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Radio refractivity estimation is paramount in the planning and design of radio link/systems for the purpose of achieving optimal performances. In this study, the monthly average daily atmospheric pressure, relative humidity and temperature data obtained from the National Aeronautics and Space Administration (NASA) during the period of twenty two years (July 1983 -June 2005 for Osogbo (Latitude 7.47 0 N, Longitude 4.29 0 E, and 302.0 m above sea level) were used to estimate the monthly tropospheric radio refractivity and to investigate its variation with other meteorological parameters of monthly average daily atmospheric pressure, relative humidity, absolute temperature, saturation vapour pressure and radio refractive index. The field strength variability (FSV) and the radio horizon distance were also computed. The monthly variation of FSV using two years data (2003 -2004) was also investigated. The results of this study revealed that the values of radio refractivity are more during the rainy season than in the dry season. It was found that the maximum average value of tropospheric radio refractivity of 370.98 N-units and minimum average value of 332.36 N-units occurred in the months of May and January during the rainy and dry seasons respectively. 71.45 % of the total value of the radio refractivity was contributed by the dry term while the major variation is by the wet term radio refractivity. The average refractivity gradient computed for the study area under investigation was −42.69 Nunits/km and the average effective earth radius (kfactor) was 1.37 which corresponds to the conditions of super refraction. The annual maximum mean value of FSV is 7.72 dB and minimum monthly mean value of 0.07 dB was obtained for the study area. The implication of this FSV values is that the output of a receiving antenna in Osogbo may generally be subjected to changes not less than 0.07 dB in a year and not greater than 7.72 dB. The descriptive statistical analysis shows that the radio refractivity, relative humidity, absolute temperature and radio refractive index data spread out more to the left of their mean value (negatively skewed), while the atmospheric pressure data spread out more to the right of their mean value (positively skewed). The radio refractivity, relative humidity and radio refractive index data have positive kurtosis which indicates a relatively peaked distribution and possibility of a leptokurtic distribution. The atmospheric pressure and absolute temperature data have negative kurtosis which indicates a relatively flat distribution and possibility of platykurtic distribution.

This study investigates the most accurate sunshine and temperature dependent models for estimating global solar radiation over Makurdi and Ibadan situated in the Guinea savannah of Nigeria by comparing nine (9) different existing sunshine dependent models. The study also proposed two temperature dependent models that took the form of quadratic logarithmic and quadratic exponential and were compared to three existing temperature dependent models (Chen, Hargreaves and Samani (HS) and Garcia). The measured monthly average daily global solar radiation, sunshine hours, maximum and minimum temperature meteorological parameters during the period of thirty one (1980-2010) years was utilized and the accuracy of the sunshine and temperature dependent models to ascertain the most suitable models in each location were tested using seven various statistical validation indicators of coefficient of determination (R 2), Mean Bias Error (MBE), Root Mean Square Error (RMSE), Mean Percentage Error (MPE), t-test, Nash-Sutcliffe Equation (NSE) and Index of Agreement (IA). The results revealed that the exponent sunshine dependent model proposed by Bakirci and the linear exponential sunshine dependent model proposed by Bakirci were found more accurate for estimating global solar radiation in Makurdi and Ibadan respectively. The proposed quadratic logarithmic and quadratic exponential temperature dependent models were found more suitable for estimating global solar radiation in Makurdi and Ibadan respectively. These recommended models can be found appropriate, if properly calibrated in regions with similar climatic information. The HS temperature dependent model evaluated in this study for Ibadan was compared with those available in literatures and was found more suitable. Furthermore, the most suitable sunshine dependent model was found more suitable for global solar radiation estimation when compared to the most suitable temperature dependent model in each of the studied locations and this was testified from the figures of the comparison between the measured and estimated sunshine and temperature dependent models as the sunshine dependent models depicts the best fitting with the measured global solar radiation data.

In this study, the shortwave solar energy balancing at the edge of the earth's atmosphere was employed to estimate the variation of albedo for Sokoto, Nigeria (Latitude 13.02°N, Longitude 05.25°E and altitude 350.8 m above sea level). The measured meteorological parameters of monthly average daily global solar radiation, minimum and maximum temperature, relative humidity and rainfall during a period of thirty one years (1980-2010) were utilized in this study. The results show that the albedo depicts direct opposite relationship with the clearness index, an inverse relationship with the emitting earth's surface temperature and direct relationship with the wavelength for the study area. The highest value of albedo simulated was in the month of August (0.4925) and the lowest in the month of November (0.3185). The emitting earth surface temperature for each month ranged between 235.2214 K in August and 253.2080 K in November, these values agrees closely to the standard emitting earth surface temperature (255.0000 K). The maximum emitting wavelength revealed that the radiation is longwave and are found within the infrared region of the electromagnetic spectrum. The albedo deduced from this study ranged from 0.3185 to 0.4925 which are consistent with the possible values of albedo of different surface covering the earth's surface and agrees closely to the standard planetary albedo (0.3000). The

In this study, five different temperature dependent models were proposed and compared with three existing temperature dependent models (Chen, Hargreaves and Samani (HS) and Garcia) using measured monthly average daily global solar radiation, maximum and minimum temperature meteorological parameters during the period of thirty one (1980 -2010) years. The comparison assessment using seven different statistical validation indices of coefficient of determination (R 2 ), Mean Bias Error (MBE), Root Mean Square Error (RMSE), Mean Percentage Error (MPE), ttest, Nash -Sutcliffe Equation (NSE) andIndex of Agreement (IA) was employed to determine the most accurate model for estimating global solar radiation in the five tropical locations situated in the coastal climatic zone of Nigeria. It was found that in each of the locations under investigation the proposed temperature dependent models were reported more suitable for global solar radiation estimation. In Akure, Ogoja, Ikeja, Benin and Owerri; the proposed quadratic logarithmic, linear exponential, multiple linear, quadratic (exponential 2) and quadratic (exponential 1) temperature dependent models respectively were found more accurate and can be applied to locations with comparable weather condition, if properly calibrated against the local data. The HS and Garcia temperature dependent models evaluated in this study for Ikeja was compared with those available in literature and was found more accurate. The figures comparing the measured and estimated temperature dependent models indicates that the most accurate models in each location exhibit the best fitting with the measured global solar radiation data.

In this paper, the monthly variation of Surface Water Vapour Density (SWVD) with meteorological parameters of monthly average daily mean temperature, relative humidity, surface pressure, cloud cover and sunshine hours during the period of sixteen years (2000-2015) for Owerri (Latitude 5.48°N, Longitude 7.00°E, and 91m above sea level) were investigated. The daily variation of surface water vapour density for the two distinct seasons considering two typical months in each during the period of year 2015 was examined. The results showed fluctuation in the amount of surface water vapour density in each day of the month for the period under investigation. The monthly average daily values indicated that the surface water vapour densities are greater during the raining season than in the dry season. It was observed that the maximum average value of surface water vapour density of 21.002gm-3 occurred in the month of June during the raining season and minimum value of 14.653gm-3 in the month of January during the dry season. The highest value of surface water vapour density was observed on 9 th May, 2015 and the lowest on 14 th January, 2015. The comparison assessment of the developed SWVD based models was carried out using statistical indices of coefficient of determination (R 2), Mean Bias Error (MBE), Root Mean Square Error (RMSE), Mean Percentage Error (MPE), Nash-Sutcliffe Equation (NSE) and Index of Agreement (IA). The developed multivariate correlation regression model that relates temperature and relative humidity with R 2 =99.9% MBE=0.1259 RMSE=0.1462 MPE=-0.6739 NSE=99.8402% and IA=99.9611% was found more suitable for surface water vapour density estimation with good fitting and therefore can be used for estimating surface water vapour density in the location under investigation and region with similar climatic information. The results of the descriptive statistical analysis revealed that the surface water vapour density, mean temperature, relative humidity, cloud cover and sunshine hours data spread out more to the left of their mean value (negatively skewed), while the surface pressure data spread out more to the right of their mean value (positively skewed). The surface water vapour density data have positive kurtosis which indicates a relatively peaked distribution and possibility of a leptokurtic distribution while the mean temperature, relative humidity, surface pressure, cloud cover and sunshine hours data have negative kurtosis which indicates a relatively flat distribution and possibility of platykurtic distribution.

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