Measuring direct solar radiation

Extra-terrestrial solar radiation impinging on the top of the atmosphere can be absorbed by the atmosphere, reflected or scattered by the atmosphere into space, or it can reach the ground. Solar radiation reaching the ground R_g may, for simplicity, be divided into two components –

1. direct radiation R_d, coming as a beam straight from the Sun, and
2. indirect or diffuse radiation R_i from the whole sky.

Many climatological stations worldwide have a long record of sunshine data measured by means of a Campbell-Stokes sunshine recorder. This instrument determines the daily duration of ‘bright’ sunshine. Bright sunshine occurs for a Cambell-Stokes recorder when R_d exceeds 120 Wm^-2 onto a surface perpendicular to the beam. This long climatological record would be more useful for climate studies if the daily total direct solar radiation R_dt (R_d integrated from sunrise to sunset) could be estimated from the daily duration of bright sunshine.

Stanhill (1) has described how to do this, using an empirical relationship between R_dt and n (the number of hours of bright sunshine per day). This relationship was developed by means of 16 years of both Cambell-Stokes daily sunshine duration data, and radiometer-based instantaneous measurements of R_d, at a place in Ireland. Monthly means of the daily average values of R_dt and n yielded the following linear relation

(to be precise, Stanhill includes a quadratic term, but its contribution is small). The correlation coefficient exceeded .95 for this relation. A similar correlation at a place in Israel gave the following -

It is remarkable that two equations above are both linear with similar coefficients, despite the great differences between the two latitudes (which determines the path lengths of the solar beam through the atmosphere) and between the two climates.

A linear equation for daily values of R_dt and n in Ireland is this -

but individual data were scattered more widely around this line.

At both sites, the data revealed an apparent small increase of cloudiness, i.e. a decline of the annual totals of bright sunshine during the period 1944-1995, by 2.99 hours each year in Israel and 4.53 h/a at the much cloudier site in Ireland. These changes resulted in a reduction of the annual total of direct radiation of 13.3 MJ m^-2 in Israel, and 10.6 MJ m^-2 in Ireland.

Reference

(1) Stanhill, G. 1998: Estimation of direct solar beam irradiance from measurements of the duration of bright sunshine. Internat. J. Climatology, in press