heat island

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The urban heat island phenomenon refers to a phenomenon in which the temperature of an area is relatively higher than that of other surrounding areas. It is named because it is thought that the isolines of the temperature distribution are in a considerable equivalence relationship with the contour lines of islands floating in the sea, and by definition it can be regarded as one of the local climates.

Compared to the surrounding areas, it heats up relatively quickly and cools down slowly, so the temperature difference is usually greater at night than during the day and in winter than summer, and is most noticeable when synoptic winds blow weakly, mixing heat north and south and weak confidence. The heat island phenomenon was first investigated and described by Luke Howard in the late 1810s.

heat island phenomenon can be obtained when the atmospheric radiological approach is taken.

When applying the concept of blackbody radiation to the Earth, Since the ratio of the absorptance and the reflectance of an object to radiation of the same wavelength at a constant temperature in thermodynamically thermal equilibrium must have a constant value regardless of the nature of the object, Planck, Stefan - Boltzman Law, E (energy) is absorbed, it must emit an equivalent value.

According to the Stefan - Boltzman law, the total radiant flux of an object emitted by a blackbody is proportional to the fourth power of the absolute temperature (K). Therefore, a greenhouse effect of 33 degrees is generated by the atmosphere.

In addition, according to Kirchhoff’s radiation law, when the wavelength and temperature are constant, the absorptivity and emissivity of a material are equivalent. The more radiation an object absorbs, the more it emits.

Albedo is used to describe the reflectivity of an object. A white object has a high light reflectance due to a large Albedo, and a black object has a small Albedo due to a higher absorption than light reflectance.

First, there is the light trapping phenomenon. Buildings in cities are dominated by gray, have fewer objects that can be reflected by using a lot of glass, and have a small albedo due to high absorption. On the other hand, the suburbs are dominated by trees, and have a high Albedo because they reflect a lot of shortwave radiation because they are green and yellow.

Buildings in the city center are not uniform in height, so there are vertical buildings and shadows between them. At this time, when light enters, reflection between buildings occurs and it is trapped inside the building in a Z shape, and in this process, the building absorbs solar shortwave radiation.

Kirchhoff’s radiation law, when the wavelength and temperature are constant, the absorptance and emissivity of a material are equivalent, so the heat absorbed by the city during the daytime emits a relatively larger heat radiation flux than the suburbs at night, resulting in a temperature higher than that of the suburbs.

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)

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