Sky Types Research Articles

The use of sky luminance and illuminance to classify the CIE Standard General Skies

Optimum use of natural light in buildings requires knowledge of the luminance distribution of the sky. Since the introduction of the CIE Standard General Skies, several methods have been proposed to find the frequency distribution of sky types. This paper presents a method whereby the ratio of the luminances of two sky elements and the ratio of the luminance of one sky element to the horizontal illuminance can be used in the determination of sky type.

Occurrences of Standard Skies and Luminuos Turbidity Factor in Bratislava

Levels of daylighting in nature continually changing and their daily courses are different day to day. Effectiveness of daylight utilisation in buildings depends on exterior luminous conditions resulting from sunny and sunless situations. Generally, prevailing interiors are illuminated by diffuse skylight during year but sunny orientated rooms can be often exposed by direct sunlight. To describe these conditions the occurrence of sky luminance patterns and light attenuation properties of atmosphere expressed e.g. by luminous turbidity factor are needed. This paper presents the comparison and evaluation of occurrences of ISO/CIE General Standard Sky and luminous turbidity factor in two distant years on the base of measured instantaneous global and diffuse horizontal illuminance, global and diffuse horizontal irradiance and zenith luminance taken during 1995 a 2002 at ICA SAS in Bratislava. The different occurrences of luminous situations described by general standard sky types were found. Results of the luminous turbidity factor calculation show that various atmospheric pollutions during sunny situations can be expected.

Synthetic generation of standard sky types series using Markov Transition Matrices

The paper describes a methodology for creating synthetic time series of the fifteen standard sky types considered by the Commission International d'Eclairage (CIE). Since the measurements of sky luminance and accordingly CIE sky types are limited to a small number of locations, it is important to develop models to generate the time series of these parameters. The method is applied to two different locations [Pamplona (Spain) and Arcavacata di Rende (Italy)] where experimental luminance data are available. Firstly, the standard sky types corresponding to the observed luminance values are determined every 10 min and from these, Markov's Transition Matrices (MTM) are obtained corresponding to the four seasons. Secondly, it is statistically verified that the process follows a Markov's chain of first order and that it is stationary. Thirdly, obtained MTMs are used as a basis for generating the synthetic series of types of sky. Finally, experimental and synthetic time series are compared for the two locations, exhibiting good fitting results. As a conclusion, it is verified that first order MTM method can be used to generate time series of occurrence of CIE standard sky types, for the two locations. To clarify the general applicability, it should be applied to different locations having different climates and in addition with longer data sets.

Análise da influência da orientação das aberturas na disponibilidade de iluminação natural no ambiente interno na extensão de sua profundidade

This study presents a discussion related to daylighting in the indoor environment, being the goal of the research to analyse the influence of the orientation of the openings in the availability of daylighting in the indoor environment to the extent of its depth. The analysis was conducted by comparing the percentage of illuminance values of the internal environment with intervals of UDI (Useful Daylight Illuminance) at selected points within the environment with openings targeted for North, South, East and West. The percentage of illuminance values were generated by simulation on computation tool TropLux, in a rectangular geometry environment inserted in an urban setting in the city of Vitoria, latitude 20° 19' S. The simulations were performed for three types of patterns skies of the CIE (Commission Internationale L´aclairage). As main result, note the following conditions so that the percentages are located in the range of UDI (500≤UDI<2000 lx): in the sky 3 (cloudy) the depth of the environment is up to 1,0m; in sky 7 (partially cloudy) this depth is up to 1,5m; in sky (clear), for orientation South, East and West, the depth is up to 2,0, and in the orientation North this depth is limited to 1,5m.

A method for selecting the CIE standard general sky model with regard to calculating luminance distributions

It is known that the best predictions of sky luminances are obtainable by the CIE 15 standard skies model, but the predictions by this model need knowledge of the measured luminance distributions themselves, since a criterion for selecting the type of sky starting from the irradiance values has not found until now.The authors propose a new simple method of applying the CIE model, based on the use of the sky index Si. A comparison between calculated luminance data and data measured in Arcavacata of Rende (Italy), Lyon (France) and Pamplona (Spain) show a good performance of this method in comparison with other methods of calculation of luminance existing in the literature.

Characterization of sky scanner measurements based on CIE and ISO standard CIE S 011/2003

The aim of this paper is to present a complete analysis of the sky luminance distribution data measured with a sky scanner at the International Daylight Measurement Program station in Lyon. Besides presentation of the sky luminance distribution, we also derive the CIE sky type based on the CIE S 011/2003 standard. The CIE sky type is based on gradation and indicatrix functions. Determination of the gradation and indicatrix group is based on the calculation of luminance ratio between two elements on every almucantar. Since there are six gradation and six indicatrix groups, we have 36 combinations but only 15 of them are listed in the CIE standard. Since some of combinations are really rare, we provide a table to show how to allocate 36 combinations to 15 standard CIE sky types. At the end of the paper, we present frequencies of occurrence of CIE sky types in Lyon.

Measuring and modelling illuminance in the semi-arid Northeast of Brazil

Abstract Illuminance measurement do not make up a part of routine measurements in meteorological stations in Brazil, therefore, they are very rare. This information is important for evaluating the potential contribution of natural illumination in commercial buildings, which would significantly reduce the consumption of electric energy that is used for artificial illumination and refrigeration systems. To face this lack of information, different models known as luminous efficacy were created, which made possible the estimation of illuminance in regions where there only exists information on solar irradiation. In general, they are statistical models that empirically correlate the relationship between illuminance and solar irradiation with other meteorological variables and/or sky conditions. In this work, an estimation of hourly luminous efficacy was made by means of several statistical models and by the MLP (multilayer perceptron) artificial neural networks (ANN). The hourly global luminous efficacy was estimated by considering a group of physical variables from the same locality and that were collected in a simultaneous way. The data input of the ANN was the following: dew temperature, precipitable water, sky brilliance index, clearness index of Perez and clearness index. The results were compared with the statistical models of Perez et al. [Perez R, Seals R, Michalsky J. All-weather model for sky luminance distribuition-preliminary configuration and validation. Solar Energy 1993;50(3):235–45], and Robledo [Robledo L, Soler A. Luminous efficacy of direct solar radiation for all sky types. Energy Conversion & Management 2000;41:1769–79], adjusted with local coefficients. The artificial neural network model shows a statistical performance slightly better than these models with RMSE of 5.8% for the city of Recife and 3.6% for Pesqueira.

Blurring the boundaries between Standard General Sky types due to multiple scattering of light

The diffuse light reaching the ground varies with instantaneous physical state of the atmosphere, specifically with atmospheric turbidity, vertical stratification of attenuating and scattering constituents, cloud coverage and many others. Despite this complexity, a set of empirical models based on so-called homogeneous sky types is widely used to simulate the real lighting conditions. However, the sky type classification can be influenced by multiple scattering of light. Even if multiple scattering is essentially important under low elevation angles and high turbidity conditions, it is quite often ignored – e.g. when modelling the luminance patterns. As a consequence, the interpretation of experimental data can suffer from the omission of multiple scattering effects. Since multiple scattering smoothes the luminance patterns, the neighbouring sky types may be subject to blurring, and thus it might be difficult to distinguish them in automated data processing. In this paper we introduce a model that lowers the uncertainties caused by multiple scattering, so the subsequent numerical routines will definitely profit from more accurate identification of sky types.

Blue Sky (review)

Reviewed by: Blue Sky Hope Morrison Wood, Audrey . Blue Sky; written and illus. by Audrey Wood. Blue Sky/Scholastic, 2012. [32p]. ISBN 978-0-545-31610-1 $16.99 Reviewed from galleys R 3-6 yrs. This simple concept book explores the various and ever-changing types of sky, with each spread offering a different characterization ("Blue sky"; "Rain sky"; "Sunset sky"; etc.) and bringing it vividly to life in the oil pastel illustrations. The sequence follows the course of a day, beginning with the titular blue sky and ending with dawn of the following day ("New sky"). The book emphasizes the changeability of the sky, with one spread depicting a midday "sun sky" ablaze in bright yellow, the following ("Sunset sky") bathed in deep orange, and the next ("Dark sky") awash in deep purple. The illustrations provide young listeners (and even burgeoning readers) with lots of visual impetus to guess what word is used for each sky shot. Additionally, the book offers a tacit encouragement for kids to pay attention to the world around them, a practice exemplified by the recurring figure of a boy with a toy monkey, who quietly observes the sky in each spread. This provides plenty of opportunity to initiate discussion about types of sky and perhaps to call on young people to add other types to the list, either with words or compositions of their own, and even to branch out into their own descriptive projects. Copyright © 2012 The Board of Trustees of the University of Illinois

天空輝度・放射輝度分布を推定するAll Sky Modelの改良

Energy conservation is an indispensable assignment in the building now. It is demanded to establish the technology that integrates the thermal environment and the luminous environment securing the indoor environmental quality. We should understand the phenomenon as minutely as possible that originates in the radiation from the sun and the sky that is a large factor of turbulence.CIE recommended CIE Standard General Sky about the sky luminance distribution. Even the method of selecting 15 Sky Type is not shown. Authors tried the improvement of previous All Sky Model based on the measurement data of IDMP in Osaka aiming the high accurate estimation method of luminance and radiance distributions of sky.In this paper, the appearance circumstance of Types of the CIE Standard General Sky and the assignment in estimating Types is shown based on the measurement data in Osaka and the improved All Sky Model is introduced.

Correlating Indian measured sky luminance distribution and Indian Design clear sky model with five CIE Standard clear sky models

This paper aims to find out representative CIE (International Commission on Illumination) Standard Clear Sky model(s) for three different seasons - winter solstice, equinox, and summer solstice applicable for prevailing clear sky climatic conditions in India [Roorkee]. Indian measured sky luminance distribution database is available only for Roorkee [29°51′ N; 77°53′ E]. To find out the best match between Indian measured sky luminance distribution and each of five CIE Standard Clear sky models, sky component of spatial illuminance distribution over the working plane of a room was simulated by MATLAB for three different seasons. The effectiveness of using Indian design clear sky model proposed in IS: 2440:1976 was also evaluated side by side. Daylight Coefficient method has been applied for the simulation using Indian sky luminance database. The simulation has been done for the room with eight different window orientations ranging from 0° to 315° with an interval of 45° to generate data for the entire sky vault. To find out the best fit CIE model(s), statistical test of significance (95% confidence level) has been carried out for the spatial illuminance distributions taking results obtained with Indian measured sky luminance distribution data as reference. Analysis revealed that CIE Standard General Clear sky type 15 described as “White-blue sky, turbid with a wide solar corona effect” is the best-fit clear sky model for both summer and equinox seasons and sky type 11 described as “White blue sky with a clear solar corona” is the best-fit clear sky model for winter season at Roorkee. Hence these sky types can be used for daylight prediction instead of Indian Design Clear Sky Model which shows large deviations.

Determination of the light tube efficiency for selected overcast sky types

Abstract The utilization of energy from classical natural sources has some limitations therefore new possibilities for its enhancement are sought. Tubular light guides (TLG) are simple devices that allow the transmission of daylight into interiors of domestic or commercial buildings. The light transmission efficiency (LTE) is important indicator of the light guide performance. It depends on the geometry and optical parameters of the light guide but it seems also on the sky conditions. LTE can be investigated by measurements or calculations. Measurements under a real sky meet with difficulties of sky luminance distribution stability because sun is continually moving and properties of atmosphere are changing. Measurements of the LTE made under the artificial sky with the precisely defined sky luminance distribution can give answer to the question: “Depends the light transmission efficiency of tubular light guides on the sky type determined by the luminance distribution?” It was found that theoretical prediction methods combined with numerical methods could be used to answer this question. In this paper are presented results of theoretical calculations of the tubular light guide transmission efficiency using ray-tracing prediction method compared with measurement results obtained under the artificial sky installed at the Institute of Construction and Architecture of Slovak Academy of Sciences in Bratislava. The main aim of this paper is to offer a method for determination of tubular light guide inner reflectance and mathematically prove that light guide transmission efficiency is different for various standardized overcast sky types.

Finding frequency distributions of CIE Standard General Skies from sky illuminance or irradiance

The CIE Standard General Sky divides the luminance distributions of skies into 15 types, ranging from overcast sky to clear sky. Finding the type of sky from the set of CIE Standard General Skies is a difficult task. This is due to the need for accurate luminance measurements over a long period of time because such data are not available in many regions around the world and to the difficulty in obtaining accurate readings for the zenith luminance. This study presents a method whereby the ratio of the vertical sky illuminance (or irradiance) on two surfaces or the ratio of vertical sky illuminance to the horizontal sky illuminance can be used to classify a sky into one of the CIE Standard General Skies.

Predicting sky types and zenith luminance from the cloud index derived from geostationary satellite data

This paper presents methods to predict sky types and hourly zenith luminance from cloud index n derived from the visible channel data of the geostationary satellites. Three sky conditions based on the CIE standard general sky can be classified by cloud index: n &gt; 0.38, 0.13 ≤ n ≤ 0.38 and n &lt; 0.13, corresponding to overcast, partly cloudy and clear sky conditions, more specifically to sky 1, sky 8 and sky 13 of the ‘Hong Kong Representative Sky’. Hourly zenith luminance can be derived as a function of the combination of relative standard overcast sky zenith luminance and relative standard clear sky zenith luminance determined by cloud index. The mean bias error and the root mean square error of hourly zenith luminance are 0.49 and 3.37 kcd/m2, respectively.

Interaction of Potential Intrinsic and Extrinsic Risk Factors in ACL Injured Recreational Female Skiers

The aim of this study was to investigate the interaction of potential intrinsic and extrinsic risk factors in ACL injured recreational female skiers. 93 female recreational skiers who had suffered a non-contact ACL injury and 93 age-matched controls completed a self-reported questionnaire relating to intrinsic risk factors (menstrual history, BMI, previous knee injuries, self reported weekly sports participation) and extrinsic risk factors (type of ski used, time of last binding adjustment, snow condition, weather and slope difficulty). A logistic regression model revealed the following independent ACL injury risk factors for female recreational skiers: icy snow conditions (odds ratio, 24.33; 95% confidence interval, 6.8-86.5, P<0.001), skiing during snowfall (odds ratio, 16.63; 95% confidence interval, 1.8-152.1, P=0.013), use of traditional skis (odds ratio, 10.49; 95% confidence interval, 2.0-54.5, P=0.005), and preovulatory phase of menstrual cycle (odds ratio, 2.59; 95% confidence interval, 1.2-5.5, P=0.013). In conclusion, ACL injuries in female recreational skiers are the result of a complex interaction of intrinsic and extrinsic risk factors.

Sky luminance measurements and comparisons with calculation models

Measurements of sky luminance in various meteorological conditions were taken in three localities: Arcavacata di Rende (Italy), Lyon (France) and Osaka (Japan), by means of a sky scanner. The data were compared with Perez, Igawa and CIE luminance calculation models. The analysis of the comparison shows that the best luminance predictions are obtained by the CIE model, but the predictions by the CIE model need the knowledge of the measured luminance distributions themselves and, consequently, this model is, at present, only suitable for the determination of the frequency of occurrence of the CIE sky types in a given place. Between the other two calculation models, Perez and Igawa, the latter appears somewhat more accurate than the former.

CIE standard sky model with reduced number of scaling parameters

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.

Změna teplot sněhu při běhu na lyžích v obraze sekvenční termografie

Twenty years ago it was shown that friction increases the temperature of snow skis. Change of state permits for water skis glide. Th e aim of this pilot study was to determine whether dynamic infrared thermography is able to capture rapid changes in temperature of snow some areas (not just at one point), the ski tracks before and aft er passing. We monitored the snow on the track aft er 60 races two runners in various weights (86 and 73 kg) on two types of skis and lubrication (for classic and skating cross) and three diff erent speeds (3,04±0,180, 3,81±0,273 a 5,30±0,314 m.s-1). Measurements of infrared radiation and the calculation of snow surface temperature immediately before and aft er passing skier was carried out thermo-graphic system FLIR SC620 with 30 Mhz. Nonparametric Wilcoxon test for paired comparison of values was found signifi cant increases in temperature aft er the snow cleared the skies: runner A from -3,71 to -3,50 °C (p=0,000000) and runner B from -3,75 na -3,50 °C (p=0,000000). To return to the baseline temperature is within 5 seconds aft er passing. Diff erences were demonstrated in the heat of snow between runners with diff erent masses and for the use of diff erent types of skis. Th e study confi rmed the ability of dynamic infrared thermography to register a very rapid temperature changes of snow in an area immediately before and aft er crossing the cross-country skiers. Dimensional dynamic thermography would seem to contribute to the choice of the correct strategy and lubrication in ski racing. However, it is necessary to make a series of validation studies.

Sky luminance distribution in the North of Iberian Peninsula during winter

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 10 min 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.

Sky luminance distribution in Pamplona (Spain) during the summer period

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 10 min 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.