Answer ยท confidence: thin
How much dust in the air turns a blue sky white or gray?
Research question: What is the quantitative threshold of aerosol optical depth required for Mie scattering to significantly alter the perceived color of the sky from blue to white or grey?
The provided records do not specify a single quantitative threshold of aerosol optical depth that causes the sky to shift from blue to white or grey. They indicate that an aerosol optical depth of 0.4 corresponds to very hazy conditions 1, while a value of 1 indicates very hazy conditions 4. The evidence notes that Mie scattering influences the spectral dependence of sky radiance, which is reflected in the red-to-blue ratio 5, but it does not link this mechanism to a specific optical depth value for color perception changes.
Checked 2026-10-13. Only documented and supported facts are used.
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[1] supported
An aerosol optical depth value of 0.4 corresponds to a very hazy condition.
"A value of 0.01 corresponds to an extremely clean atmosphere, and a value of 0.4 would correspond to a very hazy condition."SURFRAD Aerosol Optical Depth
[2] documented
The average aerosol optical depth for the United States is between 0.1 and 0.15.
"An average aerosol optical depth for the U.S. is 0.1 to 0.15."SURFRAD Aerosol Optical Depth
[3] supported
An aerosol optical thickness of less than 0.1 indicates a crystal clear sky with maximum visibility.
"An optical thickness of less than 0.1 (palest yellow) indicates a crystal clear sky with maximum visibility"Aerosol Optical Depth - Science@NASA
[4] supported
An aerosol optical thickness of 1 indicates very hazy conditions.
"whereas a value of 1 (reddish brown) indicates very hazy conditions."Aerosol Optical Depth - Science@NASA
[5] documented
The red-to-blue ratio (RBR) serves as an image-derived proxy for the spectral dependence of sky radiance associated with aerosol optical properties, specifically reflecting the particle-size-related wavelength dependence of aerosol extinction via Mie scattering.
"Thus, RBR is not merely an empirical color index, but can serve as an image-derived proxy for the spectral dependence of sky radiance associated with aerosol optical properties, particularly the particle-size-related wavelength dependence of aerosol extinction."AMT - An AI-based algorithm for retrieving aerosol optical depth and ...
[6] documented
The wavelength dependence of aerosol extinction is strongly related to particle size through Mie scattering and is commonly represented by the Angstrom exponent.
"Because the wavelength dependence of aerosol extinction is strongly related to particle size through Mie scattering and is commonly represented by the Angstrom exponent, the red-to-blue contrast can provide size-related information on the relative contribution of fine- and coarse-mode particles"AMT - An AI-based algorithm for retrieving aerosol optical depth and ...