Urban heat islands

Abstract

As global warming continues, we are facing another significant issue: The temperature increases during summer in urban areas due to a factor other than global warming. The urban heat island (UHI) is a phenomenon that occurs when the temperature in an urban area becomes higher than that in a suburban area. Moreover, as urbanization advances year by year, the area of the high-temperature zone continues to increase. The regional difference and time change of the temperature, which vary from hour to hour, are typical indexes used to indicate the UHI. While there are geographical factors that cause long-term fluctuations, artificial factors that cause short-term fluctuations also exist; therefore, we cannot fully understand this phenomenon by simply monitoring the temperature. Due to this, the numbers of days classified according to the day-to-day minimum and maximum temperatures in a year or in several months are sometimes used as a standard of climate change and the UHI. Furthermore, the degree hour (DH), which is an index for quantitative evaluation that uses both intensity and the number of hours beyond a certain constant temperature (the threshold), can show the characteristics of an urban area better.