Relative humidity figures are deceptive unless one understands that warm air has the energy to possess more water vapor than does cold air.  For example, 77˚F. air at 50% relative humidity will contain more water vapor than 50˚F air at 100% relative humidity.  If you did not know that warm air can contain more water vapor than cold air, you would probably “logically” deduce that the 50° air at 100% relative humidity had the greater amount of moisture because, after all, 100% is greater than 50% – but you would be wrong.


It is a fact that the warmer the air, the greater can be the percentage of water vapor within the whole mixture of gases, (even though the water vapor always represents but a small fraction of the gaseous content of the air).

If the relative humidity of a kilogram of 77˚F. (25˚C) air is 100%, that air will contain 20.00 grams of water vapor.

If the relative humidity of a kilogram of 86˚F. (30˚C) air is 100% that air will contain 26.50 grams of water vapor.

Therefore, if the relative humidity of 77˚F. air is 50%, the air contains 10.00 grams of water vapor in each kilogram (half of 20.00 grams).

Comparatively, if the relative humidity of 86˚F. air is 50%, that air contains 13.25 grams of water vapor in each kilogram (half of 26.50 grams).

So in this example, the warmer of the two is more humid because it contains more water vapor (EVEN THOUGH THE RELATIVE HUMIDITY IS THE SAME IN BOTH EXAMPLES).

Even if the relative humidity of the 86˚F. air was lower, say 45%, it would still contain more water vapor than the 77˚F. air at 50% relative humidity! 45% of 26.50 = 11.925 grams of water vapor per kilogram of air (which is greater than 10 grams).

The 50°F. air discussed in the first paragraph is at 100% relative humidity when it is holding only 7 grams of water vapor per kilogram.  This clearly shows the limitations of cooler air when it comes to the amount of water vapor within it, even if saturated!

So, in conclusion, as is so noticeable in humid climatic zones like where we live in Florida, the hotter the air the more moisture it usually contains. This is true so long as there are available ways for water vapor to enter the air (e.g. evaporation and transpiration). One measure important to climatologists is the “evapotranspiration” rate.

SPECIAL NOTE: This type of explanation that you just read is considered a simplification by some because water is constantly changing phases in the air. When evaporation exceeds condensation within the air, the relative humidity will be below 100% – but when condensation exceeds evaporation, the relative humidity may reach 100%.

A humid afternoon in the Smoky Mountains

A humid afternoon on a LeConte trail in the Smoky Mountains - by Cloudman23©

8 comments so far

  1. vanthey on

    can u tell me, what is definition for upslope fog?
    whatis it different upslope fog and stratus cloud?

    can u tell me about clasius-clayperon equetion?
    is it related to fog’s happen?

    thanks before

  2. cloudman23 on

    Well, your questions are very interesting – indeed. Let me address you second question first. I would enjoy knowing where you learned about the Clausius-Claperyron relation. I must share with you the chuckle I got when I read your version. You typed, “clasius-clayperon equation.” That so resembles the name of a very famous boxer, arguably the best ever, Cassius Clay, who later changed his name to Mohammed Ali, that I thought at first you were joking. Then it occurred to me that English might be your second language. If that is the case, I sympathize with you for it surely is a difficult language to master. I respect you for you attempts to communicate effectively as you have with me.

    Are you a university student studying meteorology or is it simply a special interest of yours? Whatever the case – I hope you enjoy it as much as I. If you are a university student I suggest you address your Clausius-Clapeyron question to whomever teachers thermodynamics there – probably someone in the physics department. I retired from full-time teaching in 2003 and have found it somewhat difficult to stay well-informed in meteorology or any other science for that matter and admit to being “rusty.”

    Once I learn about your needs with regard to thermodynamics, I will respond further about the Clausius-Clapeyron relation – if necessary. In the meantime, here is a link which I have only briefly examined:

    For now, let me merely say that it most certainly does play a role in cloud and fog dynamics because it deals with that thin line of back and forth pseudo-equilibrium where phase changes are occurring. Since I don’t know how much you know – here is a list of the phase changes addressed in the physics of cloud/fog dynamics and the measuring of vapor pressure (among other things).

    1. gas to liquid = evaporation (or vaporization) = heat yielding
    2. liquid to gas = condensation = heat absorbing
    3. liquid to solid = freezing = heat yielding
    4. solid to liquid = melting = heat absorbing
    5. gas to solid (liquid phase is bypassed) = sublimation = heat yielding
    6. solid to gas (liquid phase is bypassed) = deposition = heat absorbing

    Most fog is the consequence of condensation though ice fogs do occur (deposition). Frost is a good example of another weather-related consequence of deposition. Frost is NOT frozen dew. However, in my opinion, the term “fog”, verses the term “cloud” is in the “eye of the beholder.” Generally a fog is regarded as a cloud touching or very nearly touching the surface. So – when I am visiting my cabin in the mountains of the Blue Ridge I find myself surrounded by stratus at my level and thus consider that I am in a fog. However, were I to drive or hike to a lower level and look up at its underside, I would refer to it as a cloud (from that vantage point). On the other hand, I would probably say to my wife, it looks like we have a fog up there at the cabin. If she were typing now she would probably tell you that my head’s always in a fog! LOL

    One ordinarily considers stratus not touching or nearly touching the surface to be a horizontal layer but that is not necessarily true. However, any angle off the horizontal would be, in my opinion, generally imperceptible to the observer on the surface. The upslope fogs that I have seen clearly manifest an upslope movement because they have been the consequence of air rising upslope and cooling adiabatically and sometimes also by coming into contact with a colder surface. Such is the case with the upslope fog image in my weblog at https://cloudman23.wordpress.com/2008/08/25/why-does-it-feel-so-humid/

    On the Blue Ridge Parkway at Craggy Gardens, not far northeast of Asheville I have often watched upslope fog come up the northwest slope move over the ridgeline and then flow down the southeast slope for a while before evaporating (mainly due to the adiabatic heating that occurs in descending air).

    Any further questions or comments can be addressed here at this site or to my e-mail address if you prefer ( Proftoney@gmail.com ).

  3. vanthey on

    haha..so sorry ’bout that..i cant speak english fluently,
    yes, english is my second language..
    i’m a student in Akademi Meteorologi & Geofisika Jakarta. It’s BMG’s School, (*BMG=Badan Meteorologi & Geofisika. The Indonesian Meteorology Department..

    Now, i still do my exam, try to writing a scientist paper. its ’bout FOG in CITEKO BOGOR-INDONESIA. I think FOG in that place is Upslope Fog..

    Mr. Toney, could u tell what is different about upslope fog and frontal fog? and is there any relation between Clausius-Clapeyron and FOG?

    sorry maybe so much question..
    its because in Indonesia too hard to find FOG reference..

    thx b4, and sorry for my “bad english”..
    best regard..

  4. Cloudman23 on

    Thank you for the clarification. I do not know the geography of Cetiko Bogor. I suggest that you do Internet searches on both “fog types” and the “Clausius-Clapeyron relation”. You will get many “hits” on both topics. Also, don’t forget my suggestion that you need to talk to someone proficient in thermodynamics in order to get details especially if you have trouble interpreting what you are reading. Surely someone in your Physics department will entertain an appointment with you to discuss the matter. When I was a full-time college professor I thoroughly enjoyed a student’s visit when he/she was truly interested in learning. It mattered not whether the student was enrolled in one of my classes. A fringe benefit was that I would usually learn quite a lot in those sessions too.

    The simple answer to your question about a relationship between fog and the C-C relation is, “yes.” I think you need to study more about fogs, which can easily be done on line, and then, knowing the topography of the area along with the properties of the air and the wind direction you should be able to determine if the fogs are indeed upslope. They could very well be radiation fogs, particularly if you detect little if any movement.

    Upslope fogs that I have observed when the vantage point is favorable have moved upslope, whereas the radiation fogs in most humid hilly or mountainous regions have horizontal upper boundaries and are static. If there is good satellite image loop coverage of the area I also suggest you study them during those times that fogs are observed.

    Here is a link for you to do Google searches: http://www.google.com/advanced_search?hl=en

    Yours Truly,


  5. vanbedond on

    remember me?
    vanthey from Indonesia,-
    next week i have my paper exam. My “UPSLOPE FOG” paper. many thanks for your info about upslope fog. next time i’ll translate my paper to english, i hope you will read that and give me some correction.
    wish me luck.

    yours truly,
    Putu Hadi Wiguna

  6. Debbie on

    Wonderful informative article.. thanks!

    We own homes both in central Long Island, NY and south of Roanoke VA in the Blue Ridge mountains.. I was very detailed in checking every little thing, temperature, rain/snow levels, cost of living, all that.. but never evidently once noticed the humidity levels in the Blue Ridge. We’re at 1200ft in VA, while in NY we’re at approx 110ft – the ocean is ~15miles south, and the LI Sound ~15 miles north

    I am constantly surprised to see the humidity levels are sometimes as much as double in VA than in NY. Today for instance NY is at 48% 87F and VA is 72% 78F

    I was wondering if you could explain why the humidity levels are so much higher in mountains than on an island!

  7. Heidelberg Kindergeburtstag on

    Hey There. I found your blog using msn. This is a really well written article.

    I will be sure to bookmark it and come back to read more
    of your useful information. Thanks for the post.
    I’ll certainly return.

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