The notions of overpressure and underpressure are relative and depend on the observer’s point of view. By convention, we consider here that we observe the building from the outside. Unless otherwise stated, overpressure means that the pressure outside the building is higher than inside.

The wind corresponds to a balancing current between zones of different pressures. The part of the air flow that encounters a building will be dammed up in front of it, creating a state of overpressure.

The value of this overpressure is called “stagnation pressure” and is expressed in pascal. The level of pressures and their distribution over a building are the result of the interaction between the incident wind (average velocity gradient, turbulence, etc.), the building (through the interplay of its shape, dimensions, wall roughness) and the juxtaposition of immediate obstacles (or the nature of the immediate environment).

These pressure differences cause air movements through the openings of the building. This phenomenon causes air infiltration on the windward side, and air extraction from depressed areas.