Nigerian Journal of Solar Energy, Vol. 25, 2014. © Solar Energy Society of Nigeria (SESN) 2014. All rights reserved. 40 Impact of Changes in Diurnal Shortwave Solar Radiation on Air Pressure and Relative Humidity at Ilorin, Nigeria * Ajibola, T.B., Akoshile, C.O. and Fatigun, A.T. Department of Physics, University of Ilorin, Ilorin, Nigeria. Abstract - Solar Radiation incident on the Earth surface passes through the earth’s atmosphere and undergoes absorption, transmission and reflection. The fluctuation in intercepted solar radiation has effect on both living and non-living beings on the earth and is therefore worth examining. In this study, time dependent measurements of incident solar radiation flux, relative humidity (RH) and pressure using radiometers, pressure sensors and relative humidity measuring devices respectively, were made at Ilorin, Nigeria, bearing Lat. 8 0 32’ N, Long.4 0 34’ E. in the Tropics for a period of a solar cycle. The study showed that the shortwave (SW) global radiative flux has maximum in the day time between the hours of 1200 and 1400 Local Time (LT) while the RH maximum occurred at early dawn between 0600LT and 0900LT daily and minimum around 1800LT. The atmospheric pressure has 2 minima and 2 maxima per day. The plot of RH against SW gives a skewed Gaussian. The plot (RH-SW) gave a linear relationship of slope 9.79 X 10 -2 %/Wm -2 before the maximum and 4.56 X 10 -2 %/Wm -2 after the maximum with respective correlation coefficients (R) of 0.97 and 0.90. This implies that the maximum is approached sharply at about twice the rate of relaxation after the maximum before sunset. Pressure against SW plot gave a quadratic relationship before the turning point with R of 0.99 and a linear relationship after the maximum of gradient 6.6 X 10 -3 mbar/ (W/m2) and R of 0.98. Pressure against RH gave a quadratic relationship with R = .98. Daily RH has a minimum of 70% and an average of 90% in the raining season and 20% in the dry season. When the SW is high, RH dropped and Pressure increased. Keywords: shortwave solar radiation, relative humidity, climate change, solar activity, pressure 1. INTRODUCTION Solar electromagnetic radiation is recognized as the primary and almost the sole source of energy for myriads of physical and biological processes on the planet Earth. Shortwave radiation which is a component of the solar radiation supplies most significant energy needed for all forms of life on Earth (Acra, 1990). This is a study of the relationship between global shortwave solar radiation, atmospheric pressure and relative humidity for a period of 11 years between 1995 and 2005, using data obtained from measurements at the Baseline Surface Radiation Network (BSRN) Station at the University of Ilorin, Ilorin. Shortwave solar radiation in the wavelength range of 0.25 to 4 μm provides about 99% of the total incoming solar radiation (Mark, 1999). The received daily radiation per unit area peaks at about noon before reducing to a minimum as sunset approaches. The magnitude of the intercepted radiation is a function of time of day and time of year for any given latitude (McVeigh, 1976). The incoming shortwave solar radiation reaching the surface of the earth comes in as direct and diffuse radiation (Kumar et. al, 1997; Mark, 1999). Consequently, the Earth’s climatic system is constantly adjusting itself in a way that tends toward maintaining an energy balance between the energy reaching the earth from the sun and the energy that goes out from the earth back into space, in what is known as the earth’s “radiation budget” (Wilson, 2009). The relative humidity is a measure of the actual amount of water vapour in the air when compared with the capacity of water vapour that the atmosphere can hold at saturation; and atmospheric pressure is the amount of force per unit area exerted by the atmosphere on the objects on which it rests. That weight depends on density, temperature and composition of the atmosphere, which is not constant due to wind movement. Changes in the atmospheric pressure have consequent effects on weather. At the sea level, the atmospheric pressure is on the average 101.3 kPa. Atmospheric pressure decreases with height. Consequently, it is reasonable to measure SW, RH and pressure as the time changes at a latitudinal and longitudinal location. The long time variation of these parameters (in the measurements made) at a location dictates the description of the observed weather and climate of that location and this is what is being done at and concerning Ilorin. The result will be a good description of this geographical location and its surrounding latitude in the tropics. 2. MATERIALS AND METHOD 2.1. Measurements Radiometer converts incoming solar radiation to electric current in photo sensors due to release of *Corresponding author Tel: +234-8033840127 Email: bentai@unilorin.edu.ng