I'll give it my best shot, warm air rises, hits the cold air aloft, condenses to form clouds and rain.
Explain why rising air is needed to produce rain?
In simple terms, rain is produced by water vapour gathering to form clouds. I think the "rising air" you're talking about is the evaporation process, which is central to the formation of clouds.
- Heat produced from the sun causes the earth's water (from rivers, lakes, oceans, or even plants) to evaporate
- This creates water vapour, which is water in it's gaseous form, which rises in the air
- As the vapour rises, the surrounding temperature cools, turning the vapour (gas) into droplets (liquid).
- The air is filled with dust, and as the droplets collide with this dust, clouds are formed.
- As this process happens repeatedly, clouds get bigger, and water droplets collide with each other, making them heavier. When they get heavy enough, gravity takes over and the droplets fall to earth as rain.
Clouds are pretty amazing, really. Check out this time lapse vid over Cancún in Mexico:
When water is evaporated from waterbodies that heat up due to the sun's heat, they become water vapour. They then rise up till they reach the clouds where they cool down, condense, become water droplets and become heavy. Hot air is light so it rises but cold air is heavy so it settles on something.
As more water vapour condenses to form droplets, they join together to form clouds. When clouds are heavy with a lot of water droplets that they cannot hold, they turn grey and that's when they let go of all those droplets resulting in rain.