Due to Wind. This can be enabled by harnessing natural breeze or wind to freshen the home. Winds blow in through openings on the windward side of a building and exit through the openings on the leeward side and roof of the building. When implementing natural home ventilation by harnessing natural wind movement, you need to remember to remove any obstructions between the windward openings and leeward exits to ensure effective ventilation.
Due to Buoyancy. Buoyancy induced ventilation is of two types – temperature induced and humidity induced. These two can also be combined to create effective natural ventilation. Buoyancy occurs due to differences in air density caused by both temperature and humidity. This natural home ventilation system uses the cooling tower and stack ventilation techniques. Inside the home, the heat and humidity produced by both occupants and other factors results in the hot air rising and exiting through the ceiling. This results in cool air entering the home through the lower openings.
Info Trivia Resource: http://www.ventwerx.com/