Equilibrium is a state where all things are equal - where forces are equally balanced.

Air moves across the earth surface in its attempt to achieve equilibrium or balance. When equilibrium is reached the air will stop moving - this is a stable situation however the stability never lasts for long, and the air will start moving again. Air pressure is a quality that can help determine which way the air will move. Higher pressures cause the air to expand outwards and lower pressures cause the air to contract inwards.

Consider the air inside your car tyre; if you were to puncture the tyre, you know that the air will leak out through the hole. This is because the air inside the tyre is a higher pressure than the air outside of the tyre.

Remember the following: Air travels from High pressure to Low Pressure (Air travels from high density to lower density)

This fundamental principle is what allows human kind to take flight!