More heat-absorbing surfaces. Cities have lots of paved and dark-colored surfaces like roofs, roads, and parking lots, which absorb, rather than reflect, the sun’s heat.
Less evapotranspiration. With the removal of trees and vegetation the natural cooling effects of shading and evapotranspiration (the evaporation of water from leaves) is lost. Just one mature tree with a 30-foot crown transpires approximately 40 gallons of cooling water per day.pavement, and traffic. Greater concentrations of buildings and pavement produce greater temperature increases. Shade trees can significantly mitigate the urban heat island effect.
Less airflow. Tall buildings and narrow streets can heat air trapped between them and reduce airflow.
Equipment heat. Waste heat from vehicles, factories, and air conditioners can add warmth to their surroundings, further exacerbating the heat island effect.
It’s a problem because hotter air pumps up the demand for air conditioning—increasing the strain on electric supplies during times when demand is already at its peak; one-sixth of all energy consumed in the U.S. is used for cooling buildings. The higher temperatures also increase greenhouse gas emissions, water demand, and air pollution levels; and people are at greater risk of heat exhaustion, heat stroke, and other heat-related illnesses.
According to the EPA, 3-8% of urban electricity demand is used for the extra air conditioning required to compensate for the urban heat island effect.
