Solar radiation and clear sky global turbidity on the north coast of New South Wales

Measurements of global solar radiant exposure made over 17 years on the north coast of New South Wales were used to describe features of the radiation regime in a humid, subtropical, rural environment. Monthly means of daily solar radiant exposure varied from a high of 23.0 MJ m–2 in December to a low of 11.2 MJ m–2 in June. Whilst the monthly mean atmospheric transparency (ɽa) varied over a narrow range of 0.50-0.61 between months, daily values of ɽa varied widely. The frequency distributions of daily ɽa within a month were asymmetrical with a negative skew and modal values in the 0.6-0.8 range, which correspond to clear or nearly clear skies. The upper bound of clear sky atmospheric transparency (U B-CSAT) was estimated by fitting an envelope curve through the high valued ɽa, expressed as a function of day number. A second analysis identified all clear days from the smoothness of the diurnal trend in radiant exposure. A comparison of the two sets of results showed that on clear days, global turbidity could reduce the 'ta below the UB-CSAT by 0.02-0.04 units in summer and 0.07-0.09 units in winter. The magnitude of clear sky global turbidity was broadly related to the amount and variation of atmospheric humidity, but other factors could also contribute to the turbidity.