The diurnal and seasonal variations of the incoming solar radiation have been studied by analysing two years data measured between January, 2016 to December, 2017 at a Tropical station, Ile-Ife (7.53°N; 4.54°E), Nigeria. The maximum incoming solar radiation flux which occurs between 13:00 – 14:00 LT and varies in the course of the year from 639.5 ± 171.6 Wm -2 (with large fluctuations) in the wet months (March – October) to 700.7 ± 105.2 Wm -2 in the dry months (November – February). The large differences in the values, diurnal and seasonal variation of the measured incoming solar radiation between the dry and wet seasons are attributed to the attenuation of the flux by aerosol particles in the dry season and increased cloudiness and humidity in the wet season. The monthly maximum values of 760.3 Wm -2 and 732.8 Wm -2 indicated a double peak from March to May and October to November respectively while a minimum of about 492.7 Wm -2 was recorded from July to August. Similarly, the daytime average had a double peak of 412.5 Wm -2 and 361.3 Wm -2 in March/April/May and October/November respectively, equally a minimum value of about 249.8 Wm -2 was recorded in July/August. The maximum value of the air temperature (which occurs around 15:00 LT) was observed to lag behind the maximum value of the incoming solar radiation (which occurs around 13:00 LT) by 2 hours at the study site. The statistical analysis of the monthly daytime averages of the incoming solar radiation showed that the intensity of the flux received at Ile-Ife (a tropical location) is high (about 67% of the incoming solar radiation are between the interval 325 and 400 Wm -2 ) throughout the year.
En este estudio se utiliza una base de datos de radiación solar de dos años (enero 2016-diciembre 2017) para modelar índices de claridad y nubosidad en Ile-Ife, Nigeria, una localidad tropical. Los análisis de variación diaria mostraron que en la mayor parte de los días la nubosidad es uniforme en el sitio de estudio, con muy pocos días extremadamente claros o nubosos. En el plano mensual, el índice de claridad fluctuó entre 0.27 y 0.51, en tanto que el índice de nubosidad varió de 0.44 a 0.72, lo cual confirma que las variaciones en ambos índices son inducidas por cambios en la posición del sol, así como por la turbidez y las nubes. Se observó que las variaciones estacionales de ambos parámetros son inversas, de manera que los incrementos en el índice de claridad favorecidos por cielos despejados y meses sin lluvia corresponden a descensos en los valores del índice de nubosidad y viceversa. Los valores mínimos del índice de claridad (0.10 y 0.28) en el sitio de estudio fueron mayores al valor mínimo (0.5) establecido para dicho índice en condiciones de nubosidad. Se desarrollaron dos ecuaciones empíricas para calcular el máximo índice de claridad a partir del índice de claridad promedio y el índice de nubosidad a partir del índice de claridad, y se sugiere su uso para locaciones con condiciones climatológicas similares al área de estudio. También se comprobó que los modelos de Hollands y Huget y de Saunier et al. tuvieron un buen desempeño, el cual mejoró en alrededor de 23.62 y 3.66%, respectivamente, después de la calibración, por lo que se consideran adecuados para su uso en otras zonas tropicales.
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