The atmosphere functions like a greenhouse. It lets heat radiation in, but prevents it from reflecting back into space. This is known as the greenhouse effect.

Without the greenhouse effect, the planet would be a very cold place. However, if the greenhouse effect becomes too effective, the Earth will become uninhabitable. 

The Sun and the atmosphere make it possible for water to travel from place to place on planet Earth. This is called the water cycle. 

The Sun's heat radiation changes the state of Earth's water supplies. It causes liquid water to evaporate into the atmosphere. The atmosphere transports the water vapor from place to place with wind. Eventually, the water vapor cools down and condenses back into liquid, which falls back down to the planet's surface.