heat island effect

Higher-temperature 'dome' created over an urban or industrial area by hot air layers forming at building top or chimney top level. This dome is about 5°C to 7°C (about 40°F to 45°F) warmer than air above it and the ground level temperature, and can trap all polluting emissions within its confines. Most noticeable at night and early morning periods, it normally disperses by midday when the ambient temperature increases. See also temperature inversion.