Our Atmosphere In Numbers

Our atmosphere. Image from NASA.

Our atmosphere is an envelope of gases surrounding the globe, extending up to a height of around 500 km. It is mostly made of nitrogen and oxygen, but also contains water-vapour, argon, carbon dioxide, ozone and others. It may seem uninteresting at first glance, but delving into it in a little detail can reveal some remarkable and unexpected findings.

The atmosphere is divided into several layers, as shown above. As it extends to around 500 km its volume is around 275 billion km3. Its density at sea-level is 1.225 kg/m3, decreasing exponentially to practically 0 at roughly 500 km.

The total mass of all the gases is 5.1 million billion (5,100,000,000,000,000) tonnes. That’s nearly 4 times the mass of all the water in the oceans (1.35 million billion tonnes)! Around 75% of this mass lies in the bottom layer, the Troposphere, which is where almost all of the water vapour is to be found.

The amount of this water vapour varies widely but on average accounts for around 1% (about 5 million million tonnes) of the total mass of the atmosphere. If all this vapour condensed out it would be enough to fill almost 1800 Irish Seas! During strong El Niño events an extra 10% of water vapour can be evaporated into the atmosphere.

The mass of all of the atmosphere lying above Ireland is about 845 billion tonnes, of which around 8 billion tonnes are water vapour. If all that vapour condensed out and fell as rain it would cover the country to a depth of almost 1 metre!

At the mesopause (roughly 85 km) the temperature is around -85 °C and pressure is around 0.004 hPa, just 0.0004% that at sea-level. Each cubic centimetre contains about 170 million million air molecules, compared to about 25 billion billion at sea-level (about 150,000 times more).

Although the air gets thinner with height its composition doesn’t change. It is still made up of around 78% nitrogen and 21% oxygen.

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