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°.