  The Venusian clouds originate from the binary condensation of H$_{2}$SO$_{4}$
and H$_{2}$O. The two components strongly interact with each other via
chemistry and cloud formation. Previous works adopted sophisticated
microphysical approaches to understand the clouds. Here we show that the
observed vapor and cloud distributions on Venus can be well explained by a
semi-analytical model. Our model assumes local thermodynamical equilibrium for
water vapor but not for sulfuric acid vapor, and includes the feedback of cloud
condensation and acidity to vapor distributions. The model predicts strong
supersaturation of the H$_{2}$SO$_{4}$ vapor above 60 km, consistent with our
recent cloud condensation model. The semi-analytical model is 100 times faster
than the condensation model and 1000 times faster than the microphysical
models. This allows us to quickly explore a large parameter space of the
sulfuric acid gas-cloud system. We found that the cloud mass loading in the
upper clouds has an opposite response of that in the lower clouds to the vapor
mixing ratios in the lower atmosphere. The transport of water vapor influences
the cloud acidity in all cloud layers while the transport of sulfuric acid
vapor only dominates in the lower clouds. This cloud model is fast enough to be
coupled with the climate models and chemistry models to understand the cloudy
atmospheres of Venus and Venus-like extra-solar planets.
