# Change in humidity with altitude

Tags:
1. May 25, 2017

### oobgular

1. The problem statement, all variables and given/known data
So I sent a humidity sensor up in a weather balloon, which gave a reading for relative humidity. I eventually want an absolute humidity, but I am unsure whether I need to correct the output based on the changing density and atmospheric pressure with altitude.

2. Relevant equations
As I understand it:
RH = (% volume of water) * atmospheric pressure / vapor pressure.
AH = mass of water / volume of air.

3. The attempt at a solution
I want to obtain the %volume of water, and multiply by the density profile of air to obtain the absolute humidity profile.

The vapor pressure changes with height (I think) since the temperature and density changes, moving you to a different spot on the vapor dome. The atmospheric pressure also changes. So is atmospheric pressure / vapor pressure a constant? If not, how can I account for that to get the % volume of water?

Thanks!

2. May 25, 2017

### Staff: Mentor

Do you know how to get the atmospheric pressure as a function of altitude?

3. May 25, 2017

### oobgular

Yes, I can just assume the exponential atmosphere. It's the vapor pressure I'm confused about-- doesn't it change with temperature and density as well?

4. May 25, 2017

### Staff: Mentor

It changes with temperature only.

5. May 26, 2017

### Staff: Mentor

You can get a more accurate pressure profile by measuring it directly, or by knowing the temperature profile and applying the barotropic equation.