Grades 6–8 · Earth & Space
Why can a city stay warmer than nearby countryside after sunset?
A city can remain warmer than nearby less developed land, especially after sunset. Roads and buildings absorb solar energy during the day and store some of it as thermal energy. They can release that energy later.
In the diagramBuilt surfaces store and later release energystored heat releasedshade + evaporation
Vegetated land handles energy differently. Plants provide shade and release water vapor, and evaporation carries energy away. A paved area with little vegetation often has less of this cooling route.
Building shapes can also change airflow and the view of the sky through which surfaces lose heat. Vehicles, cooling equipment and other activities add waste heat. These effects combine to produce an urban heat island.
The pattern varies between neighborhoods and from one weather situation to another. A shaded park and an exposed paved square need not have the same temperature. Understanding the energy routes explains why materials, trees and layout can influence local conditions within the same regional climate.
About this explanation
Starting knowledge: Distinguish sunlight from stored thermal energy and know that evaporation can transfer energy away from a surface.
Sources
- U.S. Environmental Protection Agency — Learn About Heat Island Effects