 Atmospheric supersaturation

The fractional excess of water-vapor pressure above equilibrium saturation over a flat pure-water surface at the same temperature: $s=p_v/p_{\mathrm{sat}}(T)-1$. Positive $s$ can drive droplet growth, but activation of a particular aerosol particle requires exceeding its critical supersaturation. Cooling, condensation and mixing jointly determine parcel supersaturation.