## 1 Introduction  
The portion of the solar spectrum that plant biochemical processes use in photosynthesis for converting light energy into biomass is a composite of wavelengths between 400 and 700 nm that are diffused within the visible light spectrum band (380-780 nm). These wavelength limits define the so-called photo-synthetically active radiation that covers both photon (Photosynthetic Photon Flux Density, \(Q_{p}\) (umol\(\cdot\)s\({}^{-1}\)\(\cdot\)m\({}^{-2}\))) and energy (PAR, Photo-synthetically Active Radiation, W\(\cdot\)m\({}^{-2}\)) terms [1]. Usually, \(Q_{p}\) is recorded, and converted into energy units according to the McCree conversion factor of \(4.57\) umol\(\cdot\)J\({}^{-1}\pm 3\%\) depending on climatic factors [2]. Accurate PAR estimations are needed for modelling plant productivity and biomass production [3], natural illumination in greenhouses [4], plant physiology studies and leaf photosynthesis [5], to measure the productivity of forests [6], and to calculate the euphotic depth of the oceans [7]. Moreover, accurate PAR measurements have become central to the determination of deforestation and climate-change impacts on agriculture [8]. A global routine network for measuring PAR has yet to be established. This parameter is often indirectly calculated, due to the scarcity of PAR data, based on its relationship with global horizontal solar irradiation, \(R_{s}\). The conventional PAR/\(R_{s}\) ratio falls between 0.45 and 0.50 [9]. [PERSON] [10]estimated the PAR/\(R_{s}\) ratio at between 44% and 45% at sea level with a solar zenith angle of 30\({}^{\circ}\) over the horizon. [PERSON] [11] suggested that a constant ratio of 50% can be a good approximation for practical applications regardless of atmospheric aerosol and water vapor concentrations [12]. In terms of \(\mu\)mol\(\cdot\)J\({}^{-1}\), empirical relations [13] established \(Q_{p}/R_{s}\) ratios of between 2.1 and 2.9 \(\mu\)mol\(\cdot\)J\({}^{-1}\), depending on the location. The monthly \(Q_{p}/R_{s}\) average was calculated on a daily basis from experimental data [14] collected in an arid climate at between 2.02 and 2.19 \(\mu\)mol\(\cdot\)J\({}^{-1}\) and the mean daily value was 2.16 \(\mu\)mol\(\cdot\)J\({}^{-1}\). In Spain, [PERSON] et al. [15] estimated a mean value of 1.95 \(\mu\)mol\(\cdot\)J\({}^{-1}\) a value close to other values from different locations [3; 16]. [PERSON] et al. [17] evaluated the \(Q_{p}/R_{s}\) ratio at many locations within China at between 1.75 \(\mu\)mol\(\cdot\)J\({}^{-1}\) and 2.30 \(\mu\)mol\(\cdot\)J\({}^{-1}\). Several studies have been conducted to determine the relation between PAR/\(R_{s}\) and different parameters. In some cases, significant relationships were found, but any dependencies on site geography, climatic and weather factors, seasonal trends, and both day length and diurnal effects were very slight and negligible for practical purposes [18]. Solar elevation has no significant effect on PAR/\(R_{s}\) when greater than 10\({}^{\circ}\)[18; 19]. The variations of this ratio with sky conditions have been studied to develop weather-dependent functions. Most studies have concluded that the PAR/\(R_{s}\) ratio presents its highest values for cloud-covered skies [20; 21]. This fact is attributable to cloud-related absorption and diffusion of solar radiation across different regions of the spectrum. The observed seasonal dependence of broad-band solar radiation is essentially caused by changes to turbidity, precipitable water, ozone, and clouds within the air masses at the location throughout the year [22]. The presence of water vapor increases the absorption effects within the infrared region of the spectrum, decreasing broadband solar irradiance levels to a greater extent than PAR. A secondary effect of the atmospheric water content is the enhancement of aerosol-related diffusion, which affects PAR more than broadband solar irradiance, \(R_{s}\)[4; 23].