When discussing the impact of global climate change on our daily life and health, the Urban Heat Island (UHI) effect is often mentioned as an additional source of heat stress for urbanize areas.
In fact, the term “Urban Heat Island” can be traced back at least to 1833 when Luke Howard published his book “The climate of London”. In this book, and in thousands of studies by other authors later, this phenomena refers to the fact, that during the night, the urbanized areas are warmer compared to the rural areas. There are many reasons for this and many of them changed over the centuries, but the core reasons for the UHI can be found in missing water evaporation, increased heat storage in buildings and the ground, reduced ventilation and the obstruction of the sky which hinders the heat to escape into the open atmosphere.
However, in its original meaning, the Urban Heat Island is a large scale phenomena (Urbanized Areas versus Rural Areas) and it is a night-time effect. Actually, during the day, urban areas in general can be cooler compared to the rural landscape due to the shading effects of buildings.
In the last years, especially in the context of climate change adaption of urban areas, the term “Urban Heat Island” is used in a more general and often smaller context to describe the effect of hot spots in urban areas at any time of the day.
With respect to urban planning and architecture, it is important to understand that there is no such thing as “A” urban microclimate. The microclimate of a city is a patchwork of very different small scale climates close to each other generated by the direct environment. Hot spots can be just a few meters away from cold spots and locations, which are too cool in the morning might be too warm in the evening. It is therefore inevitable to use numerical simulations to understand which impact urban planning or design decision will have on the open space microclimate.