Why optical weight is attached to angstrom model for optimization

The term "optical weight" in the context of the Angstrom model for optimization likely refers to the influence of atmospheric optical properties (like cloud cover, aerosols, and water vapor) on solar radiation. These properties are implicitly "attached" to the Angstrom model through its empirical constants 'a' and 'b'.

Here's why this approach is suitable for optimization:

• Empirical Constants Reflect Local Conditions: The Angstrom model's constants, 'a' and 'b', are determined through local calibration using measured insolation and sunshine duration data. These constants inherently encapsulate the average "optical weight" or atmospheric transmittance characteristics of that specific location.

  • Constant 'a' represents the fraction of extraterrestrial radiation reaching the surface on a completely cloudy day (when sunshine duration is zero). This directly reflects the atmospheric attenuation under diffuse sky conditions.
  • Constant 'b' represents the sensitivity of global radiation to changes in sunshine duration, indicating how effectively the atmosphere transmits direct solar radiation.

• Optimization Through Calibration: By optimizing the values of 'a' and 'b' for a particular site, you are effectively optimizing the model to account for the local "optical weight" of the atmosphere. This ensures the model provides the most accurate estimates of ground-based insolation for that specific geographical and climatic context. This calibration process minimizes the difference between the model's predictions and actual measured insolation.

• Simplicity and Practicality: Instead of requiring complex measurements of individual atmospheric optical parameters (like aerosol optical depth or cloud optical thickness), the Angstrom model simplifies this by lumping these effects into two empirical constants. This makes it a practical and robust tool for optimization, especially in regions where detailed atmospheric data might be scarce.

In essence, the "optical weight" is not explicitly a separate variable in the Angstrom model, but its effects are implicitly integrated and optimized through the site-specific calibration of the 'a' and 'b' coefficients.