SOLAR MEASUREMENTS

Abstract

The sun's energy at the earth's surface varies continuously in time and space. The solar constant derived from ground-based measurements has varied from 1323 to 1428 W m−2 (1). Outside the atmosphere (air mass=0) the sun's energy is more predictable (American Society for Testing and Materials [ASTM, E490]). At the annual mean solar distance from the sun, we define a solar constant of radiation for the earth. This is the measured amount of solar energy flux incident normally on a unit area in a unit of time (1.373±0.008×106 erg/sec cm2). Typical atmospheric absorbers (water, ozone, CO2, dust, and industrial pollutants) scatter and absorb energy at selective wavelengths over the solar range from 0.3 to 2.5 μm. Scattered skylight plus the direct sunlight is referred to as the global radiation. It has a different spectral energy distribution than direct sunlight at the earth's surface, adding energy to blue and ultraviolet (UV) wavelengths (2, 3). The absorption effects of average amounts of atmospheric absorbers, superimposed on the solar spectral energy distribution above the earth's atmosphere, simulate solar energy at the earth's surface. A number of proposed standard solar irradiance curves (W/m2 μm) are used to characterize solar properties of materials (4). When the sun is overhead, solar energy penetrates unit thickness of air (air mass=1). The absorption increases as the path length increases. Both vary approximately as the reciprocal sine of the sun's altitude angle. When the sun is 30° above the horizon, for example, the solar energy must pass through twice as much air (air mass=2) as it would if the sun were at the zenith position. A small correction must be applied to this simple relationship to correct for atmospheric refraction, especially if the sun is lower than 30° from the horizon (2) (Fig. 1).