Powerful impact of the “Urban Heat Island” effect

 

DC's urban heat island effect

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 local geography. 

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 (UHI).”  The nation’s capital often experiences a pronounced urban heat island effect during the fall months.  Along the Potomac River and near the center of town, National Airport is often 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, the first half of October has been a largely dry month with plenty of sunny, mild days and clear, cool nights.  This past Thursday, October 12 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 52° at National, while it was only 45° at Dulles Airport.

It’s important to take into account the UHI effect when doing climate analysis.  For example, the nation’s capital and its suburbs are much more developed now than they were in the 1970s and 1980s and that contributes to a more intense UHI effect.  This, in turn, helped make the 2010s a much warmer decade in the nation’s capital than the 1980s were. 

In conclusion, the “urban heat island effect” is a phenomenon in which urban areas retain more heat compared to rural areas.  At the same time, more rural areas away from a body of water will often see more radiational cooling.