Does air have weight?

Air is a substance and all substances have weight. Air, however, is very light. A column of air an inch square extending from sea level upward as far as the atmosphere goes weighs only about fifteen pounds. This is called atmospheric pressure. Galileo first demonstrated that air has weight by weighing a glass globe, then forcing air into it and weighing it again. He correctly ascribed the extra weight to the added air. An automobile tire pumped up would weigh more than a flat tire, but the difference would be too slight to detect with ordinary scales. Since water vapor is lighter than most of the other gases in the atmosphere, moist air weighs less than dry air: Naturally the more air is compressed the more it weighs volume for volume. A tank full of compressed air is heavier and less buoyant than when it is full of air at normal atmospheric pressure.