<Definition of UHI>
BACKGROUND
ⓒNASA/JPL-Caltech <Illustration of Urban Heat Island>
ⓒDENIAL101x, 2.4.2.1 with Kevin Cowtan <Heat in the City>
Urban heat island(UHI) effect, which stands for high temperature in the urban(built up areas) than that of rural areas, happened.
<Vertical range of Urban>
1. Above big city a thermal anomaly exists, which is expressed in a ‘heat island’ effect in daytime at least up to 500 m height and in nighttime up to 100 m. Above 100 m height at night an urban atmosphere is characterized by a ‘cool island’ due to ‘cross-over’ effect, i.e. an intersection of the urban and rural air temperature nocturnal profiles.
2. The intensity both of diurnal ‘heat island’ and of nocturnal elevated ‘cool island’ gradually goes to zero with a height. Thus, at the levels of 400-500 m the spatial differences between nocturnal and diurnal air temperature are statistically insignificant.
3. In daily average the ‘heat island’ was registered at the ground level and at 100 m height. At the level of 300 m thermal anomaly above Moscow region already wasn’t detected. Thus, the level of 300 m can be considered as the vertical range of the urban thermal anomaly in average of a day.
4. The urban ‘heat island’ intensity strongly depends on weather conditions including thermal advection.
<Advanced research about UHI>
There are results from advanced research : The temperature difference is usually most pronounced at (1) night rather than day, (2) in winter than in summer, and (3) when land has greenery (4) geographic surrounding (5)initial temperature is higher
1. Day and night
During daytime, both urban and rural areas have high temperature due to strong solar radiation energy. But during nighttime, there only exists Earth radiation energy. So urban where there are a stack of buildings, heat can’t be easily released. Thus, the rate of temperature change in the urban heat island effect would be strong at night rather than in the daytime
2. Season
Compared to summer, the temperature difference between urban and rural areas is severe during winter. During winter, the temperature can drop relatively longer than in summer because the sun sets quickly. In this situation, rural areas are releasing more heat than urban areas. During summer, the temperature difference between the urban areas and rural areas is small because both areas are heated by sun before the surface cools down after sunset.
3. Land use
If an area has lots of greenery, the temperature drop significantly. In another areas with few greenery, heat island phenomenon is evident because areas with few greenery can’t release heat as well.
4. Geographical surrounding
The water has higher specific heat, and also has higher heat of vaporization. Higher specific heat means that to change temperature, it needs lots of heat. So in the condition that there is the same amount of heat by insolation, then temperature change would be slower than the place with lower specific heat. By same principle, it can be explained because water has high heat of vaporization. So in the seacoast area, temperature change would be slower than basin.