International Journal of Sustainable Energy Vol. 25, No. 2, June 2006, 107–115 Estimating net radiation using routine meteorological data at a tropical location in Nigeria O. O. JEGEDE*†, E. O. OGOLO‡ and T. O. AREGBESOLA§ †Department of Physics and §Institute of Education, ObafemiAwolowo University, Ile-Ife, Nigeria ‡Department of Physics, Adeyemi College of Education, Ondo, Nigeria §Department of Physics, Federal University of Technology, PMB 704Akure, Nigeria In this study, simple parameterizations, used to determine the net radiation, have been evaluated with dedicated field data from a tropical location at Ile-Ife, Nigeria. Separately determined were the net radiation for a clear-sky condition and a modification of this, incorporating the relative cloud amount as a multiplicative factor. Cloud amount has been computed as a ratio between the measured global radiation and that calculated for the clear-sky condition. In the absence of direct measurement of the global radiation, a simple empirical expression is given. From these estimations, differences between the clear-sky and the cloud-adjusted net radiation values were found to be substantial, thus indicating that cloudiness is an important factor influencing radiative balance of the tropical atmosphere. The scheme was compared with the actual measurements and remarkably good agreements have been obtained with correlations ranging between 0.87 and 0.94. Keywords: Net radiation; Parameterization; Tropical area; Cloudiness 1. Introduction The net radiation at a place is the algebraic sum of all incoming and outgoing components, both short-wave and long-wave radiation being radiated, absorbed, and reflected from both the sun and the earth’s surface. In general, the absorbed solar radiation exceeds the outgoing long-wave radiation in the tropical and subtropical regions, resulting in a net radiative heating of the planet, whereas in the middle to polar latitudes there is a net cooling. This equator- to-pole difference, or gradient, in radiative heating is the primary mechanism that drives the atmospheric and oceanic circulations. Thus, the net radiation budget at the surface is affected by several factors including geographic, atmospheric, and seasonal (temporal) features. The annual variation in net radiative energy follows that of the solar declination. Clouds have a strong effect on the radiative energy fluxes in the atmosphere. They scatter and absorb solar radiation, absorb and emit terrestrial (long-wave) radiation. In the long-wave, clouds generally reduce the radiation emission to space and thus result in heating of the planet, whereas in the solar (or short-wave), clouds reduce the absorbed solar radiation, because of *Corresponding author. Email: oojegede@oauife.edu.ng International Journal of Sustainable Energy ISSN 1478-6451 print/ISSN 1478-646X online © 2006 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/14786450600593261