On Earth, volcanic or man-made gases can lead to acid rain, with a bit of strong acids such as sulfuric acid (H2SO4) dissolved in water. On Venus, there is very little water. What little is left has mostly combined with SO3 to form H2SO4. The clouds and rain are made of very concentrated, even anhydrous (no water as a solvent), sulfuric acid. This rain never reaches the surface. Because of the heat, it evaporates before reaching ~30 km altitude. (A phenomenon called virga, which is common with H2O on Earth when the surface air is very dry.)