A note on the annual variability of solar radiation at Hobart Airport, Tasmania

The annual variability of solar radiation at Hobart airport is calculated. The implication of this variability for the solar energy potential of Tasmania is discussed. There is at present an increasing awareness of the potential for the use of solar energy in domestic and industrial application. An important facet in any practical use of solar energy is the day to day variability of the incoming solar radiation. This factor has largely been ignored in geographical assessments of solar radiation as they mostly deal with climatological monthly averages (Hounam 1963; Paltridge and Proctor 1976).
To illustrate the magnitude of the variability, ten years of daily solar radiation data (1968-1977) collected over Hobart airport were used in this analysis. The data was obtained from the Australian Bureau of Heteorology which maintains a network of solar energy monitoring stations throughout Australia. The data was collected by a pyranometer. At any one instant of time the instrument produces a millivolt signal proportional to all direct and diffuse solar energy incident on a unit horizontal surface per unit time (Robinson 1966). Integrating the signal over a day yields the daily flux which is commonly expressed in units of Mega-Joules per metre2 per day (MJm- 2day- 1). Cloudless conditions were investigated first since solar radiation under these conditions would represent a convenient upper boundary to the energy received at the surface. Inspection of these days gave a smooth curve for the daily distribution of solar radiation over the year (fig. 1). All data were then analyzed by grouping together daily totals of solar radiation for each day of the year with its counterparts.
The mean and standard deviation were then calculated for each group and are shown in figure 1. A ten day running mean through both data sets shows a maximum standard deviation in early January which exceeds 6 MJm- 2day-l or 28% of the incoming solar radiation. This value decreases with solar elevation reaching a minimum of 1.6 MJm-2 day-l in late June which represents approximately 35% of the incoming radiation.
The daily variability is dominantly a function of cloud cover, and to a lesser extent will depend on atmospheric water vapour and dust content (Davies et al. 1975).
Therefore it is possible to represent the cloudless solar radiation as a smooth curve which is usually within 5% of the observed daily values. On the other hand solar radiation depletion by clouds can be extremely variable since it depends on such cloud properties as type, amount, thickness, liquid water and water vapour content, etc.