The atmosphere and its layers

Look up at the sky: clouds, sunshine, a breeze on your face. All of these come from a blanket of air round the Earth, called the atmosphere (vayumandal). Gravity holds it down around the planet.

"Could we live without it?" No. The atmosphere:

• gives us the air we breathe;
• shields us from the Sun's harmful rays, such as ultraviolet radiation;
• keeps the Earth warm, by trapping some of the Sun's energy so it cannot all escape back to space;
• drives our weather and climate: temperature, humidity and air pressure.

What is air made of? (Fig. 3.2)

The 'others' include helium, neon, krypton, xenon, ozone and hydrogen. Air also holds water vapour (usually 0.1 to 0.4 per cent, which makes clouds and rain) and tiny dust particles.

Watch out: most people guess that oxygen is the biggest part of air. It is nitrogen, nearly four parts out of five. And carbon dioxide, so often in the news, is only 0.04%.

The atmosphere has layers, stacked like the floors of a very tall building. They are told apart by how temperature and density change as you go up. Air is thickest (densest) near the ground and gets thinner the higher you go. Height above sea level is called altitude.

1. Troposphere (ground to about 12 km on average): the most important layer. It has the air we breathe, most of the water vapour and clouds, and nearly all weather: rain, fog, hail. Here it gets colder as you climb; that is why hilltops are cool. Its upper edge is the tropopause.

2. Stratosphere (up to 50 km): no clouds and no storms, so it is ideal for aeroplanes. It holds the ozone layer, which filters out the Sun's harmful ultraviolet rays. Its upper edge is the stratopause.

3. Mesosphere (up to 80 km): the temperature falls again as you go up. Most meteors (rocks from space) burn up here; those are the 'shooting stars' you may see at night.

4. Thermosphere (80 to 700 km): the temperature rises very fast, because the gas here absorbs the Sun's X-rays and short-wave ultraviolet rays. Its lower part, the ionosphere, bounces radio waves back to Earth, which helps radio signals travel. The auroras happen here too.

"What is an aurora?" The word means 'dawn' in Latin. The Sun sends out charged particles, the solar wind. Near the Earth they are steered towards the magnetic poles, and when they hit the gases there, each gas glows with its own colour. In the north it is the Aurora Borealis; in the south, the Aurora Australis.

5. Exosphere (the top): the air is very thin. Gravity is so weak here that light gases like hydrogen and helium drift off into space.

Watch out: temperature falls with height in only two layers, the troposphere and the mesosphere. In the stratosphere and thermosphere it rises.

"If air has weight, why don't we feel it pressing on us?" Because it presses from all sides, and the pressure inside our bodies pushes back equally (Fig. 3.5). The two cancel out.