The pressure difference can be caused by Wind or the Buoyancy effect created by stratified warm air. In either case, the amount of ventilation will depend critically on the size and placement of openings in the building. Buoyancy ventilation is more commonly referred to as temperature-induced or stack ventilation. Buoyancy results from differences in air density. The density of air depends on temperature and humidity.
Cool air is heavier than warm air at the same humidity.
Thus, airflow is generated by the dropping of heavier air, forcing lighter air to exhaust. The greater this distance the more air pressure created, similar to a water column. Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. To equalize pressure, fresh air will enter any windward opening and be exhausted from any leeward opening. Cooler pads sit at the top of a tower with a pump re-circulating water over them.