Aerosol-cloud interactions

Aerosol-cloud interactions are the ways airborne particles affect cloud formation, droplet and ice properties, precipitation, and cloud lifetime.

Cloud condensation nuclei: Aerosol particles on which water vapor condenses to form cloud droplets. These particles provide the initial sites for liquid cloud droplets.
Aerosol indirect effect: The influence of aerosols on climate through their effects on cloud properties and behavior. It is the climate pathway encompassing aerosol-induced cloud changes.
Aerosol optical depth: A measure of the extinction of light by aerosols through a vertical column of the atmosphere. Satellite estimates help characterize aerosol loading near observed clouds.
Meteorological confounding: The distortion of an observed relationship when atmospheric conditions influence both the proposed cause and outcome. Humidity and cloud dynamics can make aerosol-cloud correlations misleading.
Aerosol-cloud interaction uncertainty: Uncertainty in the magnitude and sign of cloud responses to aerosol perturbations and their climate effects. It remains a major source of uncertainty in estimates of human-caused radiative forcing.
Köhler theory: A theory describing how particle size and solute affect the equilibrium vapor pressure over an aqueous droplet. It explains the humidity threshold at which aerosol particles activate into droplets.
Effective radiative forcing: A measure of a climate driver’s energy imbalance after rapid adjustments in the atmosphere and land. Cloud responses to aerosols contribute to the forcing estimate.
Cloud droplet number concentration: The number of liquid cloud droplets per unit volume of air. It is a key observable linking aerosol conditions to cloud microphysics.
Aerosol-cloud covariance: The statistical co-variation of aerosol and cloud properties, which may reflect interactions or shared environmental controls. Observed covariance alone does not establish a causal aerosol effect.
Cloud fraction: The proportion of a specified area or atmospheric volume covered by clouds. Whether aerosols change cloud cover remains difficult to quantify and can strongly affect radiation.

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