solar constant

The average density of solar radiation measured outside Earth's atmosphere and at Earth's mean distance from the sun, equal to 0.140 watt per square centimeter.

The total solar radiant energy flux incident upon the top of the Earth's atmosphere at a standard distance (1 astronomical unit, 1.496 × 10⁸ km or 9.3 × 10⁷ mi) from the Sun. In 1980 it was discovered that the so-called solar constant actually varies with time, though only by small amounts, around a value of about 1367 W · m⁻² (1.96 cal · cm⁻² · min⁻¹).

Both expected and unexpected items contribute to the variability of the so-called solar constant. Sunspots can produce deficits of up to a few tenths of 1% of the solar constant, on typical time scales of 1 week. Other surface manifestations of solar magnetic activity, faculae, contribute excesses rather than deficits. Global oscillations of the solar interior, analogous to seismic waves on the Earth, produce variations of a few parts per million on time scales of a few hundred seconds. Finally, and unexpectedly, there is an apparent 11-year sunspot cycle variation amounting to an approximately 0.1% increase of the solar constant during the sunspot maxima. This long-term effect has the opposite dependence from that found for individual sunspots, which block the solar radiant energy and cause decreases rather than increases. See also Helioseismology; Sun.

The rate per unit area at which solar radiation reaches the outer margin of the earth's atmosphere. This fixes the energy supply for the atmospheric heat engine. Despite its name, the solar constant probably varies slightly over time.

The average rate at which radiant energy is received by the earth from the sun; equal to 430 Btu per hr per sq ft (1.94 cal per min per sq cm); a constant employed in calculating air-cooling loads due to the effects of solar radiation on buildings.