The pressure of the atmosphere changes with a change in height. When pressure is low, the attitudes are high.
Usually the pressure of the atmospheric level can be defined as total weight of air. There are only less air molecules present in the higher altitudes when compared to lower altitudes.
Therefore at high altitudes, the air pressure will be low . This is the reason why the air pressure is low at a higher point of elevation.
The compass always points north. One can use this to know which direction North is, and therefore, in which direction south is, since south is directly opposite of north.
