All We Need Is The Air That We Breathe ...

In the next series of Reality Bit(e)s we are going to look at the atmosphere of our planet and discuss some of the common mistakes authors make regarding it and the atmospheres of other worlds.

The atmosphere is the thin (relatively) layer of gas that surrounds our planet. Human activity confines itself mainly to the twenty kilometres that is closest to the surface. Three quarters of the gas in the atmosphere lies within about the first ten kilometres, a region of the atmosphere called the Troposphere. The air of the Troposphere, the air we breathe, is composed of the following: Nitrogen(78%), Oxygen(20%), Argon(1%), Water Vapour(1%) and Carbon Dioxide(0.05%) plus other traces gases. Yes, I know this adds up to greater than 100 percent, but these are approximate values and are constantly changing as is the atmosphere.


Within the normal temperature and pressure ranges of the Troposphere, Nitrogen gas (two Nitrogen atoms tightly bound to each other), is mostly inert. It is generated in two ways. Inorganically, it is created by the breakdown of Ammonia released from the Earth by volcanic activity, by sunlight. It is also a by-product of the breakdown of organic matter. Because it is so non-reactive, and relatively heavy, it has slowly built up over the aeons so that it is now the major part of the air we breathe.


The highly reactive Oxygen atom, is kept at such a high concentration in the atmosphere, through the photosynthetic efforts of the Plant Kingdom. As the name 'photosynthesis' suggests, the energy of the light from the sun is used to strip the Carbon atom from Carbon Dioxide. This carbon atom is then used in the synthesis of organic molecules while most of the surplus Oxygen is released back into the atmosphere as diatomic Oxygen gas, O2.


The third most abundant gas is Argon. One of the so-called Noble gases (i.e. It is generally inert like the Noble metals, Gold and Silver), Argon is like Nitrogen and has slowly built up in the atmosphere over the aeons from the radioactive decay of the Potassium 40 isotope.


The fourth most common gas, Carbon Dioxide, is a by-product of animal respiration. This molecular gas is only found in small amounts in the atmosphere because the Carbon atom it contains is the basic building block of organic life on the planet. This captured Carbon also tends to stay locked in solid form for extremely long periods (think coal and oil), only being released back into the atmosphere as a by-product of combustion.


Water in gaseous form makes up about one percent of the Earth's atmosphere. Most of this water vapour is produced by evaporation from the Earth's oceans and this process is one of the main driving forces of the planet's weather patterns.

Aside: Water is normally found in liquid form on the Earth and the mass of water in the atmosphere is ... dare I say it ... only a drop in the ocean. However, it is interesting to speculate on just how much water is suspended above us. In the reference section below there is a link to a cute website where the weight of the water in a cloud is calculated and equated to an equivalent number of elephants. Did you know the average cumulous cloud weighs as much as a hundred elephants?

Next article we will look at some of the properties of the gases in our atmosphere and the consequences of changing its composition.


The Wikipedia entry for the Atmosphere.

More about the weight of clouds

N.B. Please note that I although I use the Wikipedia (and WikiMedia Commons) a lot for references, this is for expediency and the familiarity of my readers. Anyone interested in further studies should make use of the references where available and understand that the Wikipedia is a co-operative project contributable to by anyone and must always be looked at in that light.

Phill Berrie, October, 2008.