When there is so much water vapor in the air, it only takes a tiny drop in temperature to make the vapor condense and form tiny droplets of water, and we say the air is saturated.
So what happens when all these water droplets in a cloud meet a mass of cooler air? If the air is very moist, the droplets cannot evaporate. Instead they get bigger and bigger as more and more condensation takes place. Soon, each tiny droplet has become a drop and it starts to fall downward and we have rain !
