Wyllie conceded that the instrument site in Central Park does not have perfect exposure. Nearby shrubs and trees could cause the minimum temperature to be a degree or two higher than if the thermometer were located in an open, grassy field. The Weather Service has been prevented from clearing the site by many groups that do not want the vegetation disturbed. This alone does not explain the 10 degrees Fahrenheit and even 15 degrees Fahrenheit disparities sometimes noted between minimum temperatures in Central Park and nearby cities. For this, we look to the heat island effect, whereby the air in large cities is often warmer than the air in the surrounding countryside.
Central Park is in the middle of one of the world's largest cities.
It is surrounded on all sides by multistory buildings, square miles of heat-absorbing concrete, and wide rivers to the east and west, all of which cool at night much less rapidly than bare ground in open country. This explains most of the observed anomaly. The thermometer in Central Park is almost surely recording an accurate minimum temperature. It is maintained in the same way as thermometers at other ASOS sites, including those in Bridgeport, Newark, Philadelphia, Atlantic City, and Baltimore. The heat island effect that has caused elevated minimum temperatures in Central Park has been at work for many decades and will surely continue.