Ångström Exponent

The Ångström Exponent (alpha, α) is a dimensionless parameter quantifying the wavelength-dependence of aerosol light scattering, providing atmospheric scientists and photographers with critical information about aerosol particle size distributions that determine haze color, intensity, and photographic characteristics. Named after Swedish physicist Anders Ångström who pioneered atmospheric turbidity research, this exponent is derived from the power-law relationship between aerosol optical depth and wavelength: AOD(λ) = β × λ^(-α), where λ is wavelength and β is a turbidity coefficient. PhotoWeather calculates the Ångström Exponent using AOD measurements at 550nm (green light) and 865nm (near-infrared), yielding α values typically ranging from 0 to 3. The physical interpretation is straightforward: high Ångström values (α > 1.5) indicate aerosol populations dominated by fine-mode particles (diameter < 1 micrometer) such as smoke, urban pollution, and sulfate aerosols, which scatter shorter wavelengths more efficiently creating neutral-to-cool-toned grey or bluish haze. Low Ångström values (α < 1.0) indicate coarse-mode dominance (particles > 1 micrometer) from dust, sea salt, or pollen, which scatter light less selectively across wavelengths producing characteristically warm-toned amber or orange atmospheric haze. Intermediate values (α = 1.0-1.5) suggest mixed aerosol populations with contributions from both fine and coarse modes.

For photographers, the Ångström Exponent provides diagnostic insight unavailable from total AOD alone: two locations might both show AOD = 0.20 (moderate haze), but α = 2.0 at one site (fine pollution particles creating grey, color-muted conditions) versus α = 0.6 at another (coarse dust creating warm-toned golden hour enhancement) represent vastly different photographic opportunities and aesthetic outcomes. Practical interpretation for photography: α > 2.0 indicates fresh smoke or urban pollution with grey, oppressive haze reducing color saturation—generally undesirable for landscape work; α = 1.5-2.0 suggests aged smoke or fine pollution with neutral haze—acceptable for some compositions if AOD is low; α = 1.0-1.5 indicates mixed aerosols with moderate color impact—most common in continental environments; α = 0.5-1.0 suggests coarse dust or sea salt dominance with potential for enhanced warm tones during golden hour—can be photographically advantageous in moderation; α < 0.5 indicates very coarse particles or measurement uncertainties.

PhotoWeather's CAMS-derived Ångström Exponent forecasts help photographers anticipate not just how much haze exists but what type of haze and what color signature it will impart to images. Combined with component AOD forecasts (dust, smoke, sea salt), the Ångström Exponent provides a complete aerosol characterization enabling informed decisions about whether atmospheric conditions will enhance or degrade specific photographic objectives.