Frightful Halloween Weather in the Nation’s Capital ?

 

D.C. Area residents had a treat this Halloween afternoon with mostly sunny skies, breezy conditions and seasonally mild temperatures.  That may lead some to wonder what Halloween weather has historically been like in the Nation’s Capital.  For example, Halloween 2019 couldn’t have been more different than last Halloween with a high/low of 77°/53°.  It was also a wet Halloween two years ago with 1.25” of rain at National Airport.  By comparison, October 31, 2020, was unusually chilly with a high/low of only 51°/41°.

D.C.’s average high/low temperatures on October 31 are 64°/46°.  That’s compared to DC’s record October 31 high of 85° in 1950.  Meanwhile, today’s record low is 26°, which last occurred in 1917.  Since 2000, DC has had measurable rainfall on only three Halloweens: 2019, 2009 and 2002.  The warmest Halloween in recent times was in 2004 when the high/low temperatures were 79°/59°, respectively, at National Airport.  Meanwhile, DC’s chilliest Halloween in recent decades occurred in 2002, with high/low temperatures of only 47°/38°.  

As a whole, October 2021 has been a memorable weather month.  Exceedingly dry with only 0.42” of rain through October 24, this October was poised to finish among D.C.’s top 10 driest on record.  However, an exceptionally wet final week of the month helped give Washingtonians an October rainfall total of 3.27”.  Although that’s below the October average of 3.66”, it helps this month finish much closer to average rather than ranking among D.C.’s driest.

It’s also been a very warm October with only three cooler than average days in the Nation’s Capital during the entire month.  October 2021 will finish with eight days of high temperatures in the 80s.  That’s tied with October 2018 for the highest such total in the last decade.  With a monthly average temperature (combining daily high/low temperatures) of 66.5° through October 30, this October will finish as D.C.’s second warmest on record.  That will give Washingtonians three of the Top 10 warmest Octobers in the last five years.

Washington, D.C.’s Warmest October’s (Source: NOAA)

1.  67.1° (2007)
2.  65.2° (1984)
3.  64.9° (2017)
4.  64.7° (1971)
5.  64.5° (1947)
6.  64.4° (1941)
7.  64.1° (2019)
8.  63.4° (1949)
9.  63.3° (1973)
10.  63.2° (1975, 1919)
12.  63.1° (2016)

Weather Quiz

 

Hurricane Irene  (Source: NOAA)

What infamous former tropical system brought record rainfall and flooding to the Nation’s Capital on this date in 2012 ?

 

A.  Hazel

 

B.  Isabel

 

C.  Irene

 

D.  Sandy

 

A Tale of Two Seasons

 

October 2021 Has Featured Plenty of Warm Weather

As temperatures begin to cool during the fall, it’s common to see a larger range in daily temperatures.  When the air is drier, it’s easier for it to heat up and cool off then when the air is more humid.  Another factor that contributes to a high degree of variability between daily high and low temperatures is the geography of a given region. 

Given uniform atmospheric conditions, a rural area will cool off more quickly than an urban area.  Conversely, an urban area will heat up more quickly than a rural area.  This is known as the “Urban Heat Island Effect.”  The Nation’s Capital often experiences the “Urban Heat Island Effect” during the fall months.  On the water near the center of town, National Airport is often much warmer at night than the more rural Dulles Airport.  That’s because a rural area does not capture as much heat during the day the way the buildings and asphalt do in a city.  

Since there is more vegetation around Dulles Airport that absorbs less heat, Dulles also cools off more quickly at night.  Under clear skies with calm winds, any heat that accumulated at the Earth’s surface during the day will radiate upward back into the atmosphere.  This is known as “radiational cooling.” 

Urban areas or areas on or near a body of water lose less heat at night due to radiational cooling than rural, landlocked areas.  For example, October 2021 has been a largely dry month with plenty of sunny, warm days and clear, cool nights.  This past Thursday, October 21 was a mostly sunny and dry day in the Nation’s Capital.  High temperatures reached 75° at National Airport and 77° at Dulles Airport.  However, that followed a chilly start to the day with a morning low temperature of 53° at National, while it was only 44° at Dulles Airport.

In conclusion, the “Urban Heat Island Effect” is a phenomena in which urban areas retain more heat compared to more rural areas.  At the same time, more rural areas away from a body of water will see more radiational cooling.  The daily average high/low temperatures for the month of September 2021 at National Airport were 81.7°/64.7°, while at Dulles Airport they were 80.8°/59.1°.