Closed isotherms indicating an area of the surface that is relatively warm; most commonly associated areas of human disturbance such as towns and cities. The physiographic analogy derives from the similarity between the pattern of isotherms and height contours of an island on a topographic map. Heat islands commonly also possess “cliffs” at the urban–rural fringe and a “peak” in the most built-up core of the city. The annual mean temperature of a large city (say 10⁶ inhabitants) may be 1°–2°C warmer than before development, and on individual calm, clear nights may be up to 12°C warmer. The warmth extends vertically to form an urban heat dome in near calm, and an urban heat plume in more windy conditions.