There are several situations in which we measure the force of a gas. Most commonly (at least in a beginning chemistry class) we measure the pressure of the atmosphere – that is, how hard the air around you is pushing in on you. It is worth noting here that the air around you pushed in ALL directions, not just down. You can picture this if you think about swimming under water. When you go underwater, you feel the pressure of the water all around you. It pushes in against you no matter how you move. If you are deep enough to feel the pressure against your eardrums, you cannot relieve the pressure by turning the ear to the bottom of the pool, the water pushes up as well as down and sideways. Air does the same, but we don't think about it as much because air is not nearly as dense as water, so we don't feel it as much.