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Condensed water falling from clouds or deposited from the atmosphere as liquid or solid particles. It can efficiently scavenge soluble gases and aerosol particles. Gas conversion into sulfate and subsequent atmospheric precipitation removal are separate stages of an atmospheric sulfur budget.

A foehn wind is relatively warm, dry downslope air on a mountain's lee side. In the classical condensation pathway, ascending windward air expands and cools at the dry adiabatic lapse rate until the lifting condensation level. Condensation then releases latent heat, reducing the cooling rate to the moist adiabatic lapse rate. Atmospheric precipitation removes condensate, and the descending parcel warms by compression at approximately the dry rate. Its relative humidity falls.

For equal starting and finishing heights, complete condensate removal and a constant moist lapse rate above condensation height $z_c$, the precipitation-driven foehn warming isHere $H>z_c$ is the parcel's maximum height and ${\Gamma }_d>{\Gamma }_m$. The warming is adiabatic compression during descent, not heating by friction. Without condensation or a difference in source air, a reversible dry parcel returning to its original height would regain its original temperature.

Precipitation is not essential to every foehn event. Isentropic foehn drawdown can bring air of higher potential temperature from aloft down the lee slope; turbulent mixing and extra solar heating beneath clear skies can also contribute. The sketch shows the classical rainout pathway rather than asserting a unique mechanism.^[ref]

Injected sulfur dioxide is oxidized, principally through hydroxyl radical chemistry, to sulfuric acid, which nucleates or condenses into aqueous sulfate particles. A suitably sized, weakly absorbing particle population scatters some incoming sunlight back to space, increasing planetary albedo and decreasing absorbed solar energy. The sulfur dioxide molecules are precursors, not the persistent reflective population.

The albedo lifetime reflects aerosol formation, growth/aerosol coagulation, sedimentation, stratospheric transport and eventual return to efficiently scavenged tropospheric air. The stratosphere has little atmospheric precipitation, so the effect can outlast analogous tropospheric aerosol. The assumed year is an effective persistence scale, not a universal lifetime of sulfur dioxide oxidation or an exact clock for every particle.

First, weakly absorbing sulfate scatters shortwave electromagnetic radiation and tends to cool the surface/planetary energy budget, with magnitude and sign influenced by cloud background and particle optics. Second, sulfate can supply cloud condensation nuclei: at fixed cloud water, more activated droplets give smaller droplets and greater cloud albedo through the Twomey effect; atmospheric precipitation and water-content adjustments can modify that idealized cooling. Third, sulfate particles change atmospheric composition through uptake, aqueous chemistry and heterogeneous surface reactions, with their acidity and available reactants controlling what is converted. Particle surfaces alone do not guarantee polar-style stratospheric chlorine activation in an ordinary tropospheric cloud.^[ref]