wet bulb globe temperature above 30 °C tripled between 1983 and 2016. It increased by ~50% when the population growth in these cities is not taken into account. Urban cold island The same urban area that is hotter in the day can be colder than surrounding rural areas at ground level at night, leading to a new term urban cold island. Snow cover in rural areas, for example, insulates plants. This was an unexpected discovery when studying the response of plants to urban environments. The urban cold island effect takes place in the early morning because the building within cities block the sun's solar radiation, as well as the wind speed within the urban centre. Both the urban heat island and urban cold island effects are most intense at times of stable meteorological conditions. Several other studies have observed the urban cool island in semi-arid or arid regions. The reason for this phenomenon is the availability of water and vegetation in the urban region as compared to the surroundings. See also Anthropogenic heat Cool roof Urban climatology Urban dust dome Urban reforestation Urban thermal plume Tropical night References Further reading External links Lawrence Berkeley National Laboratory Heat Island Group The Surface Temperature Record and the Urban Heat Island From RealClimate.org UrbanHeatIslands.com – Urban Heat islands in Canada and the world Cool Roof and Pavement Toolkit Global Cool Cities Alliance Urban Heat Islands – introductory video by Science Museum of Virginia Climatology Climate forcing Urbanization Regional climate effects