An urban heat island is an elevation in the temperature of outdoor urban air during the daytime. This is a result of man-made structures, such as buildings and roads, that have replaced outdoor vegetation, such as trees. As a result, more heat is absorbed, and the temperature is warmer.
Urban surfaces and human activity are two main causes of urban heat islands. Urban surfaces are dark and absorb more sunlight. Human activity releases heat into the environment. Urban heat islands impair air quality and increase the amount of energy needed for air conditioning. Human health is impacted, as warmer temperature and reduced air quality can aggregate respiratory problems.