Article
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Outdoor daylight conditions in Burgos, Spain, are studied throughout a full year. The CIE standard sky type is selected in accordance with the lowest RMSD (Root Mean Square Deviation) following the comparison of both the theoretical and the experimental luminance distributions in the sky hemisphere. The selection is based on luminance distribution data, recorded every 30 min, from 145 patches of the sky hemisphere. The original criterion to determine the type of sky, the SSLD (the Standard Sky Luminance Distribution), is difficult to apply in certain places and at times when the solar elevation is higher than 35°. In consequence, two alternative procedures are used and compared in this study: the Tregenza method and the Normalization Rate (NR) introduced by Littlefair. The selection was taken from luminance distribution data of 145 patches of the sky hemisphere recorded between June 2016 and May 2017. The most frequent sky type observed in Burgos was V.5. (cloudless polluted with a broad solar corona), with a frequency of occurrence close to 20%. Notwithstanding that observation, the group of clear skies exhibited a higher frequency (in almost 50% of the cases under study, using both methods). The skies above Burgos were of an overcast sky type in less than 25% of cases, a situation with a higher likelihood in winter and in autumn, while in spring and summer the skies tended to be clear and cloud free. Both of the methodologies showed similar results in percentage terms and in confusion matrixes with almost insignificant differences when compared on a monthly, a seasonal, and an annual basis. Nevertheless, some mismatches were located in the highest solar elevation values.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Therefore, the CIE standard for sky taxonomy was selected as the reference for atmospheric conditions. A complete description of the CIE standard sky classification model and the procedure for obtaining a CIE standard sky classification from sky scanner measurements can be found in previous works [62,63,81,82]. Figure 2 shows the frequency of occurrence of each CIE standard sky type in Burgos, Spain, during the experimental campaign. Skies in Burgos are predominantly clear, the most frequent sky type in the city being CIE standard sky type 13 (cloudless polluted with a broader solar corona), with a Frequency of Occurrence (FOC) higher than 18%. ...
... As can be observed in Figure 3, the fifteen CIE categories are grouped into overcast (CIE categories 1 to 5), partly cloudy (6 to 10), and clear skies (11 to 15); the clear sky conditions have the highest FOC (44.63%). This result concurs with previous experimental campaigns developed in Burgos between 2016 and 2022 [82,83]. As CIE standard sky classification data around the world are quite scarce, an alternative sky classification model, based on the k t classification, was prepared. ...
... As can be observed in Figure 3, the fifteen CIE categories are grouped into overcast (CIE categories 1 to 5), partly cloudy (6 to 10), and clear skies (11 to 15); the clear sky conditions have the highest FOC (44.63%). This result concurs with previous experimental campaigns developed in Burgos between 2016 and 2022 [82,83]. As CIE standard sky classification data around the world are quite scarce, an alternative sky classification model, based on the classification, was prepared. ...
Article
Full-text available
Solar Ultraviolet Radiation (UVR), which is identified as a major environmental health hazard, is responsible for a variety of photochemical reactions with direct effects on urban and aquatic ecosystems, human health, plant growth, and the deterioration of industrial systems. Ground measurements of total solar UVR are scarce, with low spatial and temporal coverage around the world, which is mainly due to measurement equipment maintenance costs and the complexities of equipment calibration routines; however, models designed to estimate ultraviolet rays from global radiation measurements are frequently used alternatives. In an experimental campaign in Burgos, Spain, between September 2020 and June 2022, average values of the ratio between horizontal global ultraviolet irradiance (GHUV) and global horizontal irradiance (GHI) were determined, based on measurements at ten-minute intervals. Sky cloudiness was the most influential factor in the ratio, more so than any daily, monthly, or seasonal pattern. Both the CIE standard sky classification and the clearness index were used to characterize the cloudiness conditions of homogeneous skies. Overcast sky types presented the highest values of the ratio, whereas the clear sky categories presented the lowest and most dispersed values, regardless of the criteria used for sky classification. The main conclusion, for practical purposes, was that the ratio between GHUV and GHI can be used to model GHUV.
... In 2003, CIE [22] and the International Organization for Standardization (ISO) [23] adopted 15 Standard Sky Luminance Distributions (SSLD) as the most versatile definition of skylight in various localities and daylight climate regions, making it possible to simulate annual daylight profiles in absolute units based on typical luminance sky patterns. Despite the high interest in those measurements, very few studies at only a handful of European [24][25][26][27][28][29] and Asian [15,30,31] locations have been conducted to characterize the sky under the CIE standard, mainly due to scarce sky luminance data obtained from sky scanner devices. Different alternatives to the use of sky scanners have been proposed for classifying the skies [32] including the use of different climatic parameters [33-36], vertical [12,20] and horizontal illuminance [37,38], and satellite data [39]. ...
... As has been previously stated, when CIE standard classification is available, the sky luminance distribution can be obtained from Equation (11), and Equations (7)-(10) can be applied to calculate L dh , L dv and, therefore, VSC. In this work, the CIE standard sky type in Burgos was determined between September 2016 and January 2019, following the procedure described in a previous paper [26]. The frequency of occurrence (FOC) of each sky type during the period under study is shown in Figure 5. ...
... 2020, 10, x FOR PEER REVIEW 9 of 15 As has been previously stated, when CIE standard classification is available, the sky luminance distribution can be obtained from Equation (11), and Equations (7)-(10) can be applied to calculate L dh , L dv and, therefore, VSC. In this work, the CIE standard sky type in Burgos was determined between September 2016 and January 2019, following the procedure described in a previous paper [26]. The frequency of occurrence (FOC) of each sky type during the period under study is shown in Figure 5. ...
Article
Full-text available
Daylight availability knowledge is the first step for an energetic and visually efficient building and city design. It can be estimated with the Vertical Sky Component (VSC), which is defined as the ratio of the vertical diffuse illuminance over the unobstructed horizontal diffuse illuminance, simultaneously measured at the same point. These illuminance magnitudes are obtained from luxmeter measurements but these data are scarce. Alternatively, VSC can be obtained from prior knowledge of the sky illuminance distribution, which can be measured with a sky scanner device or by reference to the CIE (Commission Internationale de L’Éclairage) Standard classification for homogeneous skies. Both approaches are compared in this study. The coherence of the results obtained for the four cardinal orientations are analyzed by applying classical statistical parameters and luxmeter measurements as references for the results. The measurement campaign was completed between September 2016 and January 2019 in Burgos (Spain), as representative case study and specific contribution of this work. It was observed that the VSC values were higher than 100 in many cases: 21.94% for the south- and 33.6% for the east-facing vertical surfaces. The study highlights the good daylighting conditions in Burgos, mainly due to the predominance of clear skies over much of the year. This fact implies high daylight availability that, with efficient city planning and building design, could potentially lead reduction energy consumption of buildings, improvements in visual comfort, and the well-being of occupants.
... Literature reports works done in line with evaluating most probable skies at few sites: Hong Kong [26], Singapore [30], and Spain [45] over long scan periods. These works have categorized occurring ambient sky at respective locations using Root Mean Square Deviation (RMSD) approach -the lowest RMSD between theoretical and experimentally measured angular discretized incident illuminance distribution for a dataset was the identifier for closest sky-type -and thus frequency of occurrence of either of the sky-types over the scan period was analyzed. ...
... This is done so the simulation programs can predict indoor illuminance distribution with higher accuracy. Sky luminance studies in literature have reported the use of commercially available daylight scanners (Eko MS 300LR [26,30]/MS 321-LR [45]) for measurement, where the cost of device is a hindrance towards wider reach. In this work, we have used a structurally modified Fresnel-like version of our previously proposed LDR-based low-cost sky-luminance measurement device [40] for measuring sky-luminance every 5-min over 5-month period in Mumbai, India. ...
... While the RMSE approach taken for identification of closest sky-type is inherently simple, the novelty is in analysis of originally measured data to present diurnal variations between sky-types, which is presented for few sample days. RMSE based approach for defining Frequency of Occurrence has been used for Hong Kong (2003) [26], Singapore (2007) [30] and recently for Burgos, Spain (2018) [45] using commercial sky-luminance measurement setup, and we have used a low-cost device for analyzing sky-luminance measurements for Mumbai: identifying the Frequency of Occurrence along with Diurnal Variation in sky-types. ...
Article
Standard sky-types of the established Commission International de l'Eclairage (CIE) sky-models are employed for generating sky-luminance distribution datasets in most building daylight simulation packages, which are used for predicting indoor illuminance distribution. However, sky-type can vary on diurnal and seasonal basis; and there is a need for identifying closest sky-type at different time-points for better indoor illuminance predictions. This paper aims at recommending closest sky-model for measured angular sky-luminance distribution sets through comparison with established CIE standard skies. A novel design of sky-luminance measurement device using calibrated Light Dependent Resistors is proposed to measure incident illuminance from 265 discrete sky-positions every 5 min. Statistical analysis is performed on 143,100 distinct data-points corresponding to 540 representative datasets measured across 5-month period at Mumbai, and the closest CIE sky-type for various hours is recommended based on frequency of occurrence. A range of 9 sky-types of the CIE sky-model was determined as closest model sky-set, with sky-types 2, 4, 5, 6, 7, 8, 10, 11 and 13 assessed as closest sky-type for different time-points across the measured datasets. CIE sky-type 8 is identified dominating over 50% of measured datasets — 51.6% in November, 53.6% in January, 52.6% in March and 71.5% in April. Measured sky distribution is observed to generally follow trend of overcast to intermediate to clear sky-types in morning hours, and opposite in evening hours. This approach of recommending closest sky-type as simulation input — as against using generic sky-types — can enhance the accuracy of building indoor daylight predictions.
... Studies on sky luminance distributions have been carried out in a few areas around the world, such as England (53. 38°N ( Bartzokas et al., 2003Bartzokas et al., , 2005Markou, 2005;Kobav et al., 2009;Ferraro et al., 2011;Torres et al., 2010aTorres et al., ,2010bSuárez-García et al., 2018;Janjai et al., 2010;Mettanant et al., 2017;Wittkopf and Soon., 2007;Li et al., 2004;Ng et al., 2007;Luo et al, 2015). ...
... In addition, some abnormal measurement data were removed before data analysis ( Fig. 3) (Zi, 2020). Measurement scans with solar altitude angles of equal to or lower than 5° were excluded from analysis (Suárez-García et al., 2018). ...
... The Tregenza method appears to be more stable and effective, as it takes these potential issues into consideration. The method has been applied successfully by many researchers (Ng et al., 2008;Torres et al., 2010Torres et al., , 2010Suárez-García et al., 2018). ...
Article
Full-text available
Sky type classification is a significant element in daylight simulation. As a ‘winter city’, the daylight climate of Harbin is distinctive, and there is therefore a particular need to determine the most appropriate sky models to establish sky luminance distribution in the city and thereby improve the accuracy of daylight simulations. To determine the closest sky type to that of Harbin during winter, sky luminance data consisting of 145 sky elements was collected by a sky scanner at 10 min intervals from June 2018 to February 2019. The sky luminance data was compared with the 15 standard sky definitions, varying from overcast sky to clear sky, proposed by the Commission International de l’ Eclairage (CIE). The sky with the lowest RMSE was selected as the most appropriate sky type. The results show that clear skies prevail in Harbin during winter, with an occurrence of 70.56%. The dominant sky type is type 12, with an occurrence of 48.57%, followed by types 11, 10, and 8 with occurrences of 17.67%, 5.81%, and 5.36% respectively. The sum of the occurrences of these four sky types exceeds 77%, which means that they can be regarded as generally representative of Harbin’s skies during winter.
... Figure 2 shows the location of the meteorological station on the flat roof of the EPS building, where the climatic parameters are measured: ambient temperature, relative humidity, atmospheric pressure, wind speed and direction, and rainfall. A complete description of the experimental facility and its location can be found in previous papers [33,42]. dependency on sky conditions. ...
... Figure 2 shows the location of the meteorological station on the flat roof of the EPS building, where the climatic parameters are measured: ambient temperature, relative humidity, atmospheric pressure, wind speed and direction, and rainfall. A complete description of the experimental facility and its location can be found in previous papers [33,42]. Global horizontal irradiation, data were measured by a pyranometer (model SR11, Hulseflux, Delft, The Netherlands,). ...
... The Normalization Ratio (NR) introduced by Littlefair [44,45] in the original Standard Sky Luminance Distribution (SSLD) method [46], detailed and described in a previous paper [42], was used to determine the CIE standard sky types over Burgos between April 2019 and January 2020. The Frequency of Occurrence (FOC, %) of each sky type during the period under study is shown in Figure 3. ...
Article
Full-text available
Plant growth is directly related to levels of photosynthetic photon flux density, Qp. The improvement of plant-growth models therefore requires accurate estimations of the Qp parameter that is often indirectly calculated on the basis of its relationship with solar irradiation, RS, due to the scarcity of ground measurements of photosynthetic photon flux density. In this experimental campaign in Burgos, Spain, between April 2019 and January 2020, an average value of the Qp/Rs ratio is determined on the basis of measurements at ten-minute intervals. The most influential factor in the Qp/Rs ratio, over and above any daily or seasonal pattern, is the existence of overcast sky conditions. The CIE standard sky classification can be used to establish an unequivocal characterization of the cloudiness conditions of homogeneous skies. In this study, the relation between the CIE standard sky type and Qp/Rs is investigated. Its conclusions were that the Qp/Rs values, the average of which was 1.93±0.15 μmol·J−1, presented statistically significant differences for each CIE standard sky type. The overcast sky types presented the highest values of the ratio, while the clear sky categories presented the lowest and most dispersed values. During the experimental campaign, only two exceptions were noted for covered and partial covered sky-type categories, respectively, sky types 5 and 9. Their values were closer to those of categories classified as clear sky according to the CIE standard. Both categories presented high uniformity in terms of illumination.
... e experimental data used in this work were recorded at a meteorological weather station located on the roof of the Higher Polytechnic School building at Burgos University (42°21′04″N; 3°41′20″O; 856 m above mean sea level). A complete description of the meteorological facility may be found elsewhere [1,10,38]. e experimental equipment is shown in Figure 4. e sky luminance distribution for characterization of sky conditions according to the CIE Standard General Sky classification was measured with a commercial MS-321LR sky scanner (EKO Instruments Europe B. V. Den Haag, e Netherlands). e sky scanner was adjusted on a monthly basis for taking measurements from sunrise to the sunset. ...
... e first and last measurements of the day (α s ≤ 5°) were discarded, as measurements were higher than 50 kcd/m 2 and lower than 0.1 kcd/m 2 , following the recommended specifications of the sky scanner equipment. e [39], detailed in a previous paper [38], was used to determine the CIE standard sky types over Burgos during the experimental campaign. A total of 1,500 images were selected from the experimental dataset (more than 80,000 sky images), 100 from each CIE sky category, which were characterized by greater concordance with the CIE pattern for that category. ...
Article
Full-text available
Digital sky images are studied for the definition of sky conditions in accordance with the CIE Standard General Sky Guide. Likewise, adequate image-processing methods are analyzed that highlight key image information, prior to the application of Artificial Neural Network classification algorithms. Twenty-two image-processing methods are reviewed and applied to a broad and unbiased dataset of 1500 sky images recorded in Burgos, Spain, over an extensive experimental campaign. The dataset comprises one hundred images of each CIE standard sky type, previously classified from simultaneous sky scanner data. Color spaces, spectral features, and texture filters image-processing methods are applied. While the use of the traditional RGB color space for image-processing yielded good results (ANN accuracy equal to 86.6%), other color spaces, such as Hue Saturation Value (HSV), which may be more appropriate, increased the accuracy of their global classifications. The use of either the green or the blue monochromatic channels improved sky classification, both for the fifteen CIE standard sky types and for simpler classification into clear, partial, and overcast conditions. The main conclusion was that specific image-processing methods could improve ANN-algorithm accuracy, depending on the image information required for the classification problem.
... These standard annual datasets need to be created at various global locations, and need devices for measurement of angular luminance distribution of sky-hemisphere. Literature provides designs for measuring luminance of pre-defined discrete sky-patches (Casares, Ramírez-Faz, & López-Luque, 2014;Kider, Knowlton, Newlin, Li, & Greenberg, 2014;Komar, Rusnak, & Dubnicka, 2013;Shahriar, 2010), and various works have employed a commercial system for sky-analysis (Li et al., 2003;Suárez-García, Granados-López, González-Peña, Díez-Mediavilla, & Alonso-Tristán, 2018;Wittkopf & Soon, 2007) at few locations. However, these devices are cost-intensive and/or lack the ruggedness for continuous operations due to use of fragile and cost-intensive parts like spectroradiometers. ...
... Using RMSD approach (Li et al., 2003;Suárez-García et al., 2018), the normalized luminance values for each time-stamp were compared with the 15 modeled-datasets for the 265 sky-positions. The sky-type with minimum deviation with measured data was selected as the closest CIE sky-type for the time-point. ...
Conference Paper
Full-text available
Building design optimization needs information of dynamic angular sky-luminance distribution, through measurement or models, for simulating indoor illuminance. Most simulation packages use standard sky-types of the established Commission International de l'Eclairage (CIE) sky-models for calculating sky-luminance data. However, owing to dynamic conditions of sky, there is a need for more realistic sky-data incorporating solar contribution, as compared to standard overcast skies. This paper presents a low-cost device for measuring angular sky-luminance distribution, and compares the measured datasets with established CIE standard skies. Proposed calibrated Light Dependent Resistor based device is used to measure the sky-luminance at various angles over time, and statistical analysis is performed on the measured datasets to determine the frequency of occurrence of various sky-types. Indoor predictions based on various datasets are calculated to assess accuracy of closest determined sky-type, and the applicability of the device for predicting indoor illuminance is demonstrated.
... In this study, a review of the different MIs used for sky classification and their benchmarking has led to the selection of those calculated with three standard variables recorded at many meteorological stations: global, beam, and diffuse irradiance. Sky conditions and characterization of CIE Standard sky types were reported in a previous publication with a full year of data recorded at Burgos, northwestern Spain, (Suárez-García et al., 2018). The sky classifications using these MIs were compared to the CIE cloudiness categories (cloudy, partial, and clear), adapting the original intervals to match the number of categories. ...
... Various procedures to circumvent this issue are proposed, using various methods of normalization (Li et al., 2014). In a previous paper (Suárez-García et al., 2018), the Normalization Ratio (NR) introduced by Littlefair (Littlefair and Paul, 1994;Littlefair, 1994) was used to obtain the CIE Standard sky types in Burgos, Spain. ...
Article
Sky classification is a complex problem, due in part to such abstract conceptual definitions as clear, intermediate, and overcast, as well as other intermediate ranges. The CIE (Commission Internationale de L’Éclairage) Standard classification offers a solution to this problem, although its application requires data on the luminance distribution of the whole sky that are less commonly available. A benchmarking and classification system of ten meteorological indices is introduced in this study to classify the sky types from overcast to clear. The indices can be calculated from measurements of global, diffuse, and direct irradiance that are widely available from meteorological ground stations. The classification system uses confusion matrices, a machine-learning tool that generates a visual display of the results of supervised-learning algorithms. The CIE Standard skies classification, applied to half hourly sky-scanner measurements in Burgos (Spain), over the period June 2016 - May 2017, is used in this study as a baseline reference for a comparative review of the results from the meteorological indices and their results. They are classified by four performance ratings: Accuracy, Jaccard, Cohen, and Matthews, which feature both classification similarity and the randomness of any agreement. All meteorological indices yielded a high average degree of accuracy - close to 80% - in a detailed review of their classification. Neverthless, the results suggested that Perez’s Clearness Index based on global, diffuse and direct radiation measurements offered the most precise classification of the skies, followed closely by the Klucher Clearness Index and the Perraudeau Nebulosity Index.
... An underlying assumption is that the luminance efficacy is identical in sky directions of different azimuth and altitude angles, which is supported by works on diffuse luminance efficacy Soler, 2001, 2003), and has been successfully used in studies on the vertical radiation (Alshaibani, 2011;Li et al., 2011). The CIE Standard Sky model has been proven to represent the worldwide sky conditions well (Lou et al., 2019b;Suárez-García et al., 2018), and can estimate the illuminance accurately (Kim and Kim, 2019;Yun and Kim, 2018). However, the CIE Standard Skies determine the luminance (or radiance) distribution only, and the irradiance (W/m 2 ) and illuminance (lux) on a tilted plane should be determined by numerical integrations (Tregenza and Waters, 1983). ...
... The sky elements around the horizon were around one-third of that on the sky zenith for Sky 1, and the maximum ISC difference between Skies 1 and 5 for β = π/2 was around 0.12. In this connection, E βD on vertical planes tends to be lower than half of E HD under the overcast skies, considering the Sky 1 tends to be the frequent overcast sky in many places of the world (Luo et al., 2014;Ng et al., 2007;Suárez-García et al., 2018;Tregenza, 2004;Wittkopf and Soon, 2007). Estimating E βD by a uniform assumption leads to a maximum error as high as 0.12/0.379 ...
Article
Full-text available
The CIE (International Commission on Illuminance) Standard Skies depicts the typical diffuse luminance and radiance distributions over the skydome. However, it can be challenging to interpret the luminance and radiance distribution into the irradiance and illuminance contributions on tilt planes by numerical integrations. This paper proposes a surrogate model that determines the ratio of the diffuse irradiance on an arbitrary tilt plane to that on the unobstructed horizontal plane without the complicated and time-consuming numerical integrations. The model is determined using solar altitude, sky conditions, and the angular distance of the plane (surface normal) and the sun. The proposed model is validated by measurement of the vertical illuminance and irradiance that are taken in 2004 and 2005, and the irradiance on planes of different tilt angles and azimuth directions from February to May in 2015. All measurements were in the 10-minute interval. For vertical planes, the proposed approach gives the ratio of the root mean square errors to the measurement average 1.38% to 2.04% lower than a classical model for irradiance and 3.6% to 4.6% for illuminance, when the Skies can be accurately identified. The model thus accurately interprets the luminance and radiance distributions of the CIE Standard Skies, which can be essential to a fast study for the solar energy potential as well as the thermal and daylight environments under different sky conditions.
... It is important to identify the sky-luminance distribution in real time. Commercially available devices used for skyluminance modeling studies [4]- [6] are costly 1 and this paper discusses an improved design of our previously reported low-cost light dependent resistor (LDR)-based device [7], [8] for application in indoor luminance measurement and light control. Using calibrated LDRs and open-source microcontroller, we had reported a 1/200 times cost reduction as compared to commercial device [8], and the proposed modified design is a light-weight portable upgrade of our previous design -using Fresnel-like mounts for sensors, replacing the heavy circular frame -and costs about 1/600 the quoted cost of commercial device. ...
Conference Paper
A low-cost LDR-based angular sky-luminance measurement device is proposed as a centralized sensor for predicting real-time illuminance distribution in building interiors over a wide district, for application in centralized light control.
... The sky type is selected considering the RMSD between the measured and modeled luminances. Suárez- García et al. (2018) used two methods of normalized luminances described for the characterization of the skies at Burgos (Spain): the Tregenza (2004) method and that of Li et al. (2003). Both procedures were compared observing a good agreement between them. ...
Article
Since the publication of the standard sky luminance distributions (SSLD) that was consolidated in the ISO 15469:2004(E)/CIE S 011/E:2003, numerous procedures have emerged for the characterization of the sky condition according to that standard. Precisely, the use of different procedures for the classification of the skies of a certain place according to the ISO/CIE standard can lead to obtain different frequencies of sky types. The existing uncertainties in the characterization of the sky condition according to the CIE Standard General Sky as a consequence of the classification procedure used are analyzed in this study. For this, three different classification procedures are used to characterize the sky radiance and luminance distribution measurements made by means of a sky-scanner in Pamplona (Spain) from 2007 to 2013. That is, (1) a method focused on determining the relative gradation and indicatrix functions, (2) a method based on the comparison of measured and standard luminances normalized against the horizontal diffuse illuminance, and (3) a new high-spatial-resolution approach that compares measured and standard luminances relative to zenith. In general terms, it is concluded that there is some uncertainty in the classification depending on the procedure used to characterize the sky.
... One of the problems to be solved before collecting data on spatial distribution of daylight SPD is a need to build a metrological system and establish a procedure and apply them for all assumed locations. There are many solutions that could be usedstarting from classical radiometric devices (Hernández-Andrés et al., 2001), through scanning (Knoop et al., 2015;Suárez-García et al., 2018) or non-scanning devices (Riechelmann et al., 2013;Tohsing et al., 2014), multispectral synthesis based on CCD camera (Nieves et al., 2005), HDR technique (Borisuit et al., 2016;Inanici, 2019a, 2019b, Inanici 2019b. Considering the need to use the same type of the measurement system justified by the comparability of the results (Bauer et al., 2018), a synchronized network of locations for the worldwide measurements campaign would be extremely expensive. ...
Article
Full-text available
The evaluation of non-visual effects of light is a crucial topic in lighting design research and practice. Performing such analysis requires precise information about the spectral distribution of the tested light source. Assessing non-visual effects of daylight is complicated due to its spectral composition continuously changing, depending on many factors. Currently there are no available databases with spectral and spatial radiation patterns of the sky vault for locations spread all over the world, and an easy and common method to evaluate non-visual effects of daylight is lacking. The goal of the paper is to provide a simple method to evaluate the non-visual potential of daylight, accounting for its variability. In order to present it, spectral measurements were conducted in two European cities in spring and summer. Horizontal and vertical illuminance toward the four cardinal directions was measured. Daylight variability was analysed in terms of illuminance, Correlated Colour Temperature (CCT) and Melanopic to Photopic (M/P) ratio. A comparison between the measured spectra and the CIE standard illuminants was performed and it was found that the usefulness of this method to evaluate non-visual effects of light in terms of M/P is adequate and provides reliable results. Therefore, a simple method to estimate the non-visual potential of daylight based on the use of D series of illuminants was presented and validated by comparing the results with the measured data. Using this method to calculate M/P always achieves RMSPEs below 6%.
... In it, three classes of skies (cloudy, partly cloudy, and clear) are contemplated, each subdivided into five types. Consequently, it proposes a total of 15 models of the celestial dome for different conditions of cloudiness and, therefore, illumination [1]. At present, the most reliable way to analyze the type of sky, according to the CIE, is to use an expensive device that divides the sky dome into 145 sectors and measures the luminous emittance of each of them sequentially; such a device is called a "sky-scanner" [2]. ...
Article
Full-text available
Natural illumination has an important place in home automation applications. Among other advantages, it contributes to better visual health, energy savings, and lower CO2 emissions. Therefore, it is important to measure illuminance in the most accurate and cost-effective way. This work compares several low-cost commercial sensors (VEML 7700, TSL2591, and OPT3001) with a professional one (ML-020S-O), all of them installed outdoors. In addition, a platform based on the Internet of Things technology was designed and deployed as a centralized point of data collection and processing. Summer months have been chosen for the comparison. This is the most adverse situation for low-cost sensors since they are designed for indoor use, and their operating range is lower than the maximum reached by sunlight. The solar illuminance was recorded every minute. As expected, the obtained bias depends on the solar height. This can reach 60% in the worst circumstances, although most of the time, its value stays below 40%. The positive side lies in the good precision of the recordings. This systematic deviation makes it susceptible to mathematical correction. Therefore, the incorporation of more sensors and data that can help the global improvement of the precision and accuracy of this low-cost system is left as a future line of improvement.
... The distribution patterns are determined by the well-validated [39][40][41][42] ISO/ CIE Standard Skies [11]. using the non-out-of-range radiance data [43]. Mathematically, R fix can be determined by Eq. (9) [39]. ...
Article
The diffuse radiation on building envelopes can be essential to the thermal environment and the energy study of buildings. Because a part of the skydome can be obstructed irregularly in city environments, the diffuse radiance distribution over the skydome should be determined to evaluate the radiation accurately. In this study, this distribution is described by several representative sky directions to specifying the radiance variation features over the skydome, especially the attenuation around the sun. The irradiance of each sky element (normalized by the horizontal diffuse) is correlated to the routine global radiation and the solar altitude as an all-sky model for simple engineering uses. According to two sets of one-month field measurements (in different sites) of the vertical irradiance under irregular obstructions in the hot and humid Hong Kong, the coefficient of determination (R²) is 0.041 and 0.142 higher than the classical single-direction anisotropic diffuse model that is adopted by many simulation tools, and the R² difference can be up to 0.38 for partly cloudy days. When compared to the distribution models, the R² outperformance of the proposed model can still be 0.057. The proposed approach is thus effective to the solar radiation studies in the city environments.
... Analyzing sky luminance scans to obtain frequency of occurrences of the 15 CIE sky types is usually based on the best fit methods. Two widely used methods [32,36,37] will be used in this study. ...
Article
Full-text available
The International Organization for Standardization (ISO)/Commission International de l’Eclairage (CIE) Standard General Skies is a set of standard skies that can describe, mathematically, the distribution of the luminance of a sky. Since the introduction of such standard, several methods for sky type classification based on the ISO/CIE standard types have been proposed. However, each of these methods could lead to a different result. Based on sky luminance measurement and through sky type classification in Dammam city in Saudi Arabia, a new procedure is proposed that can help reach a more accurate result.
... The CIE skies have been validated through measurements [34][35][36][37][38][39][40] at few locations, and the CIE sky-ratio models have been extended from luminance to irradiance distribution predictions for PV modules at inclined planes [41]. Further calculation based enhancements have been demonstrated for urban environments where the sky is not directly visible due to dense construction; the calculation of solar heat gain and daylight intrusion through vertical fenestration was demonstrated for standard CIE skies [42], and since the amount of interior daylight depends on the light reaching the facades, a calculation procedure was proposed for assessing the luminance on a vertical plane [43]. ...
Article
Energy efficient building design requires simulated assessment of overall performance through reliable models, taking into account the daylight variations along with other significant parameters such as ventilation, plug loads and occupant behaviour. A robust indoor daylight model applicable to locations across the globe needs to incorporate local variations of sky luminance on a spatial and temporal basis, and needs an affordable system for dynamic angular daylight measurement to be used as part of a wider distributed network. In the present work, a low-cost prototype based on Light Dependent Resistors for measurement of angular daylight luminance distribution is developed and analyzed. This device was used to capture the angular analog data for 221 sky segments (20 azimuthal x 11 altitudinal segments and Zenith). The analog data was further converted to luminance data, and the visualized data was compared with sky-dome photographs and also with simulated polar daylight maps for a few Commission International de l'Eclairage (CIE) skies. The repeatability of the experiment was assessed by comparing the luminance data for selected sky segments over multiple time-segments for consecutive days. Measured data was found to be an acceptable fit with the CIE-models over various time segments, validating the prototype.
Article
There are several compilations of sky classifications that refer to Meteorological Indices (MIs) (variables usually recorded at meteorological ground stations), due to the scarcity of sky scanner devices that can supply the experimental data needed to apply the CIE standard sky classification. The use of one rather than another MI is never justified, because there is no standardized criterion for their selection. In this study, forty-three MIs, traditionally used to define different sky conditions, are reviewed. Feature Selection (FS) is a key step in the design of a sky-classification algorithm using MIs as an alternative to data from sky scanners. Four procedural methods for FS -Pearson, Permutation Importance, Recursive Feature Elimination, and Boruta- are applied to an extensive data set of MIs that includes CIE standard sky classification data, which was used as a reference. The use of FS procedures significatively reduced the original set of MIs, permitting the construction of different classification trees with high performance for the sky classification. In the case of the Pearson FS method, the classification tree only used two MIs. The advantage of the Pearson FS method is that it functions independently from the machine-learning algorithm used latter for the sky classification.
Article
This study is based on the fifteen standard sky models proposed by the International Commission of Illumination (CIE) and explores the sky luminance distribution throughout the severe cold regions of China. The experiment analysis is based on 22,829 sets of effective luminance data from January 2019 to December 2019. The Tregenza method and the normalization ratio method are used to analyze changes in the sky environment throughout the year in Northeast China. The results indicate that the standard sky models proposed by the CIE can represent the basic characteristics of the sky over a whole year in Harbin, but the frequency of each sky type is clearly different. Type V.4 (CIE standard clear sky, low illuminance probability) has a frequency exceeding 22% over the year, and is the most common type of sky in Harbin, while type VI.6 (white blue turned sky with broad solar corona) has the lowest frequency type. The use of these two methods indicates that the frequency of clear sky in Harbin over the year exceeds 50%, and this mainly occurs in spring, autumn and winter. The probability of overcast and partly cloudy skies is over 20%, and in summer an overcast sky is more probable. When comparing against different solar elevations, it is found that for solar elevations below 50°, V.4 is the most common sky type with a probability above 15%, whilst for solar elevations above 50°, type I.1 is the most frequent.
Article
The overcast, partly cloudy and clear sky conditions can determine the diffuse sky radiance and luminance distributions over the sky dome, and thus are crucial for evaluating the solar energy and daylight on and through building envelopes. It is preferable to properly identify the sky condition and determining its diffuse radiance and luminance distribution patterns by the readily available data. In this work, we propose a new approach to identifying the sky conditions, especially the cloudy and clear skies mainly by the horizontal illuminance fluctuation frequency. Illuminance on ground level may fluctuate at high frequency under cloudy skies due to the broken cloud, while vary smoothly under clear skies with few cloud blockages. For input data, the proposed approach needs the horizontal global illuminance that can be readily accessible for many places instead of the “uncommon” measurements on vertical planes or in sky zenith. The fluctuation frequency factor we propose can reduce the misclassification rate for daily and half-day representative sky conditions by 5.7% and 11.5%, respectively, compared to the classifications using the clearness index only.
Article
Potential photovoltaic (PV) production from vertical façades facing the four cardinal points of the compass are calculated from daily average vertical global insolation readings measured in Burgos, Spain. Ten-minute data sets are collected over forty-five months, from January 2014 to September 2017, in the experimental campaign to produce estimates of daily average insolation levels, from which the PV potential of the vertical surfaces was calculated. Given the scarcity of data on Global Vertical Insolation (GVI), the main sky-related variables were processed in four classic decomposition models (Isotropic, Circumsolar, Klucher, and Hay) to predict the insolation values. Both the experimentally measured GVI values and those calculated with the models were then compared using the statistical indicators RMSE and MBE. The results highlighted the economic viability of Building Integrated PhotoVoltaic (BIPV) facilities, even on the north-facing façades, in comparison with the horizontal facility at the same location.
Article
In this study, ten-minute meteorological data-sets recorded at Burgos, Spain, are used to develop models of Photosynthetic Active Radiation (PAR) following two different procedures: multilinear regression and Artificial Neural Networks. Ten Meteorological Indices (MIs) are chosen as inputs to the models: clearness index (kt), diffuse fraction (kd), direct fraction (kb), Perez's clear sky index (ɛ), brightness index (Δ), cloud cover (CC), air temperature (T), pressure (P), solar azimuth cosine (cosZ), and horizontal global irradiation (RaGH). The experimental data are clustered according to the sky conditions, following the CIE standard sky classification. A previous feature selection procedure established the most adequate MIs for modelling PAR in clear, partial and overcast sky conditions. RaGH was the common MI used by all models and for all sky conditions. Additional variables were also included: the geometrical parameter, cosZ, and three variables related to the sky conditions, kt,ε, and Δ. Both modelling methods, multilinear regression and ANN, yielded very high determination coefficients (R2) with very close results in the models for each of the different sky conditions. Slight improvements can be observed in the ANN models. The results underline the equivalence of multilinear regression models and ANN models of PAR following previous feature selection procedures.
Article
Full-text available
The diffuse radiance and luminance can be anisotropic over the skydome, which is important to the daylight and thermal environments of buildings and the urban areas. This paper proposes a model that can estimate the luminance and radiance distributions over the skydome by the basic global and diffuse irradiance, and the solar altitude measurements/data that are readily accessible for many places over the world. The radiance and luminance are normalized by the horizontal irradiance and illuminance instead of the uncommon zenith radiance and luminance. The approach is developed and tested by the mid-to long-term (a few months to two years) field measurements of several locations with various climates/latitudes, showing a good accuracy when compared to the reference models.
Article
Full-text available
The optimum utilization of solar energy requires effective harvesting of both the direct and diffuse components of ground-reaching radiation. Although solar beams are typically key contributors to the total irradiance under cloudless conditions, the diffuse component becomes important especially in regions where clear skies are not dominant. Even if the cloud cover and cloud microphysics are known, it is not an easy task to estimate the diffuse irradiance at arbitrarily oriented sloped surfaces. This situation arises from the extreme difficulty in solving the radiative transfer equation in such a heterogeneous environment.
Article
Full-text available
A theoretical model simulating the radiance field below inhomogeneous cloudy skies is derived. The model is applicable to any cloud configuration, i.e. positions of individual clouds can be arranged randomly as regularly occurs in nature. The solution follows the modified theory of successive orders of scattering in which single-scattering radiances from broken clouds and a cloud-free atmosphere are well separated, while their non-trivial superposition is used to determine a second-order approximation to the radiative field. The contributions of both single and secondary scattered beams form the total radiation field below the cloud level. Numerical simulations are performed for a cloud array with regular spacing between single clouds to document the correct model behavior for various cloud fractions, including extreme cases of cloudless and overcast skies. Despite the complexity of the derived equations, the numerical computations can be accomplished rapidly using an inexpensive Intel Pentium processor. The main strengths of the model presented here are its easy parameterization and its ability to predict more realistic radiance patterns for inhomogeneous cloudy skies. The theoretical foundation of the documented solution implies its applicability to a wide range of sky states.
Article
Full-text available
The calculation of diffuse horizonthal illuminance DV outdoors is generalized and interrelated to parameters that characterize the sky luminance distribution function. The derived analytical formula for DV,rel which is DV normalized using luminance in the zenith can be commonly applicable to all 15 ISO-standardized sky types. This makes the modelling of all 15 skies more uniform than ever before. The presented analytical solution is simplified enough because the set of 60 parameters B1,C1,D1,E1–B15,C15,D15,E15 scaling the diffuse illuminance is no more necessary in the new solution. In addition, the analytical formulae substituting the empirical ones are valid for all solar altitudes γS (including the situations with sun in zenith). On the contrary, the empirical formulae may become inapplicable for γS>75° and fail when γS approaches 90°. Compared with detail numerical integration the analytical formula guarantees more rapid calculation of DV,rel.
Article
Full-text available
The CIE Standard General Sky consists of a family of luminance distributions which can be matched to measured sky brightness patterns. The daylight climate of a site can be described concisely and effectively by the statistical distribution of the General Sky types that best fit the sky luminance patterns that occur there. This paper gives a procedure for deriving the frequency of occurrence of General Sky types from International Daylight Measurement Programme sky scan data. Measured luminances are normalized with respect to horizontal illuminance and solar elevation, then the sky types giving the best least-squares fit are found. A data structure for daylight climate simulation is proposed.
Article
Full-text available
Guidelines for the design of lighting in residential buildings used by people with dementia are derived from the results of two research projects and a literature review. General guidance on lighting for the older person is summarised and the special requirements of those living with dementia are examined under four headings: the diurnal cycle, a view of nature and the outside world, support for individual and social activities, and recognition of place. The aim is a visual environment that enhances physical well-being and enjoyment of life.
Article
Full-text available
A set of 15 standard skies proposed by Kittler, Darula and Perez was compared with a large sample of measured sky luminance distributions from Singapore, Japan and the United Kingdom. It was found that (a) the standard set gives a good overall framework for categorising actual skies; (b) subsets of about four luminance distributions were adequate to describe the skies that occurred at each site; (c) inhomogeneity of actual skies in maritime climates leads to uncertainty in interior daylight predictions (typically the room illuminance computed on the best-fitting standard sky might have a variance such that 80% of calculated values lie between 0.8 and 1.3 of the real value); and (d) this error range is significantly less than when calculations are based on an overcast sky alone.
Article
Full-text available
In this work, we study the conditions of illuminance in south England (Garston area 51.71°Ν, 0.38°W) during the winter period. This study is based on the 15 categories of sky luminance distribution defined by Kittler, Perez and Darula. These 15 sky types are represented in diagrams of the ratio of zenith luminance to diffuse horizontal illuminance against solar altitude. The data used are diffuse horizontal irradiance and illuminance, global horizontal irradiance and illuminance, zenith luminance and solar altitude, for the period 1/10/1991-31/3/1992. The classification of the observations into the 15 sky types was performed for the following situations: a) For solar altitude greater than 5° and less than or equal 35°, where the theoretical curves are not intersected. In this case, the ratio of zenith luminance to diffuse horizontal illuminance of each observation was compared with the corresponding ratios of the 15 theoretical sky types, for the same solar altitude and then the observation was classified in the sky type with the nearest value. b) For all solar altitude greater than 5°. In this case, due to the fact that the theoretical curves converge and above the 35° solar altitude they are intersected, each observation is classified in one of the 15 sky types, only when the value of the ratio of zenith luminance to diffuse horizontal illuminance lies in a zone of ±2.5% around the theoretical curve, which describes a specific category. In case that, an observation is classified in more than one sky types, the ratio of global horizontal illuminance to extraterrestrial horizontal illuminance is compared with the average ratios of the various sky types (estimated from all observations), for the same solar altitude and the observation is classified in the sky type with the nearest value. The above study has shown that the predominant sky type in south England and probable in regions with similar geographic and climatic conditions is the sky type 2 (I.2), which is an overcast sky with steep gradation and slight brightening toward the sun.
Article
There has been a growing demand on energy sector for short-term predictions of energy resources to support the planning and management of electricity generation and distribution systems. The purpose of this work is establishing a methodology to produce solar irradiation forecasts for the Brazilian Northeastern region by using Weather Research and Forecasting Model (WRF) combined with a statistical post-processing method. The 24 h solar irradiance forecasts were obtained using the WRF model. In order to reduce uncertainties, a cluster analysis technique was employed to select areas presenting similar climate features. Comparison analysis between WRF model outputs and observational data were performed to evaluate the model skill in forecasting surface solar irradiance. Next, model-derived short-term solar irradiance forecasts from the WRF outputs were refined by using an artificial neural networks (ANNs) technique. The output variables of the WRF model representing the forecasted atmospheric conditions were used as predictors by ANNs, adjusted to calculate the solar radiation incident for the entire Brazilian Northeastern (NEB) (which was divided into four homogeneous regions, defined by the Ward method). The data used in this study was from rainy and dry seasons between 2009 and 2011. Several predictors were tested to adjust and simulate the ANNs. We found the best ANN architecture and a group of 10 predictors, in which a deeper analyzes were carried out, including performance evaluation for Fall and Spring of 2011 (rainy and dry season in NEB, mainly in the northern section). There was a significant improvement of the WRF model forecasts when adjusted by the ANNs, yielding lower bias and RMSE, and an increase in the correlation coefficient.
Article
Recently, the International Commission on Illumination (CIE) has adopted a range of 15 standard sky distributions representing the whole probable spectrum of actual skies in the world. Each sky standard has its own well-defined sky luminance pattern which can be conveniently used to calculate the sky radiance and luminance for a given sky patch and the solar irradiance and daylight illuminance on inclined surfaces facing various orientations. The crucial issues are whether the skies could be correctly identified. This paper presents the work on the classification of the CIE Standard General Skies using various climatic parameters and indices. Meteorological variables namely luminance distribution for the whole sky including zenith luminance, global, direct-beam and sky-diffuse illuminance on a horizontal surface and vertical sky illuminance, and horizontal and vertical solar irradiance data are adopted for analysis. The results demonstrate that there are a number of appropriate climatic parameters for sky classification and the selection depends on their availability, accuracy and sensitivity. The approaches could contribute to the estimation of solar irradiance and daylight illuminance which are essential to the renewable and sustainable developments and energy-efficient building designs.
Article
This paper studies the classification of the International Commission on Illumination (CIE) standard skies using sky luminance and other climatic parameters measured in Hong Kong. An approach to identify the standard skies is proposed. The performance of the proposed approach and the Bartzokas et al. method was evaluated against the measured data in terms of root-mean-square error (RMSE). The statistical analysis revealed that using the Bartzokas et al. method, around 50% of the data were rejected for sky classification and the overall RMSE was calculated of around 34%. The proposed approach produced an overall RMSE of just less than 33% without data reduction.
Article
Solar irradiance and outdoor illuminance, particularly on vertical surfaces are crucial to energy-efficient building designs and daylighting schemes. In Hong Kong, only hourly horizontal global solar radiation data have been systematically recorded for a long period but no measurements of daylight illuminance exist. In 2003, the International Commission on Illumination (CIE) adopted a range of 15 standard skies covering the whole probable spectrum of skies in the world. Standard skies of the same category would have the identical well-defined sky radiance and luminance distributions. Once the skies are identified, the basic solar irradiance and daylight illuminance at the surfaces of interest can be obtained, involving simple mathematical expressions. This study presents a numerical approach to predict the vertical solar irradiance and daylight illuminance based on the CIE standard skies. Climatic parameters recorded between January 2004 and December 2005 are used in the analysis. The performance of the calculation method is evaluated against data measured in the same period. The annual RMSEs were found ranging from 17.7% to 20.8% for daylight illuminance prediction and 17.9%–19.8% for solar irradiance estimation. The findings provide an alternative to compute solar irradiance and daylight illuminance on vertical surfaces facing various orientations.
Article
This study investigated the effects of indoor lighting on occupants’ visual comfort and eye health and to contribute to the management and maintenance of buildings. The illuminance of the working plane and windows at Samsung Corporation Headquarters were measured, and 2744 healthy occupants of Samsung Corporation were surveyed regarding the indoor lighting environment via the company’s intranet for 1½ years. This building was certified with the highest ranking by Korea’s Green Building Council. The cumulative data reflected the management and maintenance of the building, such as screen-type shading devices automatically controlled by seasons and time, improvement of visual display terminal glare by the veiling reflection on monitors, efficiency of artificial lighting arrays, and so on. The data were analysed for occupants’ visual comfort and eye health. The result showed that daylighting could improve the occupants’ psychological health and productivity. The screen-type shading device could intercept direct sunlight and reduce annoyance glare. However, the indoor lighting and visual environment of the building were poor. After examining the questionnaire feedback concerning improvements, the occupants’ annoyance ratio was significantly reduced, and approximately 5% of the occupants’ annoyance ratio was deemed to be caused by personal characteristics related to the lighting of the environment.
Article
This work is based on the study of outdoor daylight conditions during the winter period in Pamplona (the Iberian Peninsula). The 15 standard sky types proposed by the Commission Internationale de l’Eclairage (CIE) are used. The selected sky type in each moment is the one exhibiting the lowest root mean standard deviation (RMSD) when the theoretical and experimental luminance distributions in the sky hemisphere are compared. The selection was derived from luminance distribution data from 145 patches of the sky hemisphere and registered every 10min from October to March from 2007 and 2008.The most frequent sky type observed in winter in Pamplona is V.5 (cloudless polluted with a broad solar corona), with a frequency of occurrence of 21%. Notwithstanding, the group of overcast skies exhibits a slightly higher frequency (40%) than the clear skies (37%). Seven out of the 15 standard sky types, i.e.V.5, I.1, III.4, II.1, IV.4, II.2 and III.3, are practically 80% of those under study. In order to provide a possible application in daylight climates studies, the frequency of occurrence of the 15 sky types is included in eleven intervals of solar elevation on the threshold corresponding to the time period and the location considered.
Article
In this work the outdoor daylight conditions in Pamplona (South Europe) during the summer period have been studied. The selected sky type (from fifteen standards) at a given moment is the one exhibiting the lowest RMSD when comparing the theoretical and experimental luminance distributions in the sky hemisphere. Two year data of luminance distribution registered every 10min in 145 positions of the sky hemisphere have been used for selecting the sky type. The most frequent sky type in Pamplona is V.5 (cloudless polluted with a broad solar corona), with an occurrence of 29.5%. This result coincides with the one observed in a previous study in Athens.Six types of sky (V.5, IV.4, III.4, III.3, V.4 y II.2) out of the fifteen standards become practically the 80% of all the studied ones. Regarding a possible use in daylight climate studies, the frequency of occurrence of the fifteen types of sky for fourteen solar elevation intervals has been included.
Article
A thorough analysis of sky distributions can help in the prediction of daylight patterns and thus provides a good understanding of architectural window and building envelope design. This paper introduces three methods of analysis for sky distributions, namely the relative indicatrix and gradation classification, and two other independent methods established by Kittler and Tregenza. All these three methods were verified for their feasibility for analysing the extreme daylight conditions in Singapore, which is located at the equator. The most suitable approach, from Tregenza, was eventually adopted in analysing the measured data from Singapore IDMP station. The prediction of frequent sky patterns in Singapore was based on the CIE Standard General Skies, as well as a complete version considering all 36 theoretical sky types.
Article
The tropical sky is highly luminous. This gives rise to expectation of its high potential for use of daylight in interior. Luminance distribution over the sky is non-uniform therefore an understanding on its distribution would help advance the movement of daylighting. R. Kittler et al. has recently proposed a set of fifteen standard sky luminance distributions (SSLD) (1). The standard skies cover a transition of skies from overcast sky through partly cloudy sky to clear sky. This paper employs SSLD to characterize luminance distribution of tropical sky. The results show that tropical sky experiences to partly cloudy sky and clear sky conditions. Luminance distribution from measurement is also determined in terms of gradation function and indicatrix function. The results are expected to contribute to knowledge on luminance of tropical sky to completion of the map of sky luminance.
Article
A good understanding of the sky luminance distribution is essential for efficient daylight-ing designs and developments. Sky luminance patterns are always modeled under various sky conditions using climatic parameters as weighting factors to indicate the degree of sky clearness. This paper presents the work on the identification of sky luminance patterns using three common climatic elements, namely cloud cover (CLD), sunshine hour (SH) and clearness index (K t ). Three sky types representing overcast, partly cloudy and clear were selected from the Kittler et al. 15 standard skies. It has been shown that sky conditions classified by individual sky indicators produced acceptable agreements with the measured data. In general, CLD shows the best results, followed by K t , then SH. Better results in terms of the frequency of occurrence and root mean square errors can be found when CLD and SH are jointly utilised to describe the sky conditions.
Article
In the present work there are studied the exterior daylight conditions in the area of Sheffield in central England (53.38°Ν, 1.50°W) during the winter period. The study makes use of the fifteen standard sky types defined by Kittler et al. These sky types are presented in diagrams of the ratio of the zenith luminance (Lz) to the diffuse horizontal illuminance (Dv) against solar altitude (γ). The illuminance parameters used are time series of half-hourly values of Dv, Lz, global (Ge) and diffuse (De) horizontal irradiance and γ for the period November 1st, 1993 to February 10th, 1994. This study shows that the most frequent winter sky types in central England are: (i) I.2 (overcast with the steep gradation and slight brightening toward sun), with frequency of occurrence of 19.5%, (ii) I.1 (overcast with the steep gradation and with azimuthal uniformity) (10.5%) and (iii) II.1 (overcast moderately graded with azimuthal uniformity) (9.7%). The most predominant cloudless sky is that of V.5 (cloudless polluted with a broader solar corona) with frequency of occurrence of 7.6%
Article
A good knowledge of the sky luminance distribution is essential for efficient day lighting designs and developments. This paper studies a set of 15 standard skies established by Kittler, Perez and Darula using the measured sky luminance data from subtropical Hong Kong. A statistical analysis of the results has shown that the standard sky set is adequate to cover the various sky conditions. A reduction of the standard set to five skies is sufficient to model the luminance distributions. The ratio of zenith luminance to horizontal diffuse illuminance (L/zD) which can be used to define homogeneous sky standards was analysed based on these five selected skies. It has been shown that the trend of L/zDv ratios under different sky types followed the ranges described by Kittler et al.
Article
In 2003, the International Commission on Illumination (CIE) adopted 15 standard skies that cover a broad spectrum of the usual skies found in the world. Each sky represents a unique sky luminance distribution, which is the most effective way to classify the 15 CIE Standard Skies. However, luminance distributions for the whole sky vault are far from being widely available. Alternatively, the standard skies can be categorized by various climatic parameters but the criteria to distinguish individual skies are not always clear-cut and may lead to ambiguous results. The artificial neural networks (ANNs) represent a powerful tool for pattern recognition. This paper presents the work on the classification of the standard skies using a new form of neural network architecture, namely the probabilistic neural network (PNN), which is particularly apposite in classification problems. Five meteorological variables, viz. zenith luminance, global, direct-beam and sky-diffuse illuminance, and solar altitude are employed as input data. Totally, 9000 samples covering the time span between 1999 and 2005 are shuffled. The findings suggest that the PNN is an appropriate tool for sky classification. Copyright © 2009 Royal Meteorological Society
Article
Daylighting is recognized as an important element in architecture and a useful strategy in energy-efficient building designs. Daylight gives a sense of cheeriness and brightness that can have a significant positive impact on the people. There is a scope for integrating daylight with electric light to reduce building energy use. The amount of daylight entering a building is mainly through window openings, which create in the indoor environment a more attractive and pleasing atmosphere, in addition to maximise visual access to the pleasant views of the outside world. Determinations of the exterior and interior daylight and lighting energy savings are key issues to demonstrate the benefits based on daylighting designs. This paper provides a review of daylight illuminance determinations and the lighting energy reductions due to daylighting schemes. The study includes daylight measurements, prediction of daylight illuminance under various sky conditions and potential electric lighting energy savings from daylight-linked lighting controls. The article aims at providing building professionals, practitioners and researchers more information and a better understanding of daylight for promoting effective daylighting designs and evaluations.
Article
Sky luminance distribution is one of the most important quantities for calculating daylight in buildings and the associated energy savings. At the Building Research Establishment, Garston, sky luminance distributions have been measured on a long term basis for the first time in the UK. These data have been used to assess a number of models which aim to predict the hourly variation of sky luminance distribution. It is concluded that the three best models analysed would all perform well in a computer program to simulate internal daylight levels and lighting energy use.
A new generation of sky standards
  • R Kittler
  • R Perez
  • S Darula
Kittler, R., Perez, R., Darula, S., 1997. A new generation of sky standards. In: Proceedings of the Eighth European lighting conference, Amsterdam, pp. 359-373., 359-373
The luminance distributions of clear and quasi-clear skies
  • P J Littlefair
Littlefair, P.J., 1994b. The luminance distributions of clear and quasi-clear skies, Proceedings of the CIBSE National Lighting Conference, Cambridge, UK. pp. 267-283.
Spatial distribution of daylight-CIE standard general sky
  • Y Uetani
  • S Aydinli
  • A Joukoff
  • J Kendrick
  • R Kittler
  • Y Koga
Uetani, Y., Aydinli, S., Joukoff, A., Kendrick, J., Kittler, R., Koga, Y., 2003. Spatial distribution of daylight-CIE standard general sky. Vienna, Austria
Using and Understanding Mathematics: A Quantitative Reasoning Approach
  • J O Bennett
  • W L Briggs
Bennett, J.O., Briggs, W.L., 2008. Using and Understanding Mathematics: A Quantitative Reasoning Approach.
Atlas Agroclimático de Castilla y León
  • Itacyl-Aemet
ITACYL-AEMET, 2013. Atlas Agroclimático de Castilla y León http://atlas.itacyl.es. (Accessed January, 2017.