Natural ventilation exchanges air through openings using pressure differences generated by wind or buoyancy. A warmer indoor column can have a different vertical hydrostatic pressure gradient from outdoor air; connected low and high openings then permit a circulation. Wind can instead create unequal external pressures on different sides of a building.
The resulting volume flux depends on opening areas, their discharge properties and the pressure differences that remain once flow is established. The stack effect and cross ventilation describe different driving geometries and can reinforce or oppose one another. Measuring indoor/outdoor temperatures alone does not determine the exchange rate without those geometric and flow conditions. A simple thermal budget combines the exchanged enthalpy with internal heating; a stratified room can require more than one representative temperature.
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