The difference in humidity is what causes the clouds to form disperse and to rain down.
Explanation:
When there is more humidity it means that there are more water droplets in the air.
This means that these water droplets can join with small particles in the air and form clouds around themselves.
These clouds go up in the air and get cooled down and condense so they then become clouds but they also become heavy because they are cold.
Now that they are heavy they begin to come down and the difference of humidity down below makes it warmer and then the clouds fall with the rain and the process begins again.
