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Optical response at fixed cloud structure. Put $b=1-g$ and write the two-stream cloud albedo as $A=b\tau /(2+b\tau )$. At fixed scattering asymmetry parameter,At zero optical depth the last quotient is understood by its finite limiting value. Optical depth counts extinction along a ray. For an approximately homogeneous, monodisperse cloud,This uses spherical droplets, independent extinction, representative constant $N,r$, and the geometric optics limit in which the droplet radius greatly exceeds the relevant visible wavelength and $Q_{\mathrm{ext}}\simeq 2$. Diffraction supplies part of the extinction cross-section; it is not legitimate to obtain the factor two from geometric interception alone. A size distribution requires its second moment rather than an arbitrary mean radius.

With liquid-water density ${\rho }_w$ and cloud liquid-water content $L$, the volume constraint gives $r^3=3L/(4\pi {\rho }_{w}N)$. ConsequentlyThis provesKeeping $L,h,{\rho }_w,g$ fixed isolates the Twomey effect:It is a partial response, not a claim that cloud water and cloud geometry stay unchanged under every real aerosol perturbation.

Absolute and fractional susceptibility. Choose $N_{\ast }$ so that $A=1/2$ there, and put $n=N/N_{\ast }$. ThenThe cloud albedo susceptibility per added droplet decreases monotonically. Its small-$n$ asymptote is proportional to $n^{-2/3}$, while its large-$n$ asymptote is proportional to $n^{-4/3}$. Thus a fixed absolute droplet-number addition has much more leverage in a clean cloud than in an already droplet-rich cloud. The formal divergence as $N\to 0$ is not a physical infinite response: fixed water content would require indefinitely large drops, eventually invalidating the optical and cloud assumptions.

For a fixed fractional number addition the response is instead $dA/d\ln N=A(1-A)/3$, maximal at $A=1/2$ and small at either very low or nearly saturated albedo. This distinction is essential when comparing interventions specified by an absolute particle addition with those specified by a percentage increase.

Brightening and net radiation. Marine cloud brightening would increase the population of suitable cloud condensation nuclei, for example by producing sea-salt aerosol from seawater and allowing transport into susceptible marine boundary-layer clouds. Activation depends on particle size, hygroscopicity and the cloud's attained supersaturation; adding particles does not guarantee a one-for-one increase of droplets. At fixed cloud water, more activated droplets mean smaller radii, larger total extinction area, larger shortwave optical depth and more reflected sunlight.

Promising physical conditions combine relatively clean, persistent low clouds, appreciable incident sunlight and a dark underlying ocean. The extensive marine stratocumulus regions over the eastern subtropical oceans are examples of this combination. An already polluted, optically saturated cloud has little remaining shortwave leverage. Bright clouds over snow or ice also add relatively little contrast with the surface. Low clouds have temperatures closer to the surface than high clouds, so their extra longwave greenhouse effect is generally less able to offset the shortwave cooling. Sunlit seasons and latitudes matter; no shortwave benefit occurs at night.

Schematically, the fixed-cloud shortwave perturbation is $\delta F_{\mathrm{SW}}\sim -\mathcal{S}{f}_c\delta A$, where $\mathcal{S}$ is the local incident solar flux and $f_c$ the affected cloud fraction. Changes in cloud fraction, precipitation, entrainment, liquid water and longwave emission must also be evaluated before equating the positive derivative above with a guaranteed net cooling. The derivative identifies susceptibility, not a complete climate-response calculation.

Denote aerosol optical depth by ${\tau }_a$ to distinguish it from cloud optical depth. Retrieve $N$ as in d, collocate it with ${\tau }_a$, and estimate an aerosol-to-droplet susceptibilityA regression should control for water content, updraught and cloud regime; aerosol optical depth is not itself a direct measurement of activated nuclei. For an approximately constant $s_a$ over a factor two in aerosol, $N_2/N_1=2^{s_a}$.

Integrate the supplied cloud albedo susceptibility rather than treating a finite doubling as infinitesimal:For a small response, $\Delta A\simeq A_1(1-A_1)s_a\log 2/3$. If $F_{\mathrm{sw}}$ is the appropriate mean incoming shortwave flux and $f_c$ the fraction of the scene occupied by these clouds, the leading top-of-atmosphere perturbation, positive downward, isThe two transmissions account approximately for the atmosphere above the cloud and may be absorbed into an effective illuminating flux. In the simplest global transparent-overburden estimate $F_{\mathrm{sw}}=L_{⊛}/4$. Positive $s_a$ gives increased reflection and negative shortwave forcing, the Twomey effect. Numerical forcing cannot be determined from the given susceptibility alone: it needs baseline albedo, cloud fraction, illumination and an empirically constrained $s_a$. Humidity swelling, cloud contamination of aerosol retrievals and common meteorological influences can create spurious correlations. Changes in cloud amount, water path or lifetime are additional adjustments, not the fixed-water Twomey term derived here.