Gaseous atmosphere

The Earth’s atmosphere is a layer of gases held close to its surface by gravity. It plays a crucial role in supporting plant and animal life, helps to maintain a global average surface temperature of about 14°C, plays a major part in determining weather and climate, screens out harmful radiation from the Sun and acts as a buffer against meteorites.

Composition of the atmosphere

The principal components of the atmosphere are nitrogen (78%) and oxygen (21%), with the remaining 1% of the atmosphere being made up of argon (0.9%), carbon dioxide (0.037%) and trace amounts of other gases. The amount of water vapour in the atmosphere varies from 0–4% depending on temperature, pressure and location.

Nitrogen, oxygen and carbon dioxide have very important associations with life:

• Oxygen is exchanged between the atmosphere and life forms through the processes of photosynthesis and respiration.
• Certain types of soil bacteria can remove nitrogen from the atmosphere by ‘fixing’ it into nitrogen compounds usable by plants. This captured nitrogen is eventually released back into the atmosphere by bacterial decay of plant and animal material.
• Carbon dioxide is an essential ingredient in photosynthesis and a major product of respiration.

Average composition of the Earth’s atmosphere (to a height of 25 km)

Vertical structure of the atmosphere

About 80% of the atmosphere lies within 16 km of the Earth’s surface. On moving out from the surface, the atmosphere thins out until it merges with outer space.

One practical way of placing a limit on the extent of the atmosphere was proposed by Theodore von Kármán (1881–1963), a Hungarian-American engineer involved in the aeroplane and spacecraft industries. He suggested that a limit be placed at 100 km above the Earth’s surface. It is at this altitude that an aircraft would need to fly at a speed equivalent to an orbiting spacecraft to give it enough lift to remain airborne. This arbitrary line is called the Kármán line

Layers based on temperature

If regular temperature measurements are taken moving up through the Earth’s atmosphere, patterns emerge that allow the atmosphere to be divided into four layers – troposphere, stratosphere, mesosphere and thermosphere – with the boundaries between each given the suffix ‘-pause’. For example, the tropopause is the boundary between the troposphere and the stratosphere.

Homosphere and heterosphere

Another way of defining the structure of the atmosphere is based on the composition of the atmosphere. Below a height of about 80 km, the gases that make up the atmosphere are thoroughly mixed. This region is called the ‘homosphere’. Above this height, the gases separate out into bands related to their molecular masses with the heavier ones like oxygen and nitrogen present near the bottom. This region is called the ‘heterosphere’.

Ozone layer and ionosphere

Two other regions of the atmosphere often referred to are the ozone layer and the ionosphere.

The ozone layer is found within the lower part of the stratosphere from about 15–35 km. Although the actual concentration of ozone is very small, it plays a critical role in absorbing ultraviolet radiation, known as UV-C, that enters the atmosphere from the Sun.

The ionosphere is the part of the atmosphere that is ‘ionised’ by incoming solar radiation. It extends from about 85 km up to 600 km. The highly energetic UV radiation entering the Earth’s atmosphere can cause the removal of outer electrons from gas molecules, creating positively charged gaseous ions. This creates a shell of electrons mixed with charged atoms and molecules that surrounds the Earth. It is in this region of the atmosphere that auroras (natural light displays) occur.