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The neutral pressure level is the height at which indoor and outdoor hydrostatic pressures agree. Flow through an opening reverses direction across this level.

Define the indoor-to-outdoor reduced gravityBecause the whole opening lies above the interface, its indoor side contains upper-layer fluid of density ${\rho }_H$. Under hydrostatic pressure, the pressure difference varies linearly about a neutral pressure level. Equal opening geometry and equal discharge coefficients for inflow and outflow put that level at the vent midpoint. At vertical distance $y$ from it, the ideal Bernoulli equation gives speed $\sqrt{2g_V^{\prime }|y|}$.

Writing $Q_V$ for either one-way volumetric flow rate, integration over one half of the opening givesThe total unsigned exchange is $2Q_V$. The ideal sharp-edged inviscid model has $C_d=1$; an empirical $C_d<1$ represents contraction and losses.

A tall opening between fluids of different densities can support simultaneous inflow below a neutral pressure level and outflow above it. For a symmetric opening with a Boussinesq hydrostatic pressure difference and equal discharge coefficients, the neutral level lies at mid-height.

The stack effect is buoyancy-driven flow caused by different vertical hydrostatic pressure profiles in connected columns. If indoor air is warmer and less dense than outdoor air, their pressure difference varies with height. Low and high openings can then admit cooler air below and discharge warmer air above.

For a characteristic opening separation $H$, the driving pressure scale is $gH\Delta \rho$, with density contrast $\Delta \rho$. A small relative temperature contrast at similar absolute pressure gives $|\Delta \rho |/\rho \sim |\Delta T|/T$ in an ideal gas. This connects thermal conditions to a mechanical driving scale. The neutral pressure level marks where the two hydrostatic pressures agree, and its location depends on the connected openings and resulting flow. A larger height difference can increase the driving scale, but the final volume flux still requires opening resistance and the full circulation balance.