Aerosol optical depth
= Aerosol optical depth
{title2=$\tau_a=\int\beta_{\rm ext}(z)\,dz$}
The dimensionless column <optical depth> from <aerosol> extinction measures attenuation by absorption and scattering at a stated wavelength. It depends on particle size, composition, humidity and vertical distribution; it is not a direct count of <cloud condensation nuclei>. Using it as a proxy for droplet-number forcing requires control of meteorological covariates and cloud-contaminated retrievals.