The Earth's layers of atmosphere differ in chemical composition and temperature. They combine to create a protective sheild that maintains our delicate energy balance essential for life on Earth. Most weather occures in the nearest layer, the troposphere (0-7 miles). The stratosphere is the level where jet airliners fly and the ozone layer resides (7-30 miles). Beyond that is the coldest part of the atmosphere, the mesosphere where only large helium balloons fly (30-50 miles). Finally, the thermosphere gradually fades into space (50-180 miles).