Autumn Freezes and Heat Islands
By Gannon Medwick
Tuesday, November 11, 2008
November 11, 2008
Of this list of cities (heavily-weighted on North Carolina), check out how many days have featured sub-freezing temperatures so far in autumn 2008:
Asheville: 9
Charlotte: 5
Greenville (PGV): 3
Raleigh-Durham: 2
Greenville (WNCT): 1
Tallahassee, FL: 1
Morehead City: 0
Hatteras: 0
Atlanta, GA: 0
Philadelphia, PA: 0
New York City, NY: 0
It may surprise you to learn that, of the non-North Carolina cities listed, Tallahassee has had a freeze and cities farther north like Atlanta, Philadelphia and New York have not. The “urban heat island” effect is the most likely reason for the seemingly odd statistic. Like islands of concrete and asphalt, urban areas usually retain daytime heat well into the nighttime. Large eastern U.S. cities exhibit strong nighttime heat islands and can stay above freezing on nights when the air in smaller cities and rural areas falls to 32 degrees or below.
The urban heat island effect can even be noticeable - on a smaller scale - within Eastern North Carolina. So far this autumn, the Greenville’s Pitt-Greenville Airport (PGV) has recorded temperatures of 32 degrees or less on three days, but here at WNCT, the temperature has fallen to 32 on just one occasion. Urban heat island effects may be used to at least partly explain this statistic. Greenville’s airport is surrounded by fields and swamps (open space) near the banks of the Tar River, but the WNCT studios are located just off Evans Street and Greenville Boulevard (heavily residential and commercial).
Urban heat islands aside, it’s typical that neither Morehead City or Hatteras has experienced its first autumn freeze yet. These places are near to a heat source more vast than any urban area on earth: the Atlantic ocean. Water temperatures of these bodies usually stay elevated well into the fall season. The warmer water positively influences the temperature of the nearby air. It may be late November or December until a strong enough blast of cold air reaches Eastern Carolina to produce freezing air temperatures right to the coast.
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