# air pressure at various height

For pressure in a liquid, the pressure

P = Po(pressure at liquid surface)+density*g*height

What about air pressure? Is there a similar formula for calculating air pressure at various and how to derive it?

Is there an air buoyant force acting on an object in air like the case immersed in a liquid?

thx

### 1 個解答

• 天同
Lv 7
8 年前
最愛解答

Atmospheric pressure various with altitude, because the temperature, and hence density, of air decreases with increase in height.

A simple (but approximate) relationship between the air density with height is given by,

p = (po).exp(-h/H)

where p and po are the densities of air at height h and sea level respectively

H is a constant which represents the mean height of the atmosphere

Commonly accepted values for po and H are 1.21 kg/m^3 and 8000 m respectively

Thus, the pressure P at height h is,

P = integral { p.g.dh} with limit of integration from h to infinitiy

where g is the acceleration due to gravity

After integration, P = Hg(po).exp(-h/H)

i.e. P = 8000 x 9.81 x 1.21 x exp(-h/8000)

or P = 94960.exp(-h/8000) Pa

This equation underestimates the actual atmospheric pressure as water vapour pressure has not been taken into account.

A more accurate equation for atmospheris pressure at various altitudes is given in the following web-page,

http://www.engineeringtoolbox.com/air-altitude-pre...

Q:Is there an air buoyant force acting on an object in air like the case immersed in a liquid?

The answer is YES. Archimedes Principle applies to fluids, which include both liquids and gases.