It has been known for some time that cities are generally warmer than the surrounding, more rural areas. Because of this relative warmth, a city may be referred to as an urban heat island.
The reason the city is warmer than the country comes down to a difference between the energy gains and losses of each region. There are a number of factors that contribute to the relative warmth of cities:
The urban heat island is clearly evident in a statistical study of surface air temperatures (Woolum, 1964). It is also apparent on cloud-free satellite images, as the 11 micron image produced with the Advanced Very High Resolution Radiometer (AVHRR) shows. The image has a spatial resolution of approximately 1 km. At this wavelength, the AVHRR measures the amount of radiant energy emitted by the surface and the tops of clouds, which is proportional to the temperature of the emitting body. The warmer the body, the greater the amount of radiant energy it emits. White portions of the image represent cold objects (e.g., cloud tops) and dark regions are warm areas.
A map is overlain the image to help orient you to the geography of the region. Find the following geographic regions: Lake Erie, Lake Ontario, Long Island, Chesapeake Bay, Delaware Bay, Hudson River, Finger Lakes, Appalachian Mountains.
Notice that on this day in April, the land is warmer (it appears darker in the image) than the Great Lakes and the Atlantic Ocean. Urban heat islands appear on the image as "dark blemishes." The city of Pittsburgh is marked to help you recognize the heat island feature. Find the following urban heat islands: Washington, New York Metropolitan Area, Philadelphia, Buffalo, Cleveland, Albany, Harrisburg, Richmond and Syracuse.
Myer, W. B., 1991: Urban heat island and urban health: Early American perspective, Professional Geographer, 43 No. 1, 38-48.
Woolum, C. A., 1964: Notes from a study of the microclimatology of the Washington, DC area for the winter and spring seasons. Weatherwise, 17, No. 6)