• Urban Heat Island

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A.  Definition

As the temperature in the urban area is higher than the surrounding suburban area (country area), the urban area has a lot of artificial heat, causing upward airflow. While the outward countryside area has a downward air flow, which causes urban winds are coming towards urban area.

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B.  Cause

Urban physical properties, material thermal properties, artificial heat generation, urban green house effect, reduced Albedo, reduced evaporation surface, reduced turbulent transmission, etc.

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C.  Generation Principle

This can be understood by comparing the surface energy balance of urban areas and suburban areas. The net radiative flux, sensible heat flux, latent heat flux, and storage heat flux at a horizontal surface satisfy the following system according to the energy conservation relationship.

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- Net radiative flux is determined by the shortwave radiation from the sun and the longwave radiant energy balance from the surface and atmosphere.

 

- Sensible heat flux represents the process of heat energy exchange caused by the difference in temperature between the surface and the atmosphere.

 

- Latent heat flux represents the process of exchanging energy used for the phase change of water when there is evaporation of moisture from the surface or when there is an increase in the action by vegetation.

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- Storage heat flux represents the energy exchange process inside a building or soil.

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Surface energy balance measured in urban areas and suburban areas has distinct characteristics.

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Urban Areas

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-  More solar radiation can be absorbed during the daytime due to the radiation trapping effect of narrow urban canyons. In the illustrated urban areas, sensible heat flux and latent heat flux occupy similar proportions. And the energy stored in the artificial structure, such as concrete and asphalt accounts for a high proportion. When the proportion of the city's impervious surface and vegetation decreases, the evaporation of soil or the increase in vegetation decreases, so the latent heat flux decreases. Instead, this energy appears as an increase in sensible heat flux and storage heat flux.

- Long-wave radiant cooling occurs during the night time, and the underground heat flux by energy stored in buildings and underground during the daytime has the effect of attenuating radiant cooling. In addition, at night, turbulence is weaker than during the day, and accordingly, sensible heat flux and latent heat flux due to turbulence show relatively low values.

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Suburban Areas

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-  The suburban area has a large vegetation area and water permeable soil, so the size of latent heat flux during the daytime is higher than that of sensible heat flux. In addition, vegetation occupies a large area in the suburban areas, so the storage heat flux is relatively lower than in urban areas.

- At night, long-wave radiated energy is emitted most, and the effect of underground storage heat is small, so surface radiant cooling is active.

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▷▶ As a result, the surface cooling rate of urban areas at night is attenuated by artificial structures and the storage heat of impervious surfaces, resulting in slower rises than suburban areas. Due to this, the difference in temperature between the urban area and the suburban area becomes larger as the night gets deeper.

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D.  Urban Heat Island Intensity

『Defined as the difference in temperature between urban and suburban areas, it is more pronounced at winter rather than summer and at night rather than day.』

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 < Modulator of Urban Heat Island Strength >

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1) Synoptic meteorological conditions

- wind speed

- cloudiness

- humidity

- solar radiation quantity

 

2) Urban morphology and structure

- city size

- building arrangement

- building aspect ratio

- building materials

- land cover and vegetation distribution

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3) City function

- energy/water usage

- anthropogenic air pollution

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4) City’s geographical location

- climate zone

- topography

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• Solar Radiation Quantity 

It is called a single ray of energy emitted from the sun to the earth. In the measurement of the quantity of the solar radiation, the measurement is performed by copying the quantity from 1 cm2 width perpendicular to the advancing direction of the solar radiation for one minute.

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Assuming there is no air on Earth, the amount of solar radiation is 1.94 cal per minute, a constant of the sun. The sun's ordinary water is not uniform, but changes to some extent. This value has been considered one of the main factors causing climate change on the earth's surface, and research has been active.

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In fact, the amount of solar radiation measured on the earth's surface is only about 70 percent compared to the amount of solar radiation outside the atmosphere because it is reduced by absorption and scattering of dust and water vapor.

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• Cloudiness

It represents the amount of clouds covering the whole sky. A day without any cloud is displayed as '0', and when the cloud is full, '10' is displayed, and 11 classes ranging from '0-10' are displayed according to the degree of cloud quantity. 

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The cloudiness measured by the Seoul area's longitudinal weather observation equipment was classified as "clear sky" when the amount of clouds is zero to two days, "partly cloud sky" when the amount of clouds is three to five days, "Most cloud sky" where the amount of clouds is six to eight days, and "Cloudy sky" when the amount of clouds is nine to ten days. 

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Usually, the cloudiness is measured by the eye, and the sky where the stars are invisible at night is assumed to be hidden in the clouds. If the sky is not seen due to the dense fog, the carriage is displayed as a parenthesis (10).

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The relationship between solar radiation and cloudiness

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-  In general, as the cloudiness increases, duration of sunshine decreases, and this relationship is especially consistent in autumn and winter. The highly correlated whether element of cloudiness is duration of sunshine.

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- The increase in solar radiation was mainly seen in clouds of broken cumulus, where the traffic was generally scattered at 4 to 9. The increase in solar radiation occurred when the optical thickness was less than 1.0 with a maximum of 37% or higher, even if the cloud covered the sun or covered the sun.

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- Therefore, the increase in solar radiation can only be seen when  the sun light of a certain intensity enters the gap between the clouds and multiple scattering and reflection of light occur.

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* duration of sunshine: The time when a solar light illuminates the surface without being blocked by clouds or fog. Marked as a unit of time, expressed as a value up to the tenth-percentile.

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• Mixing Ratio

It means the mass ratio of water vapor to dry air in the atmosphere. Usually, it refers to the number of g of water vapors that coexist with 1 kg of dry air. It is used as an indicator of the amount of water vapor in the atmosphere. Mix ratio is independent of changes in air pressure or temperature without water vapor access.

 

South Korea is basically humid with three sides surrounded by the sea. High humidity in the air traps heat in the atmosphere, so the temperature is higher than in places with low humidity.