Dictionary
Atmosphere

The Earth is surrounded by a blanket of air, called atmosphere, which reaches up to 560 Km from the surface.

What is it made of?

The atmosphere is mainly composed of Nitrogen (N2, 78%) Oxygen (O2, 21%) and argon (Ar, 1%). Many other very important components are also present, such as water (H2O 0-7%) Ozone (O, 0-0.01%) Carbon dioxide (CO2 0.01-0.1%).

 

What happens in the atmosphere?
Life is supported by the atmosphere, that absorbs energy from the Sun, working to provide moderate climate and protecting us from high energy radiation. For example, a very important phenomenon that takes place in the atmosphere, is the evaporation of the oceans, that leads to a constant influx of water vapor (mostly concentrated in the lowest parts of the troposphere) allowing a constant recycling of water and chemical elements.
Another important role is also played by the atmosphere when talking of motion. In fact the air, which is present in the lower layers of the atmosphere, opposes a resistance to the motion of any body. This, is one of the main causes of friction.


How light penetrates the different layers of the atmosphere

How is it composed?

This envelope of gas changes going upward from the ground. Using thermal characteristics, chemical composition, movement and density, four distinct layers have been identified:


Image: courtesy NASA
Troposhere is the first layer, going from the ground to 4.5 Km. It is the most dense part of the atmosphere: almost all climatic changes happen in this region. Together with the tropopause, which separates it form the next layer, it is called the lower atmosphere.

Stratosphere starts above tropopause, up to 50 Km high. It is a dry and less dense region, with a higher temperature due to ultraviolet radiation absorption, by the ozone layer. 90 % of what is called air is located here. The stratopause separates this layer from the next.

Mesosphere extends up to 85 Km, with a temperature that falls progressively up to -93 degrees Celsius. The chemical elements present in this region are in an excited state, due to the absorbed radiation from the Sun. The mesopause separates this layer from the next. The region covered by stratosphere and mesosphere is called middle atmosphere.

Thermosphere is the last layer, going toward space, extending up to 600 Km. Due to the Sun's energy, the temperature grows while increasing in altitude and chemical reactions occur much faster than on surface of the Earth. This is known as the upper atmosphere.

Exosphere starts beyond the atmosphere, and continues until it merges with interplanetary space, with very low density of Hydrogen and Helium.