**4. Discussion**

Hanoi is one of the hottest cities in Asia. It is highly vulnerable to the detrimental consequences of heat islands on urban ecology and human health. On 4 June 2017, Hanoi experienced a heat wave to the hottest level in recent 40 years, reaching 42 ◦C [40]. According to the study by Nguyen et al. [41], Vietnam's average temperature has increased at a rate of 0.26 ◦C per decade since the 1970s, which is approximately twice the rate of global warming over the same period of time. Over the last century, Hanoi has been experiencing a drastic increase in population, high-speed urbanization, and transformation of natural forests into urban built-up areas. All of these socioeconomic and biophysical changes have had a critical impact on the urban microclimate. As a result, severe heat waves have become common during the summer months in recent years. UHI is mainly caused by the modification of land surfaces and the concentration of the population [42]. The process of land modification generally increases the use of materials that retain heat (concrete surfaces) and sacrifices air-conditioning elements such as trees and water surfaces and moreover, waste heat is generated by energy usage [43]. In tropical cities, where there are a great number of very hot days, UHI causes the temperature to rise dramatically, negatively affecting the quality of the living environment in these cities.

By assuming that the unit of urban land management is around 25 ha (510 × 510 m), on a very hot day such as 4 June 2017, the LST (◦C) can be predicted with respect to land-use composition using the regression model derived in this research, as shown in Table 10.


**Table 10.** Land-use composition-driven prediction of land surface temperature.

A substantial impact of land-use composition on LST and UHI is apparent from Tables 8 and 10. This analysis shows that the LST at each management unit (e.g., 25 ha) can be decreased from 45.23 ◦C (100% urban built-up cover) to 37.91 ◦C (100% vegetative cover), and then to 28.24 ◦C (100% water cover). The urban planner and designer indeed can neither change the LST directly to mitigate the UHI effects, and nor would it be possible to convert all urban built-up coverage in the cities to water bodies or vegetative areas. However, even a 20% conversion of urban built-up areas into vegetative (10%) and water bodies (10%) could reduce LST by 2.43 ◦C. The results of this research present an opportunity for urban planners and designers—the LST and associated effects of SUHI can be adjusted by managing the land-use composition and percentage coverage of the individual land-use types (%U, %V, and %W) in each urban land management unit. Reduction in the greenery has been described as a major cause of the rising temperature in cities by many researchers. It has been suggested that urban greening activities are the efficient nature-friendly solutions for mitigating UHI effects [44–46]. Such suggestions are also justified by the observed negative correlations between LST and NDVI, resulting in increased eco-environmental vulnerability [47–51].
