Observations of the Urban Heat Island Effect in New York City

Differences in the temperature fields through the lowest 700 m of the atmosphere in and around New York City during the hours near sunrise are analyzed. Data were obtained by an instrumented helicopter on 42 predetermined test mornings from July 1964 to December 1966. Results show urban surface temperature inversions to be less intense, and far less frequent, than those in the surrounding non-urban regions. A high frequency of weak elevated inversion layers at an average height of 310 m was observed over the city. The average intensity of the urban heat island, i.e., urban temperature excess, was a maximum near the surface and decreased to zero at 300 m. On mornings with relatively strong urban elevated inversion layers the heat island extended to well over 500 m. For more than two-thirds of the test mornings there existed an elevated “cross-over layer” in which rural temperatures were higher than urban temperatures. The magnitude of the cross-over effect was less than that of the heat island effect.