How to build your world: Atmosphere

So, I’m finally back around to my World Building Series again. If you missed the earlier ones I’ve covered The Sun & Seasons, Landscapes and Water.  Today I’m going to discuss “atmosphere”.

Let’s get the basics out the way (yup, it’s another science lesson. Just go with it people).


What is ‘Atmosphere’?

It’s a layer of gas that circles the planet. Actually, it’s a mix of different gases and it’s held in place by gravity.

Clouds, sky, atmosphere. Image from Pixabay

Earth’s atmosphere contains around 21% oxygen, 78% nitrogen, 1% argon and trace amounts of other gases such as carbon dioxide, neon, helium, hydrogen, ozone, and others.


Tell me about gas

Gas is a fluid substance that expands to fill available space. So if contained, it will fill the container in which it is held in a uniform density, irrespective of the amount of gas.

Gas molecules are able to move freely, they are not trapped in a structure. They are always in motion and bounce off each other.

The higher the temperature, the more violent the movement – the molecules become “excited” so vibrate and rotate faster (and the higher the temperature the faster they move too!)


What about escape velocity?

Since we are talking about the gases in the atmosphere, we need to discuss escape velocity.

Remember the atmosphere is gas, and gas is not bound in a structure, so gravity is holding it in place.

However, these molecules are moving fast and so we need to address escape velocity, which is the minimum speed an object needs to reach in order to break free from the gravitational pull of a massive body (such as a planet).

Velocity, image of the earth and the sunrise. Image from Pixabay

So these tiny molecules of gas can reach speeds (especially if heated) that allow them to escape.

Now, there is always a certain amount of gas that will escape into space. However, if the percentage increases too much then larger amounts of the atmosphere can start leaking into space.

Not to mention, some gases are heavier than others so the lighter gases such as helium and hydrogen might escape whereas the heavier gases such as oxygen remain.



As well as gravity you need to consider the temperature of the exosphere (outer atmosphere).

If it’s too hot, say you create a world and give it like 3 huge suns… that outer atmosphere is going to be filled with incredibly excited gas molecules that could be zipping off into space.

So, if you have a massively high temperature in the exosphere, then you need more gravity to compensate.

This can be an interesting concept to bring in for say a sci-fi idea, whereby a sun could be growing hotter (which occurs as suns age) and cause a devastating effect on the atmosphere.

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If we use our Sun as an example, fusion is where hydrogen is fused into helium. However, helium is a denser gas than hydrogen so the Sun becomes denser as more helium is produced.

This will make the Sun hotter (though the process takes millions of years).

Therefore, over a long period of time, the Earth will get hotter (due to the increasing temperature of our sun) and the atmosphere will be affected (e.g. because lighter molecules and atoms will escape more easily, the atmosphere may slowly become denser).


Different Atmospheric conditions

When we look at the Earth and the atmosphere, we often do so from our own human perspective. However, there are many other animals that can survive in very different atmospheres than us.

In fact, back in 2010, the microscopic marine animal (phylum Loricifera) was reported as being the very first oxygen-free multi-cellular organism.

Discovered at the bottom of the Mediterranean sea, these animals go through their whole life cycle in the complete absence of oxygen and light.

For more on this, check out the Wiki article


Ozone Layer

The Ozone Layer is a thin layer in the stratosphere heavy in the gas ozone (O3). This layer is an important barrier that protects the Earth. Ozone partially absorbs UV-B radiation that comes from the Sun.

UV radiation has high energy and is capable of damaging living cells.  Heavy exposure to this radiation increases risks to humans such as cancer as well as causing damage to other life on Earth.

Ozone Layer. Earth protective layer. Vector image from Pixabay

The discovery of the hole in the ozone layer over Antarctica had a large impact on the world.

After a report was published by the United States National Academy of Sciences, concluding there was evidence of ozone depletion several countries took steps. The first being a move to eliminate CFCs in aerosol cans.

So this (comparatively) thin layer of gas shook the world when it was shown to be depleting.  Something as simple as a hole in a layer of gas could have damning results.


The Greenhouse Effect

This is a natural process where the atmosphere traps in some of the Sun’s energy.  This energy warms the Earth.  However, this effect has been increased artificially by human activity.  Many different activities have increased this effect.

The most well-known greenhouse gas is carbon dioxide.  This can be increased by burning fossil fuels such as coal.  When coal is burned in a power station the carbon it contains combines with oxygen in the atmosphere and forms carbon dioxide.

The increase of the Greenhouse Effect means more heat trapped, and the Earth gets warmer, hence the term Global Warming (now referred to as Climate Change).

Global warming harms the environment in a number of ways, the one we read about the most is the melting of the polar ice caps.  There is also desertification, a rise in sea level and extreme storms.

There is also the issue of ocean acidification.  The greater carbon dioxide levels have increased the acidity of the world’s oceans by over 30%.

If you remember at the start of the article, I mentioned that CO2 is only present in trace amounts in the atmosphere.

This gives you some idea of how delicate the balance is. All the problems caused by increasing CO2 are due to an increase from around 0.028 % (around 250 years ago) to 0.041 % (today).



Venus is a great planet to study if you want to make some horrifically uninhabitable planet in your novel… or if you want to have your world BECOME a horrifically uninhabitable planet.

100 times hotter than the Earth, Venus is not the beauty it’s named after.  It has no protective magnetosphere so (according to NASA) in 2006 the Sun sent a small, slow-moving CME (coronal mass ejection) out into space.

Four days later this solar material hit Venus and ripped away huge amounts of oxygen out of Venus’ atmosphere.

Planet Venus with its devastated atmosphere. Science. Surface of Venus. Image from pixabay

It has a crushing atmosphere, so thick (around 90 times more massive than Earth’s) that a little over 2 hours, is the longest any spacecraft has managed to survive on its surface.

The atmosphere is around 96% CO2 and 3% nitrogen, with a little water vapour thrown in. Weather on Venus is a balmy 460 °C helped by the thick sulphuric acid clouds.

So, when you are world building, give some thought to the atmosphere of your planet(s). It’s kinda important 🙂

Want more articles on my World Builder Series?


Have you thought about your World’s atmosphere?

Share your Thoughts image.

Happy writing

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3 thoughts on “How to build your world: Atmosphere

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