<p>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.</p>
<p>The answer to your question hinges on the difference between:</p>
<ul>
<li><strong>absolute humidity</strong> — how much water vapour ('moisture') the air holds irrespective of temperature; and</li>
<li><strong>relative humidity</strong> — how much vapour the air holds relative to how much it <em>could</em> hold at that temperature.</li>
</ul>
<p>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 <em>could</em> contain. That is, the absolute humidity stays the same, but the relative humidity changes:</p>
<ul>
<li>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.</li>
<li>Conversely, cooling air down will push its relative humidity up. (If you cool to the <a href="https://en.wikipedia.org/wiki/Dew_point" rel="noreferrer"><strong>dew point</strong></a>, you'll hit 100% relative humidity and water will condense out as dew or frost.)</li>
</ul>
<p>I drew <a href="https://docs.google.com/presentation/d/1cTxUhB4zsRiqvXFigNMe9bDN3L99aYKAGqrWR8sfxLY/edit?usp=sharing" rel="noreferrer">this</a>, maybe it helps...</p>
<p><a href="https://i.sstatic.net/s4kw8.png" rel="noreferrer"><img src="https://i.sstatic.net/s4kw8.png" alt="Diagram to illustrate the concepts of relative and absolute humidity" /></a></p>
<hr />
<p>† 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. <a href="https://en.wikipedia.org/wiki/Vapor_pressure" rel="noreferrer">Read about vapour pressure.</a></p>

CC BY-SA 4.0
2021-12-02T03:50:13.923Z
Nitpick: "...liquid water will condense out as dew", or frost if the dewpoint is below freezing.
jamesqf
CC BY-SA 4.0
2021-12-02T07:43:41.953Z
"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.
nanoman
CC BY-SA 4.0
2021-12-02T08:58:51.227Z
That's a beautiful diagram. How did you do it?
Eric Duminil
CC BY-SA 4.0
2021-12-02T19:47:52.430Z
@EricDuminil Thanks a lot... I wish I had a cooler answer than Google Slides.
Matt Hall
CC BY-SA 4.0
2021-12-02T19:57:07.853Z
@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.
Matt Hall
CC BY-SA 4.0
2021-12-02T20:25:50.927Z
@kwinkunks: The less impressive the tool, the more impressive your result!
Eric Duminil
CC BY-SA 4.0
2021-12-01T11:47:05.970Z
2021-12-02T20:53:14.020Z
2021-12-02T20:53:14.020Z
Matt Hall
<p>Because cooling air tends to take out moisture, while heating air doesn't change the actual moisture levels.<br />
When air gets colder, it can't &quot;hold&quot; as much water vapor (lower saturation vapor pressure).<br />
When air gets warmer, it can &quot;hold&quot; more water vapor (higher saturation vapor pressure).</p>
<ul>
<li><p>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).</p>
</li>
<li><p>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)</p>
</li>
</ul>
<p>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.</p>
<p>(Dew point is really reflected more by the <em>lowest</em> 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)</p>
<p>(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.)</p>
<p>(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.<br />
...<br />
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 &quot;puddle&quot; 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.)</p>

CC BY-SA 4.0
2021-12-03T15:35:11.447Z
You shouldn't have made the last two paragraphs a parenthetical remark. This is key.
David Hammen
CC BY-SA 4.0
2021-12-04T12:53:29.430Z
Letters are conserved between "conversation" and "conservation", but they do not converse. ;)
Eric Duminil
CC BY-SA 4.0
2021-12-04T20:38:54.853Z
@EricDuminil hahaha, oops!
JeopardyTempest
CC BY-SA 4.0
2021-12-04T20:39:24.760Z
You means there's no conversation of mass?  The liquid said to the plasma....
JeopardyTempest
CC BY-SA 4.0
2021-11-30T23:30:01.567Z
2021-12-04T20:38:43.160Z
2021-12-04T20:38:43.160Z
JeopardyTempest
<p>A <a href="https://en.wikipedia.org/wiki/Psychrometrics#Psychrometric_charts" rel="noreferrer">psychrometric chart</a> is very useful for those kind of calculations.</p>
<p><a href="https://en.wikipedia.org/wiki/Psychrometrics#/media/File:PsychrometricChart.SeaLevel.SI.svg" rel="noreferrer"><img src="https://i.sstatic.net/kuvLY.png" alt="enter image description here" /></a></p>
<p>There is a lot of information and the chart looks overwhelming at first. You can click on the charts to see a larger version.</p>
<ul>
<li>35°F is ~2°C, just above freezing point. What we call temperature is &quot;Dry Bulb Temperature&quot;, and it is displayed on the x-axis, in green.</li>
<li>The relative humidity is displayed on the red curves.</li>
</ul>
<p>Let's put a cross on the chart for the outdoor air:</p>
<p><a href="https://i.sstatic.net/aaUbM.png" rel="noreferrer"><img src="https://i.sstatic.net/aaUbM.png" alt="enter image description here" /></a></p>
<p>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 &quot;Humidity ratio&quot; on the chart. This ratio does not depend on the dry-bulb temperature.</p>
<p>If you take outdoor air and raise its temperature from 35°F to 72°F (~2°C -&gt; ~22°C), you'll land on this point:</p>
<p><a href="https://i.sstatic.net/wiDJm.png" rel="noreferrer"><img src="https://i.sstatic.net/wiDJm.png" alt="enter image description here" /></a></p>
<p>The relative humidity is now a bit below 20%. Which, by the way, is too dry, and your <a href="https://en.wikipedia.org/wiki/Thermal_comfort#Relative_humidity" rel="noreferrer">nose will probably notice</a>.</p>

CC BY-SA 4.0
2021-12-01T10:21:35.290Z
Downvoter: constructive criticism is welcome!
Eric Duminil
CC BY-SA 4.0
2021-12-02T10:03:12.110Z
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
Toivo Säwén
CC BY-SA 4.0
2021-12-02T10:05:42.167Z
@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.
Eric Duminil
CC BY-SA 4.0
2021-12-04T02:23:53.687Z
"the basics of air humidity" aren't a thing. Only answer with *bulb* in it, +1.
Mazura
CC BY-SA 4.0
2021-12-01T10:10:31.237Z
2021-12-01T10:22:19.850Z
2021-12-01T10:22:19.850Z
Eric Duminil
<p>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.</p>
<p>During the warmer months the humidity in the house never goes below 35% relative humidity.</p>
<p>So, it appears that somehow the outdoor temperature is affecting the relative humidity inside the house. How could this be?</p>

CC BY-SA 4.0
2021-12-03T15:35:36.907Z
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.
David Hammen
<temperature><humidity>
Why would it be drier indoors during the winter vs the summer?
Tyler Durden
# Why would it be drier indoors during the winter vs the summer?

(Asked by: username_0 on 2021-11-30)

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?

**Tags:** temperature, humidity

> **Comments:**
> **username_1**: 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.  

---

## 3 answers

### Answer by username_2 on 2021-12-01. Score: 41

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**](https://en.wikipedia.org/wiki/Dew_point), you'll hit 100% relative humidity and water will condense out as dew or frost.)

I drew [this](https://docs.google.com/presentation/d/1cTxUhB4zsRiqvXFigNMe9bDN3L99aYKAGqrWR8sfxLY/edit?usp=sharing), maybe it helps...

[![Diagram to illustrate the concepts of relative and absolute humidity](https://i.sstatic.net/s4kw8.png)](https://i.sstatic.net/s4kw8.png)

---

† 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.](https://en.wikipedia.org/wiki/Vapor_pressure)

> **Comments:**
> **username_3**: Nitpick: "...liquid water will condense out as dew", or frost if the dewpoint is below freezing.  
> **username_4**: "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.  
> **username_5**: That's a beautiful diagram. How did you do it?  
> **username_2**: @username_5 Thanks a lot... I wish I had a cooler answer than Google Slides.  
> **username_2**: @username_4 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.  
> **username_5**: @kwinkunks: The less impressive the tool, the more impressive your result!  

---

### Answer by username_6 on 2021-11-30. Score: 14

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.)

> **Comments:**
> **username_1**: You shouldn't have made the last two paragraphs a parenthetical remark. This is key.  
> **username_5**: Letters are conserved between "conversation" and "conservation", but they do not converse. ;)  
> **username_6**: @username_5 hahaha, oops!  
> **username_6**: You means there's no conversation of mass?  The liquid said to the plasma....  

---

### Answer by username_5 on 2021-12-01. Score: 11

A [psychrometric chart](https://en.wikipedia.org/wiki/Psychrometrics#Psychrometric_charts) is very useful for those kind of calculations.

[![enter image description here](https://i.sstatic.net/kuvLY.png)](https://en.wikipedia.org/wiki/Psychrometrics#/media/File:PsychrometricChart.SeaLevel.SI.svg)

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](https://i.sstatic.net/aaUbM.png)](https://i.sstatic.net/aaUbM.png)

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](https://i.sstatic.net/wiDJm.png)](https://i.sstatic.net/wiDJm.png)

The relative humidity is now a bit below 20%. Which, by the way, is too dry, and your [nose will probably notice](https://en.wikipedia.org/wiki/Thermal_comfort#Relative_humidity).

> **Comments:**
> **username_5**: Downvoter: constructive criticism is welcome!  
> **username_7**: 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  
> **username_5**: @username_7 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.  
> **username_8**: "the basics of air humidity" aren't a thing. Only answer with *bulb* in it, +1.  

---

