# How to calculate atmospheric pressure?

## Main Question or Discussion Point

Let's say I had the composition of an atmosphere, the mass of the planet it was on, and the diameter/density of said planet. First, what else, if anything, would I need to find the atmospheric pressure? Second, what would the equation for doing so be?

Related Other Physics Topics News on Phys.org
You need the mass of the atmosphere.

You need the mass of the atmosphere.
By composition, I meant quantity of each gas.

jbriggs444
Homework Helper
2019 Award
By composition, I meant quantity of each gas.
If you know the total mass of each gas separately then you know the total mass of all the gasses put together.

If you know only the percentages (by weight) then what you know will not help you find the pressure you seek.

Chestermiller
Mentor
To get the pressure at the surface, which is usually referred to as the atmospheric pressure, just divide the weight of the atmosphere mg by the total surface area of the planet.

To get the pressure at the surface, which is usually referred to as the atmospheric pressure, just divide the weight of the atmosphere mg by the total surface area of the planet.
In what units? I tried this with Earth's atmosphere and I didn't get the right answer.

jbriggs444
Homework Helper
2019 Award
In what units? I tried this with Earth's atmosphere and I didn't get the right answer.
What units did you use for the weight of the Earth's atmosphere and the surface area of the planet and what answer did you get?

What units did you use for the weight of the Earth's atmosphere and the surface area of the planet and what answer did you get?
I used 5.1x1018 kg for the atmosphere's weight and 4*π*12,742,0002 m2 for the surface area of the Earth. I got 2,499,684 kg/m2

jbriggs444
Homework Helper
2019 Award
I used 5.1x1018 kg for the atmosphere's weight and 4*π*12,742,0002 m2 for the surface area of the Earth. I got 2,499,684 kg/m2
Check your arithmetic and check your figure for the radius of the Earth in meters. I get around 11,000 kg/m2 on the back of an envelope.

Edit: 7 significant figures of computed output on an input that is only stated to 2 digits reflects more precision than is justified.

Radius. Sigh. I hate when I make stupid mistakes like that. Thank you all for your help.

Chestermiller
Mentor
In what units? I tried this with Earth's atmosphere and I didn't get the right answer.
Pa

Chestermiller
Mentor
Check your arithmetic and check your figure for the radius of the Earth in meters. I get around 11,000 kg/m2 on the back of an envelope.

Edit: 7 significant figures of computed output on an input that is only stated to 2 digits reflects more precision than is justified.
Don't forget to multiply by g.

jbriggs444