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Anne Marie Helmenstine, Ph.D.

Does Atmospheric Pressure Affect Humidity?

By May 12, 2006

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Does atmospheric pressure affect relative humidity? My inclination would be to say 'yes'. The answer, as you will see, might not be so simple. Following is an e-mail I received. After reading it, I was able to convince myself of both sides of the argument, which doesn't help the person asking the question at all. Therefore, I'm opening it up for discussion. Please feel free to add your own comments and observations.

"I have a question for you that concerns the preservation of millions of dollars of art and priceless archives in many libraries and museums in the Utah, Colorado, Arizona, Idaho and Texas. Because of the nature of the question, I ask that you please answer in complete thoughts, sentences and paragraphs. Although I have requested for completeness, it has been difficult for me to apply the same rules and be concise.

Problem: understanding the definition of relative humidity, its measurement, and finally the calibration of RH sensors. Below are two opinions, which is correct and why?

My opinions:
1) Relative Humidity is defined as a ratio of mole fraction of actual water vapor, to a mole fraction of water vapor that can be saturated in dry air, where the two values are obtained at the same temperature and pressure. Key word to focus is ... Pressure.
2) Mole fraction values are obtained from water density values.
3) Water density values vary with atmospheric pressure.
4) Atmospheric pressure varies with altitude.
5) The temperature boiling point of water varies with atmospheric pressure (or altitude).
6) Saturated Water Vapor pressure value is dependent on the boiling point of water (such that the values of the boiling point of water is lower at higher altitudes).
7) Humidity in any form is the relationship between the saturated water vapor pressure, and the sample-air's partial water vapor pressure. Partial water vapor pressure values are dependent on pressure and temperature.
8) Since both saturated water vapor property values and partial water pressure values are observed to non-linearly change with atmospheric pressure and temperature, then the absolute value of atmospheric pressure is required to accurately calculate the water vapor relationship as it applies to the perfect ideal gas law (PV = nRT).
9) To accurately measure humidity and use the principles of the perfect gas law, one must obtain the absolute atmospheric pressure value as a fundamental requirement for calculating relative humidity values at higher altitudes.
10) Since the majority of the RH sensors do not have built-in pressure sensor, they are inaccurate above sea level, unless a conversion equation is used with a local atm. pressure instrument.

Other people's opinions as they were stated to me:
A) Nearly all humidity related processes are independent of total air pressure, because water vapor in air does not interact with oxygen and nitrogen in any way, as first demonstrated by John Dalton early in the nineteenth century.
B) The only RH sensor type that is sensitive to air pressure is the psychrometer, because air is the carrier of heat to the wet sensor and the remover of evaporated water vapor from it. The psychrometric constant is quoted in tables of physical constants as a function of total air pressure. All other RH sensors should not need adjustment for altitude. However, the psychrometer is often used as a convenient calibration device for hvac installations, so if it is used with the constant for the wrong pressure to check a sensor that is in fact correct, it will indicate a sensor error."

Worked Pressure Conversion Problems | Ideal Gas Law Problems

Comments

May 20, 2006 at 3:21 pm
(1) Duncan says:

I to am working to better understand this subject and unfortunately have little to accurately add on the subject.
However something I have learned on the subject is that science can often be quite certain about various parameters before and after an event and is quite good at stating as fact what occurs in the interveening period, despite not exactly knowing what is happening provided before and after situation is repeatable. Why this is of relevance to your text, I am unable to completely describe (due to my time constraints, and lack of your contact details) however Thermodynamics is often quite happy to to perfrom the above ‘trick’ because it works in many engineering application, applications that are closed system with measurable inputs and outputs. The real world, outside the closed systems of engineering, is what we live in and are surrounded by, and is prehaps less easily defined accurately, due to its variblity and various scales of time and space. I’d be happy to converse further on this subject more specifically if I had your contact details, I am at yahoo as ‘napincat’ but get lots of junk mail, so inclusion of the word ‘humidity’ in the subject may well help, cheers.

April 4, 2008 at 2:49 pm
(2) Garrett says:

In needing to calculate a ‘virtual’ wet bulb temperature inside a vacuum chamber using an absolute pressure sensor and a relative humidity sensor, I spent a decent amount of time researching this. I eventually crafted a formula which approximated the wet bulb temperature, and a component of it was the atmospheric pressure, although it had a relatively small impact of the final value.

You can see this for the type of source material I used: http://www.4wx.com/wxcalc/formulas/rhTdFromWetBulb.php

I basically had to reverse what everyone else is doing, most are taking a wet bulb value and calculating the RH, I am taking an RH and calculating the wet bulb…

February 3, 2009 at 7:23 pm
(3) Shannon says:

OMG!!! thank you so much!!!! i’m working on a science fair project, and i needed that answer! once again
THANKS!!!!! =)

April 13, 2010 at 8:42 pm
(4) ME says:

THANKS

July 27, 2010 at 1:07 am
(5) ET says:

Does barometric pressure affect humidity?

I would say ‘yes.’

One may see one’s exhalations either in Death Valley or at the summit of a very tall mountain, when the temperature at both locations is low enough.

Note that barometric pressure in the valley will be far higher than at the summit of the mountain.

For a comparison, consider: Water boils sooner at a lower pressure (high altitude) than a high pressure (low altitude).

At a lower pressure then, water vapor (humidity) may form faster than at a higher pressure, relative to temperature.

November 18, 2010 at 11:32 am
(6) kay says:

does humidity affects air pressure

December 19, 2010 at 12:08 pm
(7) Bill says:

I also would like to know if humidity affects air pressure readings. i.e. if I run a dehumidifier in a room will the pressure in that room be higher than other rooms or the outside air pressure by any discernable amount? Thanks.

February 9, 2012 at 10:15 am
(8) Gary Marszalek says:

Contrary to what one might assume, a doubling of pressure will also double RH but “ONLY” when the composition has not changed and… the “TEMPERATE REMAINS THE SAME”!!! In the real world, an immediate doubling of pressure also raises temperature, which also changes saturation level and RH%. However, once it cools back to it’s same original temperature at the newly doubled pressure, its RH% will also have doubled. Same situation is responsible for why air compressors collect water in their storage tanks. When moderately humid air is compressed and eventually equalizes back toward room temperature or “cools”, the pressure amplified RH% can potentially exceed 100%, causing water to form within the tank and on it’s cooler tank walls. Make sense?

March 3, 2014 at 4:33 pm
(9) KC says:

Does high reflective heat from the sun effect the atmosphere? and if so could vast solar fields reflect or change weather and harm the ozone layer.

MOST STUDIES ON OZONE DAMAGE (LIKE AEROSALS) WHERE DONE IN LABS, DID ANYONE DO A STUDY ON SOLAR PANELS REFLECTION (HEAT) ON AIROR OZONE?

March 3, 2014 at 4:39 pm
(10) KC says:

Does high reflective heat from the sun effect the atmosphere? and if so could vast solar fields reflect or change weather and harm the ozone layer.

studies on ozone damage was done in a lab with aerosols, has anyone done studies with the solar panel reflection? I would just like to make sure before we spend billions and billions of dollars that we make sure we are not creating a new atmospheric issue. Thanks

May 16, 2014 at 6:39 am
(11) Kev says:

For the readers:
SIMPLE ANWSER IS: YES
–> RH= partial vapor pressure/ saturated vapor pressure
–> RH = function(amount of water in the air, pressure, temperature)

If you need a 10 steps logical steps for that kind of matters you have all chances to have a wrong conclusion, especially humidity, temperature and pressure effects can be counter-intuitive due to simplifying the question.

But, key word to focus is TEMPERATURE. Under normal ambient pressure the effect on relative humidity will be quite small. Under a normal (human living conditions) temperature range the effect on relative humidity will be HUGE. See a Mollier diagram

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