## 1 Introduction  
Knowledge of solar radiation data finds application in many fields of science, including agricultural science, bioreeteorology, micrometeorology and environmental photochemistry. To mention a specific example, information relating to global solar irradiance is very important in estimating equilibrium surface temperature [1], which in turn is important in the study of the urban heat island effect. The latter phenomenon affects urban air quality through an increase in the emission of primary and precursors of secondary air pollutants [2].  
A literature survey revealed many solar radiation models, but they all seem to fall into one of two categories. The first category consists of 'detailed' models which require information on constituents of the atmosphere and are generally theoretical in nature. Researchers outside the physical sciences may find these models difficult to implement in their own specific models or even to use them because of their mathematical and computational complexity. Hence, detailed solar radiation models may not be suitable for routine applications. An example of such models is that of [PERSON] et al. [3]. This is a versatile radiation scheme developed to calculate radiative fluxes and heating rates for the solar and infrared emission spectrum, which incorporates the effects of atmospheric water vapour, carbon dioxide, ozone, pollution gases, aerosol particles and multilayered clouds of arbitrary cloud cover. The model is intended for use within circulation and climate models. Two other detailed models, which are more comprehensible to the non-mathematician, are those of [PERSON] and [PERSON] [4], and [PERSON] et al. [5]. The models of [PERSON] et al. [3], [PERSON] [4], and [PERSON] et al. [5] can be used for specific solar spectral regions, and this makes them suitable for applications such as outdoor photosynthetic studies and the effect of ultraviolet radiation on the skin. As mentioned earlier, the only disadvantage is their complexity and, hence, difficulty in implementation.  
The second category consists of solar radiation models which require routine meteorological data as input; hence, they may be termed'simple'. The models are not necessarily simple in structure; an example of this is the numerical solar radiation model of [PERSON] and [PERSON] [6]. In general, however, simple models can be very easily implemented into other specific models simulating some aspect of the environment since they normally consist of a small number of regression equations (or parametrisations) and a simple procedure which links the equations together. The only problem with simple models is that most of them are based on statistical analysis of empirical meteorological data collected at one or more stations, making them very site specific. Examples of models of the second category include those of [PERSON] et al. [7], [PERSON] and [PERSON] [8; 9], [PERSON] and [PERSON] [10], [PERSON] et al. [11], [PERSON] et al. [12] and [PERSON] and [PERSON] [13].  
The model developed and described in this paper has input requirements that consist of routine meteorological data. It has a theoretical basis with assumptions on atmospheric optical characteristics, making it less site specific. Generally speaking, in this work, the approach adopts relatively simple 'textbook' material, i.e., basic theory/information, leading to reasonably good solar radiation modelling, with outcomes that can be used for applications where accuracy is not a major requirement.