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Yesterday during the day, inside my house it was 16% relative humidity. Outside it was 69% relative humidity and cold, around 35 degrees Farenheit. It is always 72 degrees Farenheit inside the house.

During the warmer months the humidity in the house never goes below 35% relative humidity.

So, it appears that somehow the outdoor temperature is affecting the relative humidity inside the house. How could this be?

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    $\begingroup$ Re How could this be? A well designed house has indoor air being completely replaced with outdoor air on an hourly basis, or shorter. The summertime high humidity air in a house is long gone well before winter comes around. $\endgroup$ Dec 3, 2021 at 15:35

3 Answers 3

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The only physics you need to know is that if the temperature is higher the air† can hold more water vapour (the gaseous phase of water). Cool molecules don't have enough energy to free themselves from intermolecular forces.

The answer to your question hinges on the difference between:

  • absolute humidity — how much water vapour ('moisture') the air holds irrespective of temperature; and
  • relative humidity — how much vapour the air holds relative to how much it could hold at that temperature.

Changing the temperature of the air, e.g. by bringing it into your house, does not change how much vapour it contains but it does change its capacity, i.e. how much vapour it could contain. That is, the absolute humidity stays the same, but the relative humidity changes:

  • If you take cool air and warm it up, the warmer air could hold more vapour than it contains, so the relative humidity goes down.
  • Conversely, cooling air down will push its relative humidity up. (If you cool to the dew point, you'll hit 100% relative humidity and water will condense out as dew or frost.)

I drew this, maybe it helps...

Diagram to illustrate the concepts of relative and absolute humidity


† Okay, this is a simplification. The conversation is about conditions in and around a house on the surface of the Earth. We don't really care about the air, per se. Read about vapour pressure.

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    $\begingroup$ Nitpick: "...liquid water will condense out as dew", or frost if the dewpoint is below freezing. $\endgroup$
    – jamesqf
    Dec 2, 2021 at 3:50
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    $\begingroup$ "the air can hold more water vapour" -- no, the air has nothing to do with it and doesn't "hold" vapor. At a given temperature there is a limit to the density or pressure of water vapor that can exist without condensing (saturation). The presence of other gases is irrelevant. $\endgroup$
    – nanoman
    Dec 2, 2021 at 7:43
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    $\begingroup$ That's a beautiful diagram. How did you do it? $\endgroup$ Dec 2, 2021 at 8:58
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    $\begingroup$ @EricDuminil Thanks a lot... I wish I had a cooler answer than Google Slides. $\endgroup$
    – kwinkunks
    Dec 2, 2021 at 19:47
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    $\begingroup$ @nanoman Okay, sure, your point is reasonable... but we are talking about a house on the surface of Earth here, and I tried to use fairly approachable language. I added a footnote for the, er, sticklers. $\endgroup$
    – kwinkunks
    Dec 2, 2021 at 19:57
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Because cooling air tends to take out moisture, while heating air doesn't change the actual moisture levels.
When air gets colder, it can't "hold" as much water vapor (lower saturation vapor pressure).
When air gets warmer, it can "hold" more water vapor (higher saturation vapor pressure).

  • So when air cools into the 30s °F, that drops the limit of moisture. Then you heat it to the 70s, it still has that lower limit on moisture (the dew point can't be higher than the 30s).

  • Whereas when the summer airmass heats into the 80s, 90s, and beyond outside, that airmass isn't changing moisture. When you cool it down on hot days, it still basically has its original ceiling limit (the dew point is whatever it was beforehand in the summer... which varies by location/airmass, but in reasonably moist regions like the eastern US and Europe, is usually up in like the 50s/60s/70s)

So your 72 °F in the summer will have dew points in like the 60s and so the ratio of actual to possible moisture is quite high, higher relative humidity... but in winter, your 72 °F will have dew points down in like the 20s or 30s or lower = lower relative humidity.

(Dew point is really reflected more by the lowest temperature in the day. So if it's 30 °F outside in the daytime, but 10 °F at night... the 10 °F is the ceiling limit of the dew point of that air mass. Likewise, if it's 90 °F during the daytime, but 65 °F at night, the dew point is capped around 65 °F. But that's quite moist compared to winter air masses)

(Unlike a house heater, quick major outdoor warmups are driven by warm fronts/warm air advection, which tend to bring in a different airmass, pulled in from more tropical regions, where the air also has also had plenty of time to evaporate moisture into it. So not the same dryness.)

(And air outdoors inevitably infiltrates/mixes indoors over time. So that colder, drier air will eventually come inside. You'll heat it, but it still doesn't have the moisture in it that the summertime air does.
...
Plus even apart from that, with it quite cold outside and warm and moist inside, the windows and other boundaries will cause the inside air touching it to condense because the surfaces get cold. Conservation of mass suggests it'd generally reevapoprate when warmer [unless you clean it up], but given most areas get cold regularly during the winter, the "puddle" would tend to be a pretty permanent feature if your inside warm air managed to be moist enough [try steaming some vegetables for a while, or running a humidifier, and you'll see this... also the same idea as the condensation that forms on the outside of a cold drink bottle/cup]. Your walls may even get damp if the temperature difference is great enough. All that liquid water is moisture taken out of the air.)

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  • $\begingroup$ You shouldn't have made the last two paragraphs a parenthetical remark. This is key. $\endgroup$ Dec 3, 2021 at 15:35
  • $\begingroup$ Letters are conserved between "conversation" and "conservation", but they do not converse. ;) $\endgroup$ Dec 4, 2021 at 12:53
  • $\begingroup$ @EricDuminil hahaha, oops! $\endgroup$ Dec 4, 2021 at 20:38
  • $\begingroup$ You means there's no conversation of mass? The liquid said to the plasma.... $\endgroup$ Dec 4, 2021 at 20:39
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A psychrometric chart is very useful for those kind of calculations.

enter image description here

There is a lot of information and the chart looks overwhelming at first. You can click on the charts to see a larger version.

  • 35°F is ~2°C, just above freezing point. What we call temperature is "Dry Bulb Temperature", and it is displayed on the x-axis, in green.
  • The relative humidity is displayed on the red curves.

Let's put a cross on the chart for the outdoor air:

enter image description here

The water content (in gram of water per gram of dry air) is displayed on the y-axis of the chart. It's approximately 0.003, or 3g/kg. It's called "Humidity ratio" on the chart. This ratio does not depend on the dry-bulb temperature.

If you take outdoor air and raise its temperature from 35°F to 72°F (~2°C -> ~22°C), you'll land on this point:

enter image description here

The relative humidity is now a bit below 20%. Which, by the way, is too dry, and your nose will probably notice.

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    $\begingroup$ Downvoter: constructive criticism is welcome! $\endgroup$ Dec 1, 2021 at 10:21
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    $\begingroup$ While technically true that a psychrometric chart is useful for such phenomena, I wouldn't recommend it to teach the basics of air humidity. My 2 cents $\endgroup$ Dec 2, 2021 at 10:03
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    $\begingroup$ @ToivoSäwén I agree. kwinkunks' answer is much better than mine. Once you know a bit more about temperature and humidity, this chart is a treasure of information. I just wanted to mention it, since it might be helpful or clear to OP someday. $\endgroup$ Dec 2, 2021 at 10:05
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    $\begingroup$ "the basics of air humidity" aren't a thing. Only answer with bulb in it, +1. $\endgroup$
    – Mazura
    Dec 4, 2021 at 2:23

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