Sky conditions at Ilorin as characterized by clearness index and relative sunshine

Solar Energy ◽  
2000 ◽  
Vol 69 (1) ◽  
pp. 45-53 ◽  
Author(s):  
S.O. Udo
Keyword(s):  
Clearness Index ◽  
Sky Conditions ◽  
Relative Sunshine

scholarly journals Validation of Hourly Global Horizontal Irradiance for Two Satellite-Derived Datasets in Northeast Iraq

2018 ◽  
Vol 10 (10) ◽  
pp. 1651 ◽  
Author(s):  
Bikhtiyar Ameen ◽  
Heiko Balzter ◽  
Claire Jarvis ◽  
Etienne Wey ◽  
Claire Thomas ◽  
...  
Keyword(s):  
Climate Data ◽  
Clearness Index ◽  
Clear Sky ◽  
Station Data ◽  
Data Points ◽  
Recorded Data ◽  
Monitoring Service ◽  
Sky Conditions ◽  
Station Location

Several sectors need global horizontal irradiance (GHI) data for various purposes. However, the availability of a long-term time series of high quality in situ GHI measurements is limited. Therefore, several studies have tried to estimate GHI by re-analysing climate data or satellite images. Validation is essential for the later use of GHI data in the regions with a scarcity of ground-recorded data. This study contributes to previous studies that have been carried out in the past to validate HelioClim-3 version 5 (HC3v5) and the Copernicus Atmosphere Monitoring Service, using radiation service version 3 (CRSv3) data of hourly GHI from satellite-derived datasets (SDD) with nine ground stations in northeast Iraq, which have not been used previously. The validation is carried out with station data at the pixel locations and two other data points in the vicinity of each station, which is something that is rarely seen in the literature. The temporal and spatial trends of the ground data are well captured by the two SDDs. Correlation ranges from 0.94 to 0.97 in all-sky and clear-sky conditions in most cases, while for cloudy-sky conditions, it is between 0.51–0.72 and 0.82–0.89 for the clearness index. The bias is negative for most of the cases, except for three positive cases. It ranges from −7% to 4%, and −8% to 3% for the all-sky and clear-sky conditions, respectively. For cloudy-sky conditions, the bias is positive, and differs from one station to another, from 16% to 85%. The root mean square error (RMSE) ranges between 12–20% and 8–12% for all-sky and clear-sky conditions, respectively. In contrast, the RMSE range is significantly higher in cloudy-sky conditions: above 56%. The bias and RMSE for the clearness index are nearly the same as those for the GHI for all-sky conditions. The spatial variability of hourly GHI SDD differs only by 2%, depending on the station location compared to the data points around each station. The variability of two SDDs is quite similar to the ground data, based on the mean and standard deviation of hourly GHI in a month. Having station data at different timescales and the small number of stations with GHI records in the region are the main limitations of this analysis.

scholarly journals Analysis of Measurements of Direct Normal and Global Solar Radiation for Maceió, Alagoas State, Northeastern Brazil

2020 ◽  
Vol 35 (4) ◽  
pp. 659-674
Author(s):  
José Marcelo Lopes Júnior ◽  
José Leonaldo de Souza ◽  
Ricardo Araujo Ferreira Junior ◽  
Cícero Manoel dos Santos ◽  
Gustavo Bastos Lyra ◽  
...  
Keyword(s):  
Solar Radiation ◽  
Solar Irradiance ◽  
Cloud Cover ◽  
Global Solar Radiation ◽  
Northeastern Brazil ◽  
Human Knowledge ◽  
Clearness Index ◽  
Study Region ◽  
Maximum Value ◽  
Sky Conditions

Abstract Studying solar radiation is essential for human knowledge, since it is present in practically all its activities. Thus, the aim of this work was to analyze the climatic and seasonal variation of direct normal and global solar radiation in the region of Maceió, Alagoas State, Northeastern Brazil with sky conditions characterized by clearness index (Kt). The Kt was determined by the ratio between global solar irradiance and solar irradiance at the top of the atmosphere. The highest occurrences of daily direct normal solar irradiance under conditions of Kt ≥ 0.6 were recorded between 400 W m−2 and 700 W m−2 for all seasons. Under conditions of 0.4 ≤ Kt < 0.6, the daily direct normal solar irradiance occurred between 200 W m−2 and 500 W m−2 and for conditions of Kt < 0.4, its maximum value was 200 W m−2. It was observed that the levels of solar incidence in the study region depend on cloud cover conditions, with little influence of seasonality.

scholarly journals A Novel Hybrid Model of WRF and Clearness Index-Based Kalman Filter for Day-Ahead Solar Radiation Forecasting

2019 ◽  
Vol 9 (19) ◽  
pp. 3967 ◽  
Author(s):  
Yuzhang Che ◽  
Lewei Chen ◽  
Jiafeng Zheng ◽  
Liang Yuan ◽  
Feng Xiao
Keyword(s):  
Kalman Filter ◽  
Solar Radiation ◽  
Hybrid Model ◽  
Weather Prediction ◽  
Numerical Weather Prediction Model ◽  
Energy Utilization ◽  
Mean Bias Error ◽  
Bias Error ◽  
Clearness Index ◽  
Sky Conditions

Day-ahead forecasting of solar radiation is essential for grid balancing, real-time unit dispatching, scheduling and trading in the solar energy utilization system. In order to provide reliable forecasts of solar radiation, a novel hybrid model is proposed in this study. The hybrid model consists of two modules: a mesoscale numerical weather prediction model (WRF: Weather Research and Forecasting) and Kalman filter. However, the Kalman filter is less likely to predict sudden changes in the forecasting errors. To address this shortcoming, we develop a new framework to implement a Kalman filter based on the clearness index. The performance of this hybrid model is evaluated using a one-year dataset of solar radiation taken from a photovoltaic plant located at Maizuru, Japan and Qinghai, China, respectively. The numerical results reveal that the proposed hybrid model performs much better in comparison with the WRF-alone forecasts under different sky conditions. In particular, in the case of clear sky conditions, the hybrid model can improve the forecasting accuracy by 95.7% and 90.9% in mean bias error (MBE), and 42.2% and 26.8% in root mean square error (RMSE) for Maizuru and Qinghai sites, respectively.

scholarly journals Effects of Sky Conditions Measured by the Clearness Index on the Estimation of Solar Radiation Using a Digital Elevation Model

2013 ◽  
Vol 03 (04) ◽  
pp. 618-626 ◽  
Author(s):  
Marcelo de Carvalho Alves ◽  
Luciana Sanches ◽  
José de Souza Nogueira ◽  
Vanessa Augusto Mattos Silva
Keyword(s):  
Solar Radiation ◽  
Digital Elevation Model ◽  
Clearness Index ◽  
Digital Elevation ◽  
Elevation Model ◽  
Sky Conditions

Standard Skies Classification Using Common Climatic Parameters

2004 ◽  
Vol 126 (3) ◽  
pp. 957-964 ◽  
Author(s):  
Danny H. W. Li ◽  
Chris C. S. Lau ◽  
Joseph C. Lam
Keyword(s):  
Cloud Cover ◽  
Measured Data ◽  
Mean Square ◽  
Climatic Parameters ◽  
Clearness Index ◽  
Luminance Distribution ◽  
Sky Conditions ◽  
Mean Square Errors ◽  
Climatic Elements ◽  
Sunshine Hour

A good understanding of the sky luminance distribution is essential for efficient daylighting 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 Kt. 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 Kt, 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.

scholarly journals Using a field radiometer to estimate instantaneous sky clearness

2006 ◽  
Vol 10 (2) ◽  
pp. 369-373 ◽  
Author(s):  
Eduardo G. Souza ◽  
Peter Scharf ◽  
Ken A. Sudduth ◽  
James. D. Hipple
Keyword(s):  
Vegetation Index ◽  
Global Radiation ◽  
Practical Application ◽  
Clearness Index ◽  
Band Ratio ◽  
N Application ◽  
Solar Noon ◽  
Cloud Effect ◽  
Sky Conditions ◽  
Reflectance Measurements

Reflectance measurements of crop plants and canopies show promise for guiding within-season, variable-rate nitrogen (N) application. Most research results have been obtained around solar noon with clear skies. However, for practical application, the system must work under cloudy skies or away from solar noon. The objective of this work was to assess the effect of cloud conditions on reflectance measurements of a corn canopy. The approach was to estimate an instantaneous sky clearness index (ICI) which could be used to correct field radiometer data for variations in cloud cover, such that the same reflectance reading would be obtained (and the same N recommendation made) for the same plants regardless of cloud conditions. Readings were taken from morning until night over 11 days with a range of sky conditions (sunny, overcast, partly cloudy). Data from clear days were used to estimate the theoretical expected spectral global radiation incident on a horizontal surface. The ICI was calculated as the ratio between the actual spectral global radiation and the corresponding theoretical global radiation. Analysis of the ICI for each band showed that the influence of cloudiness was different for each band. Thus, the cloud effect could not be compensated by the use of a band ratio or vegetation index.

scholarly journals Estimation of direct normal irradiance under various sky condi-tions in data sparse tropical ecological zones in Nigeria

2017 ◽  
Vol 5 (2) ◽  
pp. 90 ◽  
Author(s):  
Samuel Nwokolo ◽  
Julie Ogbulezie
Keyword(s):  
Performance Test ◽  
Model Performance ◽  
Hadley Cell ◽  
Clearness Index ◽  
Ecological Zones ◽  
Direct Normal Irradiance ◽  
Correlation Models ◽  
Condi Tions ◽  
Calculation System ◽  
Sky Conditions

In this study, models for estimating direct normal irradiance (Hb) under various sky conditions in data sparse tropical ecological zones in Nigeria were fitted. The evaluated values of clearness index indicate that the prevailing sky condition in the southern tropical zones is heavily overcast while the northern zones experience partly overcast under all sky conditions. From the analyses of the influences of clearness index on Hb, it was observed that higher values of Hb were recorded in the far north zone of Sahel savannah (FNZSS) of Sokoto while lower values were registered in the far south zone of mangrove swamp of Port Harcourt revealing an increasing trend from FSZMS to FNZSS due to the trends in cloudiness and associated atmospheric moisture with the movement through the Hadley cell calculation system along the equatorial line. The regression correlation models developed from the model performance test indicates that the proposed models could be used to estimate Hb accurately between latitude 4 to 10oN, that is, mangroves swamp and Guinea savannah tropical zones in Nigeria and other locations with comparable sky conditions.

scholarly journals Clouds over Hyytiälä, Finland: an algorithm to classify clouds based on solar radiation and cloud base height measurements

2020 ◽  
Vol 13 (10) ◽  
pp. 5595-5619
Author(s):  
Ilona Ylivinkka ◽  
Santeri Kaupinmäki ◽  
Meri Virman ◽  
Maija Peltola ◽  
Ditte Taipale ◽  
...  
Keyword(s):  
Solar Radiation ◽  
Global Radiation ◽  
Cloud Base ◽  
Aerosol Formation ◽  
Clearness Index ◽  
Detection Range ◽  
Clear Sky ◽  
Overall Performance ◽  
Sky Conditions ◽  
Cloud Base Height

Abstract. We developed a simple algorithm to classify clouds based on global radiation and cloud base height measured by pyranometer and ceilometer, respectively. We separated clouds into seven different classes (stratus, stratocumulus, cumulus, nimbostratus, altocumulus + altostratus, cirrus + cirrocumulus + cirrostratus and clear sky + cirrus). We also included classes for cumulus and cirrus clouds causing global radiation enhancement, and we classified multilayered clouds, when captured by the ceilometer, based on their height and characteristics (transmittance, patchiness and uniformity). The overall performance of the algorithm was nearly 70 % when compared with classification by an observer using total-sky images. The performance was best for clouds having well-distinguishable effects on solar radiation: nimbostratus clouds were classified correctly in 100 % of the cases. The worst performance corresponds to cirriform clouds (50 %). Although the overall performance of the algorithm was good, it is likely to miss the occurrences of high and multilayered clouds. This is due to the technical limits of the instrumentation: the vertical detection range of the ceilometer and occultation of the laser pulse by the lowest cloud layer. We examined the use of clearness index, which is defined as a ratio between measured global radiation and modeled radiation at the top of the atmosphere, as an indicator of clear-sky conditions. Our results show that cumulus, altocumulus, altostratus and cirriform clouds can be present when the index indicates clear-sky conditions. Those conditions have previously been associated with enhanced aerosol formation under clear skies. This is an important finding especially in the case of low clouds coupled to the surface, which can influence aerosol population via aerosol–cloud interactions. Overall, caution is required when the clearness index is used in the analysis of processes affected by partitioning of radiation by clouds.

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