Have you ever flown through clouds in an aircraft, or perhaps been high up on a mountain where the clouds swirled act about you? Then you must have gained a fairly good idea of what a cloud is: just an accumulation of mist.

As you know, there is always water vapor in the air. During the summer there is more of this vapor in the air because the temperature is higher. When there is so much water vapor in the air that just a small reduction in temperature will make the vapor condense Form tiny droplets of watery, we say the air is saturated.

It takes only a slight drop in temperature to make water vapor condense in saturated air. So when saturated warm air rises to an altitude where the temperature is lower, condensation takes place and we have a cloud. The molecules of water have come together to form countless little droplets.

What happens if act these water droplets in a cloud meet a mass of warm air? They evaporate and the cloud disappears! This is why clouds are constantly changing shape. The water in them is changing back and forth from vapor to liquid.

The droplets of water in a cloud have weight, so gravity gradually pulls them down and they sink lower and lower. As most of them fall, they reach a warmer layer of air, and this warmer air causes them to evaporate. So here we have clouds that don’t produce rain. They evaporate before the drops can reach the earth as rain.

But suppose the air beneath a cloud is not warmer air? Suppose it’s very moist air? Naturally, the droplets won’t evaporate. Instead, the droplets will get bigger and bigger as more and more condensation takes place

Pretty soon, each tiny droplet has become a drop and it continues falling downward and we have rain!