heat island

medium fama

The urban heat island phenomenon refers to a local climate effect in which the temperature of a built-up area is noticeably higher than that of its surrounding regions. The term comes from the way temperature distribution isolines resemble the contour lines of islands in the sea.

Urban areas tend to heat up more quickly and cool down more slowly than their surroundings. This makes the temperature difference more pronounced at night than during the day, and in winter more than in summer. The effect is strongest under calm weather conditions, when weak synoptic winds fail to mix the air effectively. The phenomenon was first studied in the late 1810s by Luke Howard.

From an atmospheric radiative perspective, the heat island effect can be explained through blackbody radiation principles. According to the Stefan–Boltzmann law, the total radiant flux emitted by a blackbody is proportional to the fourth power of its absolute temperature. For the Earth, this principle, combined with atmospheric effects, leads to an average greenhouse warming of around 33°C. Kirchhoff’s radiation law further states that, for a given wavelength and temperature, a material’s absorptivity equals its emissivity, meaning that surfaces which absorb more radiation will also emit more.

Albedo, or reflectivity, plays an important role. Light-coloured surfaces, with high albedo, reflect a larger proportion of incoming solar radiation, while darker surfaces absorb more. In cities, grey materials, extensive glass surfaces, and asphalt tend to have low albedo, absorbing more shortwave solar radiation. By contrast, suburban areas, dominated by vegetation in green and yellow hues, reflect more shortwave radiation and have higher albedo.

Another contributing factor is the light-trapping effect. Buildings of varying heights create vertical surfaces and deep shadows. Sunlight entering between these structures is reflected multiple times in a zigzag path, increasing the likelihood of absorption. As a result, cities store more heat during the day. At night, this stored energy is released as longwave radiation, which, according to Kirchhoff’s law, is greater than that emitted by cooler suburban areas, sustaining higher urban temperatures even after sunset.

References

1.   The climate of London, deduced from Meteorological observations, made at different places in the neighborhood of the metropolis (Howard, 1818-20)
2.   An approach to heat island according to the law of Kirchhoff's (Weather Center of Seoul, 2019)
3.  The energetic basis of the urban heat island(Oke, 1982)
4.   LANDSAT image used Sejong Special Self-Governing City urbanization and heat island effect Analysis (Kim, 2014)

---