The term "Urban Heat Island phenomenon" in which the air the temperature of a city part increases more markedly in the inner city than that in the suburbs comes from the isothermal line that forms an “island”.
The heat island and its effect on human health, such as heat stroke and sleep disorders, resulting from the increased number of tropical days and tropical nights has been reported.
It is said that heat island phenomenon is also a contributing factor to the increased occurrence of torrential rain in the urban area.
In Tokyo, because of the global warming and the heat island phenomenon, the annual mean temperature has increased by about 3°C over the past 100 years.
In July 2005, Tokyo Metropolitan Government (TMG) prepared Guidelines for Heat Island Countermeasures, including a heat environment map, countermeasures by municipalities, and countermeasures to be implemented in the private business institutions and the residents of Tokyo can take effective measures. TMG issued these guidelines to provide helpful information as followings.