2. First, we need two basic ingredients: water and dust.
On Planet Earth, naturally occurring clouds are composed primarily of water in its liquid or solid state. (On other planets, clouds may form from other compounds such as the sulphuric acid clouds on Venus.) Thus, we begin our recipe by collecting a sufficient quantity of water in the vapour state that we will soon transform into the liquid or solid states. The water vapour content of the atmosphere varies from near zero to about 4 percent, depending on the moisture on the surface beneath and the air temperature.
Next, we need some dust. Not a large amount nor large particles and not all dusts will do. Without "dirty air" there would likely be no clouds at all or only high altitude ice clouds. Even the "cleanest" air found on Earth contains about 1000 dust particles per cubic metre of air. Dust is needed for condensation nuclei, sites on which water vapour may condense or deposit as a liquid or solid. Certain types and shapes of dust and salt particles, such as sea salts and clay, make the best condensation nuclei.
With proper quantities of water vapour and dust in an air parcel, the next step is for the air parcel mass to be cooled to a temperature at which cloud droplets or ice crystals can form. And, voila, we have clouds.http://www.islandnet.com/~see/weather/elements/makerain.htm