Mitigating urban heat Island effects with increased evaporation

Abstract

Heat stress of the natural enviroment as well as of the urban environment is increasing with climate change and global warming. The urban heat island effect is emerging in densely built up areas. We are approaching the problem of heat islands by focusing on the root causes and dealing less with the consequences. The significant role of the heat properties is at the center of our attention: the warming up of water and concrete surfaces with the same amount of energy. Concrete surfaces gets 5 times warmer within heat islands. Even more important is the evaporation of water.The process requires twice as much energy in the transformation. Analysing a precipitation event in summer we demonstrate the outstanding heat properties of water and its irreplacable role in the energy distribution process. We suggest to refine terminology and introduce terms ’passive shielding’ (where we cast a shadow e.g. by a canvas) and ’active shielding’ differentiating a case where vegetation provides the shade accompanied by evapotranspiration with cooling effect. These terms can help to promote the importance of green walls and roofs, arbors, parks, and contribute to increase the size of green areas. The objective of this paper is to call attention to the need of a balanced hydrological cycle, as well as to our responsibility to preserve or restore its smooth operation. It is a prerequisit for sustainability both in urban and in rural areas alike.