Cities suffer from higher temperatures

June 2017 broke quite a few weather records in Switzerland. Above all the month was extraordinarily hot, the second hottest June on records, peaking in a heat wave between 19 and 23 June. Cities heat up even more than the surrounding regions, an effect known as urban heat islands. To better understand the causes of - and, more importantly, develop effective mitigation measures against - urban heat islands scientists at Empa and ETH Zurich combined weather forecast models with the effects of buildings and streets and came up with a very detailed heat map of Zurich, which can be used to accurately predict local air temperatures. The latest heat wave in Switzerland in late June 2017 brings back memories of the heat waves of 2015 and 2003. Urban areas are most strongly affected by heat waves, since these regions experience an urban heat island (UHI) effect characterized by higher air temperatures compared to the surrounding rural environment. Reasons for urban heat islands are darker colors of, say, roofs and streets leading to a higher absorption of solar radiation energy, less shadowing, lack of evaporative cooling by vegetation, lack of open spaces, lack of ventilation and nighttime cooling due to urban densification.