Air can be thick or thin

At the top of Mt Everest, the air is quite thin making it difficult to breathe while on the surface the air is relatively thick. There are areas on the earth where the air is thinner (more sparse) and other areas where the air is very thick.

The thickness of air is an easy way to understand the 'Density' of air. Density can be measured in all objects by how much it is made up of. For example compare a small rock from the ground outside to a piece of cork of the same size. The rock seems rather solid and nearly impenetrable while the cork has little holes in it and is somewhat light and flimsy. The rock is more dense than the cork.

It can be hard to fathom air having different densities, but it does and if you have been fortunate to climb a mountain then you will understand how the thinner air at altitude can affect you.

Thicker air is denser while thinner air is less dense.

A quality of air you must understand is its need to 'Equalise' or 'find equilibrium.'

Air is constantly trying to achieve balance or equalisation, and it would seem on a global scale this never happens. Like pulling the plug on the bottom of the bath, the water falls to fill the space that is made in the piping beneath, so makes air attempt to fill the empty spaces around it. Pulling the plug out of sports ball or puncturing a car tyre causes the air to rush out. Opening a window in your house allows the air to rush in or out. Which way the air moves depends on the density of air. More dense air naturally wants to spread out, so it flows towards lesser dense air.

This is constantly happening. Air will always attempt to fill in areas where it is less dense. This occurs on a grand scale and when air moves across the surface of the earth (of a horizontal nature), we call it WIND!

What you must remember today is that air is forever attempting to achieve equilibrium by filling areas of lower density or areas of lower pressure around it.