye inspired me to examine all months that were relatively close to average, temperature-wise, to see how many had highs and lows both with departures from average of the same magnitude, and how many diverged significantly, but neutralized each other (like August 1985).  For my purposes I included all months that were either right at the average, or 0.1, 0.2 or 0.3 degree from average (+/-).  Since 1960 (thru 2020) there have been 63 months that fit this qualifier (which translates to an occurrence of about once a year).

 

 

So, what were my findings?  Only six of the 63 months that reported a mean temperature close to average had highs and lows with departures from average that were nearly identical.  But perhaps the biggest finding was that March 2009 was actually closer to a 0.0 difference from average than April 2013, as its high temperature was 0.1 below average and its low was right at the average.  However, because of rounding, it ended up being 0.1 below average and not at 0.0 as April 2013 was. 

 

		Departure from Average		Spread Between
		High	Low	High/Low
September	1989	-0.2	-0.2	0.0
March	1980	+0.1	+0.1	0.0
MARCH	2009	-0.1	0.0	0.1
August	1990	-0.2	-0.1	0.1
June	1995	+0.2	+0.3	0.1

 

On the other hand, as the chart below shows, there are "average" months that have highs and lows that diverge wildly from their averages, but balanced each other to produce a monthly mean temperature that was "average".  The most extreme case was November 1977.  Although its mean temperature was 0.1 degree below average, the month's high was 2.5 degrees below average while the low was 2.3 above average.  Two months before that, September 1977, exhibited a similar wide divergence.

 

		Departure from Average			Spread Btwn
		Mean	High	Low	High/Low
November	1977	-0.1	-2.5	+2.3	4.8
September	1977	-0.2	-2.1	+1.8	3.9
September	1996	-0.2	-1.9	+1.6	3.5
June	1996	-0.2	-1.8	+1.4	3.2
April	1983	-0.1	-1.7	+1.4	3.1
October	1998	+0.1	-1.4	+1.5	2.9
March	2007	-0.3	+1.1	-1.6	2.7
August	1985	0.0	-1.3	+1.3	2.6
September	1982	-0.1	-1.3	+1.2	2.5
June	2006	-0.2	-1.3	+1.0	2.3
September	1960	-0.3	-1.4	+0.9	2.3

 

It's interesting to note that in all cases but one, the highs were below average while the lows were above average.  This greater propensity for low temperatures to increase is a classic sign of global warming, whereby a more persistent cloud cover, promoted by increased carbon dioxide levels, keeps heat from escaping at night.  

 

 

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