Thesis document - Jana Milosovicova - Urban Design English

On the premise of appointed alleviation of the extreme summertemperature events in the cities in cool moderate climate zone,the aim is to eliminate the temperature differences betweenthe urban area and its surrounding environment by loweringthe urban temperatures. The subject of this Thesis is to composea set of measures that would help achieve this goal.2.2 Basic energy, heat and water cyclecharacteristicsTo understand better how the energy processes and the water cycleare influenced by cities (both affecting the temperatures and climate),a few factors related to the energy flux and water cycle in rural andurban areas in moderate climate are explained in this section. Thesefactors are the incoming solar radiation (and its interaction with waterand vegetation), storage heat, surface reflectance (albedo), and thewater cycle.Solar radiation and its impact onEarthIn the moderate zone, the annual input of solar radiationimpinging the surface of the Earth reaches avalue of about 1,100 kWh/m 2 per year. The amountof solar energy falling on the Earth’s surface ondays of clear skies differ from that on days of overcastskies – as solar radiation is being partially absorbedby gases and water vapour in the atmosphere(Kravčík et al. 2007, p. 24) (fig. 8). Of the amount ofenergy that reaches the Earth’s surface, about 7.3%is reflected back into space in the form of shortwaveradiation, 12.7% radiates as long-wave (thermal)radiation and 41.8% is consumed in the circulationof water for evaporation (Schmidt 2010). The latter,also called ‘latent heat’, is the only form of theEarth’s surface reaching transformed solar radiationthat is not connected with increasing the temperatures(Kravčík et al. 2007, p. 25). Especially in ruralareas, if solar radiation falls on a surface well stockedwith water, a significant part of the incident solar radiationis transformed into latent heat by plants inthe process of evapotranspiration. Only the remainingpart is converted into sensible heat and resultsin temperature differences, another small part beingreflected, or used for photosynthesis (Givoni 1998,p. 270), see also fig. 9.In sealed and drained urban areas, as in the figure10, most of the incident solar radiation is convertedultimately into sensible (thermal) heat, which raisesthe air temperature and contributes to the heat islandeffect (Givoni 1998, p. 270-271).Fig. 9 Global radiation balance as annual mean of one m 2 on earth(Schmidt 2010)Fig. 10 Global radiation balance of a black bitumen roof as indicatorfor urban radiation changes (Schmidt 2010)2 Literature review: The impact of built structures on climate, energy flow and the water cycle15