The Impact of Façade Orientation and Woody Vegetation on Summertime Heat Stress Patterns in a Central European Square: Comparison of Radiation Measurements and Simulations

Received Sep 11, 2017; Accepted Nov 12, 2017

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1. Introduction

Regional climate change is expected to bring rising air temperature values and to increase the frequency, length, and severity of heat waves in Central Europe, and thus in Hungary too [1, 2]. Combined with the peculiar climate of cities, characterized by increased air temperature and reduced ventilation due to the great amount of artificial materials, low vegetation rate, and the complex surface morphology [3], extreme heat events are expected to have more serious impacts on urban environments [4]. Without adaptation to heat waves people shall face deteriorating thermal comfort conditions, which in turn lead to declining working efficiency [5]. Moreover, intensification of heat stress is expected to increase the mortality rates, especially among the vulnerable groups, like infants, elderly people, and those with cardiovascular diseases [6]. In this respect it is worth emphasizing that, among the continents, Europe has the greatest percentage (24%) of its population aging 60 or over [7]. Furthermore, 73% of the European population already lives in urban areas, and by 2050 this proportion is expected to rise over 80% [7]. In the light of the mentioned warming, aging, and urbanization tendencies, mitigating the impact of extreme heat events should be one of the most important issues in urban planning [8–10].

Researchers in the field of urban human-biometeorology demonstrated that radiation heat load, quantified usually as mean radiant temperature ($T_{\mathrm{m}\mathrm{r}\mathrm{t}}$) [[11–13] and see Section 2.2], is the main factor of daytime heat stress in summer in midlatitudes, and therefore shading, that is, the reduction of $T_{\mathrm{m}\mathrm{r}\mathrm{t}}$ is the most effective mean of heat stress mitigation in outdoor urban spaces [14–18]. Field measurements and simulation studies conducted at various climate zones (continental, arid, and tropical) have shown that larger tree canopy coverage and higher street aspect ratio (that is, shading by buildings) are generally the most effective design strategies against urban heat stress [19–27]. Studies from cities with temperate climates commonly found that shading delivers the greatest human-biometeorological improvement [28–34]. It must be emphasized...