Air, namely, is a gas. Since it is a gas, it can be compressed by applying outer force. Compression of a gas raises its density. In a place above or under sea level, the atmospheric pressure will be above or below 1 atm, accordingly. On the surface of a lake which is high above sea level, the atmospheric pressure will be lower, since it has a small amount of air above it. At 5 km above sea level, for example, the atmospheric pressure will only be 0.5 atm. At the Dead Sea, which is hundreds of meters below sea level, the atmospheric pressure will be higher, due to the weight of the air layers above it.