Permanent gases maintain the same proportion to each other.
Variable gases are characterized by variations in time and space. Generally, the permanent gases tend to be non-radiatively active whereas the variable gases are radiatively active, a point we will consider in more depth when we talk about atmospheric radiation and the greenhouse effect. The invisible atmosphere makes its presence known by the force per unit area it exerts on all surfaces. The roof doesn t collapse because the air pressure at any point is the same in all directions - up, down and sideways. The air pressure within the house counterbalances the air pressure outside the house and the net pressure acting on the roof is zero.