The Relationship between Air Temperature, mean Radiant Temperature and Albedo in the Reduction of Thermal Island in Cities

Urban population, urbanization growth, Reduce green spaces, High fossil fuel consumption, greenhouse gas emissions, and Use of inappropriate materials They have created a heat island over the cities That cause the city's air several degrees Celsius warmer surroundings. Nowadays, science has proven that there has been a lot of climate change throughout the life of the planet Earth, and many indicators show that human intervention in nature is accelerating natural processes. Humans have been known to be involved in the occurrence of many natural disasters including global warming, sea-level rise, forest destruction, ozone depletion, acid rain, and biodiversity decline. Population growth, urbanization, green space decrease, fossil fuels overuse, greenhouse gas emission, and the use of inappropriate building materials have caused the emergence of thermal island microclimates over cities which make cities warmer than their vicinities (villages) by several centigrade degrees.

This phenomenon is influenced by several factors such as city location, city size and population, urban density, urban geometry, thermal properties of urban surfaces, waterproof surfaces, human-caused heat and air pollution. Some of these factors require long-term urban planning, but others such as using appropriate building materials can be considered as an early yielding decrease strategy to mitigate the destructive effects of heat islands. Therefore, this paper, after identifying the factors affecting the creation of heat islands and introducing the decrease strategies, tries to examine parameters such as mean radiant temperature and air temperature in three high albedo (0.8), medium albedo (0.5) and low albedo (0.3). In this regard, by choosing Poonak street in Qazvin, which has a variety of building materials, and by studying through Envi_met simulation software.

Materials used in urban environments absorb and accumulate solar and infrared rays and transmit them to the atmosphere, so materials play an important role in reducing the reception, storage of heat, and its transfer to the urban environment.
 The findings show that, it was concluded that changes in the amount of ambient radiant and temperature power can have a significant effect on the mean radiant temperature and consequently on outdoor thermal comfort. The results show that although cool materials with high albedo are suitable for roadbeds and roofs, but high albedo materials in the façade, due to their high reflectivity, bring lower thermal comfort and warmer environments in cities. In cities with heat islands, cool materials can warm the environment, but in cities with no heat islands, use of high albedo materials reduces indoor energy consumption in summer.

The results indicate that in order to reduce the mean radiant temperature (the most important factor of thermal comfort), direct and indirect radiant flux values must be controlled. Therefore, with the decrease in the amount of albedo in the facade materials, the radiant flux of the environment decreases leading to a simultaneous decrease in air temperature and the mean radiant temperature (Tmrt). Materials with albedo have the highest reflectivity, as a result of which the materials remain cold and heat is not transferred to the interior; however high albedo is not suitable for facade materials in cities that have a heat island, because it causes an increase in ambient temperature. The solution is to use materials with low albedo; thus, the ambient temperature does not increase and because the heat from the materials to the interior is not transferred. Use suitable thermal insulation in the wall so that the heat absorbed by the materials is not transferred to the interior.