# Why we consider pressure do not vary with height incase a gas in a vessel?

Why pressure do not vary with respect to height incase a gas contained in a vessel and atmospheric air pressure varies with respect to height.

for example we consider constant pressure in a mercury barometer over the volume above mercury level, but pressure of merucry changes with respect to height, why pressure of gas wont change with respect to height in barometer.

• It is instructive to calculate the height over which you would expect a 1% change. Useful figure: $\rho_{\text{air,stp}} \approx 1.2 \,\mathrm{kg/m^3}$. Commented Feb 14, 2019 at 3:32
• why atmospheric pressure is calculated only by considering weight of air above earth surface? should we need to consider kinetic theory of gases also to measure atmospheric pressure.
– teja
Commented Feb 14, 2019 at 3:36
• It is instructive to calculate the scale height for a 1% change in pressure by both static weight and kinetic theory. Under what conditions do you expect them to give the same answer? How well are those conditions met for some allegedly typical point on the Earth's surface? Commented Feb 14, 2019 at 3:39
• Im asking that whether atomspheric pressure is sum of pressures due to static weight pressure and kinetic pressure.
– teja
Commented Feb 14, 2019 at 3:44
• If that is the question you intend then you should probably edit for improved clarity. However, the point of the second suggested exercise is that under the right conditions (equilibrium, constant density and adiabatic lapse) you get the same answer from the two calculation. Because these are not two distinct pressures but two ways of expressing the same physics (though the connection between them is not necessarily obvious when you first encounter them). Commented Feb 14, 2019 at 4:24