What Is the Atmosphere?

The atmosphere surrounding our planet is full of air! The air in our atmosphere is made of molecules of different gases. The most common gases are nitrogen and oxygen. Nearly four out of five molecules in the atmosphere are nitrogen. About one in five molecules is oxygen. There are small amounts of other types of air molecules as well. Air is important for almost all life on Earth, including plants and animals. Animals need to breathe air to get oxygen. Plants need gases from the air for photosynthesis, which is how they get the food they need. 

When air molecules are packed closely together, they are at a higher pressure than when air molecules are more spread out. High in the atmosphere, air molecules are spread out and at lower pressure. 

Layers of the Atmosphere

The atmosphere surrounds the planet and has five layers. The lowest layer of the atmosphere, the troposphere, is where weather happens. The air in the troposphere layer is heated by the surface of the Earth, which is warmed by sunshine.

Above the troposphere is the stratosphere, where jet airplanes fly. In the stratosphere is the ozone layer. There are more ozone molecules in the ozone layer than anywhere else. And they protect us by blocking some of the Sun's strongest and most harmful rays. Scientists watch this layer closely because some of the ozone was destroyed by pollution. This made the ozone layer so thin near the South Pole that it's called an ozone hole. 

Above the stratosphere is the mesosphere, which includes the coldest part of our atmosphere. Above that is the thermosphere, where there are few air molecules, a lot of heat from the Sun, and the astronauts orbiting Earth in the International Space Station. Above the thermosphere is the exosphere at the edge of space.