Urban heat island is the more documented phenomenon of climatic change. Experimental studies carried out the last 50 years have offered scientific evidence and knowledge on the characteristics of the urban heat island phenomenon in more than 400 cities around the globe.
Urban heat island phenomenon has an important effect on the quality of life of urban citizens. In particular, it highly affects the energy consumption spend for cooling purposes, thermal comfort conditions in cities, the concentration of pollutants, althogh it has a serious impact on health, economy and the global environmental quality.
Recent studies have shown that urban warming has a significant effect on the peak and global electricity demand in cities. As reported in , when ambient temperature increases by 1 K, the corresponding increase of the peak electricity load is between 0.45 and 4.6%. This is equivalent to an additional peak electricity load penalty of 21 (±10.4) W per person and degree. Also, the increase of the total electricity consumption varies between 0.5 and 8.5% per degree of temperature increase.
Analysis of the energy impact of urban heat island on buildings reported in , shown that in average urban heat island increases the cooling load of urban buildings by 13% compared with the load of similar buildings in adjacent rural areas. Analysis of all existing studies has also shown that average Global Energy Penalty per square metre of city surface is close to 2.4 kW h/m2 while the average Global Energy penalty per square metre of city's surface and degree of UHI intensity is 0.74 kW h/m2/K. Additionally, the mean Global Energy Penalty per person is 237 kWh/p and the Global Energy Penalty per Person and per degree of the UHI intensity is 70 kWh/p/K. Finally, between 1970 and 2010, the average increase of the cooling demand as reported by the existing studies is close to 23% while the decrease in the heating demand is close to 19%. In total, between 1970 and 2010 the total energy consumption of typical examined buildings has increased by 11% .
Urban warming has a serious effect on the concentration of gaseous pollutants and in particular ozone. Specific studies have shown that an almost linear correlation exist between the ozone concentration and the corresponding ambient temperature . In parallel, studies have shown that because of the urban heat island the ecological foot print of cities increases considerably . In particular, in Athens, it is calculated that the ecological footprint caused by the heat island ranges 1.5–2 times the city's political area.
Urban warming has a serious impact on indoor and outdoor thermal comfort and on the health of human beings. Various studies have shown that because of the urban warming outdoor thermal conditions are seriously deteriorated, while in urban areas thermal comfort conditions are seriously worsened during the recent years [5, 6]. In parallel, monitoring of low-income houses during heat waves shown that indoor thermal stress was extremely high and spells of about 216 continuous hours above 30°C were recorded . Urban warming affects human well-being and health in many ways. It is accepted that higher urban temperatures are increasing the number of hospital admissions and the human mortality rates. Most studies showed that during extreme hot periods the number of hospital admissions is increasing, without however to obtain a statistically significant correlation between discomfort levels and the number of visits in cardiology clinics . On the contrary, during high temperature events mortality rates increase significantly due to respiratory, cardiovascular and cerebrovascular problems .
To face the specific problems created by the global urban warming in a city, mitigation and adaptation techniques may be employed. Intensive research carried out in the recent years has provided evidence and proves that utilization of efficient mitigation technologies like the use of cool materials in roofs and pavements, the use of additional urban greenery like the creation of new parks and green spaces and the extensive use of green roofs can contribute significantly to cool cities, decrease their energy consumption and improve their global environmental quality .
The use of cool materials characterized by high reflectivity in the solar spectrum as well as high emissivity value seems to be an excellent option for roofs and pavements . In particular, cool roofs can decrease the cooling load of buildings without an important heating penalty both in southern and northern climates [12, 13]. In parallel, the extensive use of cool roofs in cities can contribute to decrease the ambient temperature up to 1.5 K . Cool pavements based on the use of reflective or water retaining materials have a very high mitigation potential . Large-scale applications of cool pavements have shown that ambient temperature may decrease significantly while thermal comfort may improve considerably .
The use of green roofs may contribute to important reductions of the energy load of the buildings but also to a significant decrease of the ambient temperature . Efficient planted roofs should present a high latent heat transfer and exchange to efficiently mitigate urban heat island. Urban parks and greenery contribute to lower ambient temperatures inside their periphery and the surrounding areas. Their mitigation potential is highly determined by the thermal balance of the considered area and examples of very high but also reduced mitigation potential are reported .
The present special issue of the Journal devoted to urban mitigation technologies aims to present some of the major scientific achievements on the topic. The issue includes eight papers of excellent quality describing the recent progress on various domains of the mitigation technologies.
I would like to thank the Editor in Chief of the Journal Prof S. Riffat for offering us the opportunity to prepare and publish this special issue.