The term “urban heat island” refers to the fact that cities (urban areas) tend to get much warmer than their surrounding rural areas, particularly during the summer. This occurs because cities have more impervious surfaces, like roads and buildings, that absorb and retain more heat than natural surface (grass and trees). Also fewer trees in cities results in less cooling through shade and evapotranspiration. 

However, the coverage of impervious surfaces and trees is not uniform in most cities. This is the case for Baltimore, where there are fewer trees and more impervious surfaces in East Baltimore than other parts of the city, see maps below. Because of this the air temperature is not uniform across the city, and it is more appropriate to think of a city being made up of a collection of islands of heat (referred to as an urban archipelago).