From what I know, the pressure of a gas is due to the collisions of its molecules against a surface. So I don't understand why atmospheric pressure is measured by taking the weight of all the air molecules above a surface including the ones that aren't contacting it. Gases have plenty of space between their molecules so I don't understand how I can feel the weight of a gas molecule one mile above me that isn't in contact with anything else. Can somebody please enlighten me? Thanks in advance.