Urban microenvironment affects indoor temperatures during heatwaves

Researchers at Aalto University and Finnish Meteorological Institute investigated how the indoor temperatures in residential buildings in Helsinki are affected by urban microenvironment during heatwaves. The study shows that greenery in the streets can consistently lower indoor temperatures both during short and long heatwaves.

Picture: Tuija Vuorinen

The impact of urban microenvironment on indoor temperatures was assessed by using three parameters: green view index, floor area ratio, and distance from the sea. Green view index is used to represent vegetation level on the sides of the street, the shading of trees and surrounding buildings at the height of 1–5 m. Floor area ratio on the other hand reflects the urbanization of the area and takes into account, for example, the building height. The distance to the sea reflects the effect of sea temperature on the local microenvironment.

The analyzed data was collected from over 2000 apartments in 409 buildings in the Helsinki region during heatwaves in 2018 and 2021. The buildings were separated into several groups of closely located buildings with the same level of urban microenvironment parameters to analyze their influence on the district level. From both years, four time periods were investigated: early summer, short and long heatwaves, and late summer.

During the short heatwaves in midsummer, the effect of microenvironment parameters was visible the most clearly. In the groups defined by the microenvironment parameters, the coolest group was on average 0.4 degrees below average; the warmest group was on average 1.2 degrees warmer than the coolest.

In the warmest group, the green view index was low, floor area ratio was high, and the apartment was near the sea. In the coolest group, the green view index was high, floor area ratio was low, and the apartment was far from the sea.

However, the indoor temperatures in the apartments near the sea were 0.3 degrees cooler than average in 2018, and 0.5 degrees cooler in 2021. Similarly, a high green view index lowered the indoor temperatures on average of 0.53 degrees in 2018 and 0.5 degrees in 2021.

The features of microenvironment can be utilized in city planning

In the Nordic region the buildings need to have both good insulation during the cold winters and good ventilation during the warmer periods of the summer. The study shows that high green view index has a lowering effect on the indoor temperatures both during long and short heatwaves.

When tackling the effects of heatwaves, the distance to sea is more difficult to take into account than the other parameters, but, according to the research, building new apartments near the coast helps the residents to endure heatwaves better. Overall, incorporating urban microenvironment analysis into building and city planning enhances sustainability and reduces overheating risks.

Researchers at the Finnish Meteorological Institute run high-resolution numerical weather prediction experiments with the HARMONIE-AROME model, and results from those experiments were compared to the observed indoor temperatures. The weather model experiments were performed in finer detail than in daily weather forecasts: the horizontal resolution of the model was 750 metres, while its resolution in the weather forecast is 2.5 kilometres. The results show a clear correlation between the modelled outdoor temperatures and the observed indoor temperatures and between the spatial distributions of the temperatures.

The numerical weather prediction model experiments performed at the Finnish Meteorological Institute were carried out in the HERCULES-project (Health, Risk and Climate change: Understanding Links between Exposure, hazards and vulnerability across spatial and temporal Scales), which is under the CLIHE-programme funded by the Research Council of Finland.

Further information:

Ilia Kravchenko, Aalto University, ilia.kravchenko@aalto.fi

Researcher Olli Saranko, Finnish Meteorological Institute, p. +358 50 529 9520, olli.saranko@fmi.fi

The scientific article is available in the journal Building and Environment.

Reference: Ilia Kravchenko, Azin Velashjerdi Farahani, Risto Kosonen, Simo Kilpeläinen, Olli Saranko, Carl Fortelius. (2023). Effect of the urban microenvironment on the indoor air temperature of the residential building stock in the Helsinki region, Building and Environment, Volume 246, 110971, ISSN 0360-1323

https://doi.org/10.1016/j.buildenv.2023.110971

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