Understanding the Characteristics of the U.S. Standard Atmosphere 1976 Source

This page describes the characteristics, specificities and typical use case scenario of the U.S. Standard Atmosphere 1976 Ambient Source.

The Ambient Source U.S. Standard Atmosphere 1976 is an ambient source that simulates a sky with a spectrum ranging from 280nm into ultraviolet to 4µm into infrared.

This Ambient Source is mainly used to compute irradiance. The visual rendering may then not appear realistic, and may not be considered as such. For a realistic rendering, rather use the CIE Standard Overcast or General Sky Ambient Source.

Characteristic of the Source

  • Sky, sun spectra and radiance vary according to the sun position:

    • Sky spectrum depends on the sun elevation and is constant over the whole sky for a given position of the sun.
    • Sun spectrum depends on its elevation.
    • Radiance levels depend on the solar zenith angle. Azimuthal variations are not considered.

  • The atmospheric and meteorological conditions are defined in the U.S. Standard Atmosphere, 1976 and cannot be modified.

    The conditions are a mid-latitude northern hemisphere model of daytime atmospheric temperature and pressure, for a clear sky with large visibility distance.

The chart below shows the radiance of the sky for a given sun position according to the solar zenith angle.

Note: The radiance of the sun is not considered in the chart.
Figure 1. Figure 1. Radiance behavior according to solar zenith angle.

The more the solar zenith angle, the more radiance:

  • When the sun is at the zenith, the radiance level is high and increases when the sensor orientation approaches the horizon.
  • When the sun is on the horizon, the radiance is low (dawn or dusk condition) and presents a flat profile (with a slight increase at the horizon).

Typical Use Case

The U.S. Standard Atmosphere Ambient Source can be used to compute the irradiance on solar panels.

To be realistic, the Ambient Source follows the ASTM-G173 Standard when used in the conditions of this standard, that is when the solar zenith angle is 48.236° (air mass 1.5).

Figure 2. Figure 2. ASTM-G173 Standard conditions representation.