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Highlights: [•] Very low indoor temperatures in low and very low income housing in Athens Greece. [•] Very low energy consumption because of the energy crisis. [•] Important environmental social and health problems associated to low indoor temperatures. [ABSTRACT FROM AUTHOR]

Highlights: [•] We collected meteorological data from 26 fixed stations placed in Athens. [•] We examined the thermal comfort in five geographical zones. [•] Highest air temperature values are observed in the western and southern section. [•] Highest relative humidity is observed in June–July in the southern section. [•] Daily evolution of relative humidity is opposite to that of air temperature. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the indoor thermal conditions in fifty low income non air conditioned houses in Athens, Greece, during the extremely hot summer of 2007. Hourly indoor temperature data have been analysed during three extended heat waves. Indoor temperatures as high as 40°C were recorded and the average indoor minimum temperature was always above 28°C. Mean indoor temperatures during the hot season were up to 4.2K above the normal climatic period, while the daily increase rate varied from 0.2K/day during the first up to 0.8K/day for the peak days of the heat wave. The exposure of inhabitants to high temperatures is very important because for almost 85% of the hot period, indoor temperature exceeded 30°C. Periods of about 216 continuous hours above 30°C, and six days above 33°C were recorded in many buildings. The dynamics and the evolution of indoor temperatures as a function of the outdoor climatic characteristics and the thermal capacitance of the buildings are analysed. Possible techniques to improve the indoor environmental quality in low income housing during extremely hot periods are discussed. [Copyright &y& Elsevier]

Abstract: In summer 2007, in the city of Athens, Greece, extremely high air temperatures were recorded, inducing heat discomfort conditions in the urban environment. Four biometeorological indices were calculated in order to evaluate human thermal sensation and thermal comfort: Actual Sensation Vote (ASV), Thermal Sensation-Ginovi method (TS), Discomfort Index (DI) and Heat Load Index (HL). Data included measurements of ambient temperature, temperature of the surrounding ground surface, relative humidity, air pressure, wind velocity and solar radiation obtained from National Observatory of Athens (NOA) station. During this period the daily number of patients probably affected by heat in emergency department units of cardiac clinics of four public general hospitals in Athens was recorded. The results revealed high values of DI and HL indices, demonstrating severe heat stress conditions during the last ten day period of June and July, while the ASV tends to classify too many cases into the comfort zone compared to TS, DI and HL. The statistical analysis revealed a negative relationship between the number of heat affected patients and the estimated indices values. [ABSTRACT FROM AUTHOR]

Abstract: Data from 25 fixed temperature stations placed in the major Athens area have been used to analyse the characteristics of the heat island phenomenon during the summer season. The city has been divided in five geographic zones presenting different thermal balances and it is found that the differences between the mean and maximum daily air temperatures, among the five areas, were statistically significant, showing that the five areas of this analysis had definitely different temperature conditions. Higher air temperatures are found in the industrial western part of the city and also the center while the lower values were presented at the northern and the eastern parts. The intensity of the phenomenon is found to be close to 5C. Statistical methods have been developed and the mean and maximum nocturnal air temperatures as well as the number of hours where air temperature exceeds 30°C, can be estimated using the corresponding diurnal temperature values. The overall analysis is important to identify energy and comfort problems in the city and plan in a sustainable way the rehabilitation of the zones presenting important thermal problems. [Copyright &y& Elsevier]

The urban heat island effect and its consequences is one of the most important problems cities worldwide have to face in an effort to become more sustainable. This paper presents a study of a major open area in a densely built and populated area in the centre of Athens that was part of a Municipality of Athens program to improve thermal comfort conditions in open spaces. The bioclimatic rehabilitation design process was based on a specific strategy involving detailed monitoring of the thermal characteristics and climatic conditions in the area, extensive analysis of the results in order to define the problem, design and use of efficient passive cooling techniques (cool materials, green spaces, solar control and earth to air heat exchangers), and the use of advanced simulation tools to evaluate and optimise the planed microclimatic improvements. The results indicate that the proposed measures contribute to a decrease in local temperatures of up to 2°C for summer conditions.Practical application:• The knowledge on perceptions of microclimate in outdoor space can serve as a basis for urban spatial design.• To evaluate the application of cool materials in public urban squares.• Strategies to improve local microclimate with monitoring and modelling the design features. [ABSTRACT FROM PUBLISHER]

The present paper investigates the impact of canyon geometry on the temperature regime and nocturnal heat island development in the very dense urban area of Athens, Greece. Detailed measurements of air temperature have been carried out within three deep urban canyons of different aspect ratios ( H/ W = 3, 2.1 and 1.7) during the night period of the summer and autumn of 2007. An analysis was carried out to investigate the relative impact of the canyon geometry, the undisturbed wind velocity, ambient temperature, and cloud cover on the development of a nocturnal heat island. A clear increase of the median, maximum and minimum values of the cooling rates has been observed for decreasing aspect ratios. Under low ambient temperatures, high wind speeds correspond to a substantial rise of the cooling rate in the urban canyons mainly because of the increased convective losses. On the contrary, cooling rates decrease substantially under high-undisturbed wind speeds and ambient temperatures because of the important convective gains. The impact of cloud cover was found to be important as cloudy skies cause a substantial decrease of the cooling rates in the urban canyons. Comparisons were performed between the temperature data collected in the three studied urban canyons and temperatures recorded in an urban as well as a suburban open space station. [ABSTRACT FROM AUTHOR]

The intraurban temperature variation in the center of Athens, Greece, was investigated in relation to urban geometry. This paper describes two main tasks: 1) Air temperature was recorded in the center of Athens and at the Meteorological Service Station at the University of Athens. Experimental data were collected through extensive monitoring at four different heights inside five different urban canyons in the center of Athens during the summer period. A measurement uncertainty analysis was carried out to estimate critical threshold values of air temperature below which differences were not significant. 2) The correlation between urban–suburban air temperature differences was assessed, using the geometrical characteristics of each urban street canyon. Urban–rural air temperature differences were considered to be not important if they were below the threshold value of 0.3°C. It was concluded that the major factor controlling urban–suburban air temperature differences was the geometry of the urban area. Other factors were the orientation of the observational sites, the current weather conditions, and the inversion of air masses adjacent to the ground level. An increase in the value of aspect ratios leads to a decrease in the difference between air inside the canyons and at the suburban station. The air temperature profile in an open-space area was the most important defining factor for the stratification of the urban–rural air temperature differences. [ABSTRACT FROM AUTHOR]

The role of urban green areas in the microclimatic conditions of cities, during summer, is investigated in this paper through monitoring campaigns carried out at the National garden, at the city centre of Athens. Two types of investigations were carried out: i) a microscopic one that investigated the thermal conditions inside the Garden and the immediate surrounding urban area and ii) a macroscopic one that compared the temperature profile of the Garden with that of the greater city centre area. It was concluded that in microscopic level, the temperature profile inside the National Garden and the immediate surrounding urban area did not showed a clear evidence of the influence of the Garden and it was dependent on the characteristics of each location. In a macroscopic scale, the Garden was found cooler than the other monitored urban locations and temperature differences were mainly greater during the night, especially in streets with high building height to street width (H/W) ratio and low traffic, while in streets with high anthropogenic heat during the day, the biggest temperature differences were recorded during the day. [ABSTRACT FROM AUTHOR]

High-spatial resolution multispectral satellite images collected over the metropolitan Athens area in Greece were used to generate (a) a shortwave albedo map depicting the albedo spatial variations across the metropolitan area, (b) a fractional vegetation cover map showing the spatial distribution of urban vegetation and (c) a daytime and night-time land surface temperature (LST) map depicting the spatial variations of the surface temperature across the city. From LST maps, cooling and heating regions were identified and analysed to reveal relationships between surface heat islands and urban surface characteristics. Based on the data acquired with the use of satellite images and in order to better define the heat island problem and the mitigation measures that need to be taken, the most common building and paving materials used in the urban fabric of Athens were examined. Their optical properties were measured using a UV/VIS/NIR spectrophotometer fitted with an integrating sphere, an emissometer and their thermal performance was evaluated. Furthermore, measurements of the spectral reflectance help explore the possibility of increasing the near-infrared reflectance of materials in order to increase their total solar reflectance. The solar reflectance index of the samples was calculated in order to characterise them as 'cool' or 'warm'. Cool materials, with high albedo and thermal emittance values, attain lower surface temperatures when exposed to solar radiation, reducing the transference of heat to the environmental air. [ABSTRACT FROM AUTHOR]

The mitigation of the heat island effect can be achieved by the use of cool materials that are characterized by high solar reflectance and infrared emittance values. Several types of cool materials have been tested and their optical and thermal properties reveal that these materials can be classified as “cool” with the ability to maintain lower surface temperatures. Cool materials can be used on buildings and other surfaces of the urban environment. Based on these results, a modeling study was undertaken to assess the urban heat island effect over Athens, Greece, a densely populated city, by trying to analyze the impacts of large-scale increases in surface albedo on ambient temperature. Numerical simulations were performed by the “urbanized” version of the nonhydrostatic fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5, version 3-6-1). Two scenarios of modified albedo were studied: a moderate and an extreme increase in albedo scenario. It was found that large-scale increases in albedo could lower ambient air temperatures by 2°C. Furthermore, the impact of high albedo measures on heat island magnitude was estimated by creating a spatial representation of the urban heat island effect over the modeled area. The results of this study can help to promote the adoption of high albedo measures in building energy codes and urban planning regulations. [ABSTRACT FROM AUTHOR]

Abstract: Air flow and the associated indoor carbon dioxide concentrations have been extensively monitored in 62 classrooms of 27 naturally ventilated schools in Athens, Greece. The specific ventilation patterns as well as the associated carbon dioxide concentrations, before, during and after the teaching period are analysed in detail. During the teaching period, only 23% of the measured classrooms presented a flow rate higher than the recommended value of 8l/p/s while the mean daily fluctuation was close to 40%. About, 52% of the classrooms presented a mean indoor CO2 concentration higher than 1000ppm. The specific experimental data have been compared against existing ventilation rates and carbon dioxide concentrations using published information from 287 classrooms of 182 naturally ventilated schools and 900 classrooms from 220 mechanically ventilated schools. The relation between the air flow rates and the corresponding indoor carbon dioxide is analysed and then compared to the existing data from naturally and mechanically ventilated schools. It is found that all three data sets present a CO2 concentration equal to 1000ppm for air flows around 8l/p/s. Specific adaptive actions to improve the indoor environmental quality have been recorded and the impact of indoor and ambient temperatures as well as of the carbon dioxide concentration on window opening is analysed in detail. A clear relation is found, between the indoor temperature at which the adapting action takes place and the resulting air flow rate. In parallel, a statistically significant relation between window opening and the indoor–outdoor temperature difference has been established. [Copyright &y& Elsevier]

Abstract: For low ambient wind speeds, airflow in deep urban canyons is characterized by a high scatter and important fluctuation as no coupling is established between the undisturbed wind flow and the flow inside the canyon. Thus, thermal and mechanical forces determine the wind speed characteristics. Existing studies based on experimental comparison have shown that under the above boundary conditions, deterministic models may predict with sufficient accuracy the mean wind speed but not the fluctuation caused by the thermal phenomena. In the present paper, data have been collected through an extensive experimental campaign in seven canyons. Then, data driven techniques, to predict the more probable wind speed in deep urban canyons as a function of the prevailing thermal and inertia phenomena, have been developed. The proposed methodologies are strictly valid inside the limits of the experimental data, i.e. aspect ratios between 1.7 and 3.25, and can be used to estimate the more probable wind speed close to the facades of urban canyons. [Copyright &y& Elsevier]

Abstract: The present paper presents a computational methodology to calculate the wind speed in canyons when there is a coupling between the undisturbed wind speed and the airflow inside the canyon. This can happen when the undisturbed wind speed above the canyon exceeds a threshold values, i.e. 4m/sec. The proposed algorithms are presented in details. Experimental data collected through extensive monitoring in four urban canyons in Athens Greece, are used to validate the accuracy of the proposed model. A very good agreement between the experimental and the theoretical is found for most of the cases. The proposed model can be used to calculate the wind speed in canyons for natural ventilation purposes or for any other type of studies where the wind speed is required. [Copyright &y& Elsevier]

Abstract: This paper presents the results of an urban measurement campaign performed in a street canyon in Athens, Greece. A number of field experimental procedures were organized during hot weather conditions, on a 24-h basis for five consecutive days during July 2002. Wind velocity measurements were conducted inside and outside the street canyon together with air and surface temperature measurements. Based on the results of air and surface temperature measurements, a further analysis is performed for the investigation of airflow inside the canyon when the ambient flow is parallel, perpendicular and oblique relative to the long canyon axis. The observed airflow characteristics are associated with the impact of thermal effects mainly induced from ground heating due to the incident solar radiation. However, the role of the finite length canyon effects related to wind circulation near street intersections, on the observed airflow patterns, is also identified. [Copyright &y& Elsevier]

Heat island is a very well documented climatic phenomenon that has an important energy and environmental impact in the urban environment. The main energy problems are related to the important increase of the energy consumption for cooling purposes as well as to the important increase of the peak electricity load. Heat island in Athens, Greece, is measured during the last decade and its energy impact is calculated in details. The aim of the present paper is to estimate the direct and indirect environmental impact of the heat island effect in Athens. This is achieved through the estimation of the additional ecological footprint caused by the urban heat island phenomenon over the city. The ecological footprint estimation is performed at a first step by calculating the increase of the cooling demand caused by the heat island over the whole city and then by translating the energy to environmental cost. Two years annual experimental data from many urban stations have been used. The results show that the ecological footprint because of the heat island ranges 1.5-2 times the city's political area that have to be reserved every year to compensate the additional CO2 emissions caused by the presence of the heat island effect while the maximum potential ecological footprint, provided that all buildings are air conditioned, is almost 110,000 hectares. [ABSTRACT FROM AUTHOR]

Studies the existence of the heat island effect in urban places in Athens, Greece. Thermal structures between the urban and rural environments; Measurement of climatic parameters; District variations of the heat island intensity.

Assesses the impact of the urban climate on the energy consumption of buildings using climatic measurements from urban and suburban stations in Athens, Greece. Relative position of the measuring stations; Spatial and hourly distribution of the cooling degree hours in Athens for August 1996; Impact on the natural ventilation potential.

• Advanced mitigation technologies under heat-wave conditions. • Maximum cooling effect in the case of 'cool/reflective' roofs and roads. • Increase of latent heat by green roofs and a cooling effect during the night. • Cool/reflective and green roofs result in changes of air motion and circulation. • Deciduous broadleaf street trees have an evident reduction in surface temperature. We assess the impact of advanced heat-mitigation technologies in a coastal temperate city under heatwave conditions. For the first time urban-heating mitigation scenarios that refer to 'cool/reflective' roofs and roads, 'green/living' roofs and shading by replacing low urban vegetation with deciduous broadleaf trees are considered at the highly dense-populated city of Athens (Greece). Numerical simulations are performed for a typical see-breeze and a heatwave day with the Weather Research and Forecasting (WRF) model coupled to an urban-canopy model. Highresolution data on vegetation and urban land use, derived from satellite image analysis, are considered. All scenarios show a cooling effect, with the maximum mean daytime temperature reduction in the case of 'cool/reflective' roofs and roads. During daytime, the mean ambient-temperature reduction reaches up to 1 °C while for the surface-temperature up to 9.5 °C and 11.5 °C, on the see-breeze and heatwave day respectively. In the case of 'green/living' roofs, the mean daytime latent-heat flux is increased (e.g. up to 140 W/m2 on the heatwave day) due to increased evapotranspiration while the surface temperature is more affected during nighttime. Both scenarios result in a sea-breeze attenuation of 0.5–1 m/s. The presence of deciduous broadleaf street trees has a minor impact on mean ambient temperature but an evident reduction in surface temperature. The mean urban-heating reduction ranges from 0.1 °C to 0.8 °C and from 0.3 °C to 1.7 °C during the sea breeze and heatwave day respectively, with the maximum reduction shown in 'cool/reflective' roofs and roads and the minimum in 'shading trees' scenarios. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes an effort to estimate the impact of a cool roof (characterised by increased solar reflectance compared to the initial roof and high infrared emittance) on the energy performance and thermal behaviour of a non-cooled school building in Athens, Greece. An experimental monitoring campaign has been carried out before and after the application of a white elastomeric coating with a solar reflectance of 0.89 on the roof of the building. The air and surface temperature of the building have been measured and a set of meteorological parameters have been collected. An infrared camera has been used to depict the differences in the surface temperature before and after the cool roof application. The monitoring results are complemented and supported by building simulation. The building has been modelled into TRNSYS software and the model has been calibrated and validated using the experimental data. Simulation results show a decrease in the air temperature in the classes that reaches 2.8°C and a decrease in the annual cooling load by 40%. The corresponding heating penalty (i.e. the increase of heating load) is 10%. Additionally, the impact of the cool roof on thermal comfort, peak power and surface temperature has been assessed. [Copyright &y& Elsevier]

The present paper describes a holistic energy efficient retrofit of low income multiple social housing located in Athens, Greece. A holistic analysis was conducted in order to determine the optimum retrofit plan that includes innovative and state of the art commercially available technologies, passive techniques as well as renewable energy sources, aiming to reduce its energy consumption and carbon footprint, improve indoor environmental conditions and be cost effective at the same time. An extensive experimental campaign including air leakage measurements, thermal imaging, energy consumption and indoor environmental quality measurements was conducted before and after the implementation of the retrofit. In addition, advanced building simulation, occupant surveys and socioeconomic analyses were performed in order to evaluate the impact of the retrofit and estimate specific performance indicators. [ABSTRACT FROM AUTHOR]