# Dewpoint vs. relative humidity
**URL:** https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539
**Category:** Factual Questions
**Created:** [July 13, 2010, 3:46pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539 "2010-07-13T15:46:30Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)
#### Post date: [July 13, 2010, 3:46pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/1 "2010-07-13T15:46:30Z")
I have noticed over the last several years the tendency of local weather forecasters to use dewpoint as an indication of humidity rather than relative humidity. Am I imagining this or is there a shift in the meteorological community to do this? If so, why? It seems less intuitive to me as the average guy in the street how the dewpoint affects my perception of the heat.
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### Author:  [@ftg](https://boards.straightdope.com/u/ftg)
#### Post date: [July 13, 2010, 4:23pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/2 "2010-07-13T16:23:21Z")
I _much_ prefer dewpoint over relative humidity. For one thing, relative humidity changes as the air heats up. So just knowing that it’s 60% humidity isn’t enough. You also need to know the temp and do some math to get a feel for how bad it is.
But if I know the dewpoint is 72 degrees, I know that’s getting fairly humid. It’s going to feel muggy no matter what the time of day. I also know the overnight low is probably not going to drop below 72 unless a front moves by. And if the dewpoint is 60 degrees, that’s pretty good weather for working outdoors in the summer here. A single number, no math required.
It’s also handy in the winter. A dewpoint of 36 degrees means little chance of frost. And if it’s in the 20s, **Mrs. FtG** can make eggwhite cookies.
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### Author:  [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)
#### Post date: [July 13, 2010, 4:54pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/3 "2010-07-13T16:54:15Z")
> [@ftg](#):
>
> But if I know the dewpoint is 72 degrees, I know that’s getting fairly humid. It’s going to feel muggy no matter what the time of day.
Help me out here–does that mean that a dewpoint of 72 degrees if the temp is 80 is going to feel as muggy as a dewpoint of 72 when it’s 90? Is it normal for the dewpoint to be the same through the day as temperatures rise?
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### Author:  [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)
#### Post date: [July 13, 2010, 5:32pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/4 "2010-07-13T17:32:50Z")
> [@CookingWithGas](#):
>
> Is it normal for the dewpoint to be the same through the day as temperatures rise?
Generally, yes, unless a front comes through or you find yourself under a different air mass. Barring those sorts of events, the dewpoint may vary a few degrees over the course of the day, but it won’t move significantly.
Part of the reason that meterologists have started talking about dewpoint more is that it’s more directly meaningful. You might have a relative humidity of 90% at 7 a.m., and of 50% at 1 p.m. Did it get less humid out? No…you’re in the same airmass, and the dewpoint didn’t change…it’s just that the air warmed up (as it does during the day), and that caused the “holding capacity” of the air to increase.
As ftg notes, dewpoint is usually a better indicator of how humid the air **feels** to you. For most people, in warm weather, a dewpoint in the 50s or below will feel “dry”. Once you get a dewpoint in the 60s, most people will feel that it’s noticeably humid. And, when the dewpoint gets up into the 70s, most people will feel that the air is positively tropical.
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### Author:  [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)
#### Post date: [July 13, 2010, 5:36pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/5 "2010-07-13T17:36:28Z")
> [@ftg](#):
>
> I also know the overnight low is probably not going to drop below 72 unless a front moves by.
That’s another good point…the nighttime temperature **cannot** drop below the dewpoint. If the dewpoint during the day is 72, unless you have a frontal passage (which likely will bring in a different (and possibly drier) airmass), your nighttime temperature won’t get any lower than 72 degrees (and probably will bottom out a few degrees above 72).
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### Author:  [@luvrbcs](https://boards.straightdope.com/u/luvrbcs)
#### Post date: [July 13, 2010, 5:54pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/6 "2010-07-13T17:54:08Z")
And for a bit of trivia, if you know the temperature and dewpoint, you can calculate the height of the cloudbase.
(temperature-dewpoint/4.4)\*1000
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### Author:  [@KneadToKnow](https://boards.straightdope.com/u/KneadToKnow)
#### Post date: [July 13, 2010, 6:29pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/7 "2010-07-13T18:29:09Z")
A simple definition:
> [@Wikipedia](#):
>
> The dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point.
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### Author:  [@tim-n-va](https://boards.straightdope.com/u/tim-n-va)
#### Post date: [July 13, 2010, 7:29pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/8 "2010-07-13T19:29:53Z")
No real experience here but did a few scans of some websites.
I could just be mis-reading but it seems to me that while the temperature can’t go below the dewpoint, that is not a barrier on the temperature. If the temperature gets to the dew point, dew will form and there will be less moisture in the air and the dew point will be lower.
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### Author:  [@Chessic\_Sense](https://boards.straightdope.com/u/Chessic_Sense)
#### Post date: [July 13, 2010, 8:02pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/9 "2010-07-13T20:02:37Z")
> [@kenobi\_65](#):
>
> That’s another good point…the nighttime temperature **cannot** drop below the dewpoint. If the dewpoint during the day is 72, unless you have a frontal passage (which likely will bring in a different (and possibly drier) airmass), your nighttime temperature won’t get any lower than 72 degrees (and probably will bottom out a few degrees above 72).
Are you being pedantic (i.e. if the temp drops, the dew point drops) or are you saying the temperature _really can’t_ go below the dew point? Are you saying I can’t find dew on my lawn in the middle of the night? I mean, I’ve never checked, so I don’t know.
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [July 13, 2010, 8:19pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/10 "2010-07-13T20:19:45Z")
> [@luvrbcs](#):
>
> And for a bit of trivia, if you know the temperature and dewpoint, you can calculate the height of the cloudbase.
>
> (temperature-dewpoint/4.4)\*1000
That assumes the relative humidity stays constant up to that height, right? Is that usually a realistic approximation?
> [@Chessic Sense](#):
>
> Are you being pedantic (i.e. if the temp drops, the dew point drops) or are you saying the temperature really can’t go below the dew point? Are you saying I can’t find dew on my lawn in the middle of the night?
I think he/she is saying that when the air starts to cool down and get down to the dew point, dew starts to form, which releases heat. So the temperature drop is halted, at least until a non-negligible fraction of the moisture in the air has condensed.
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### Author:  [@kenobi\_65](https://boards.straightdope.com/u/kenobi_65)
#### Post date: [July 13, 2010, 8:22pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/11 "2010-07-13T20:22:35Z")
> [@scr4](#):
>
> I think he/she is saying that when the air starts to cool down and get down to the dew point, dew starts to form, which releases heat. So the temperature drop is halted, at least until a non-negligible fraction of the moisture in the air has condensed.
That’s pretty much it.
As I understand it, once you cool air down to the dewpoint, it simply cannot be cooled any further, unless some of the water vapor is removed from the air (i.e., the water condenses, such as with dew forming).
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### Author:  [@Delayed\_Reflex](https://boards.straightdope.com/u/Delayed_Reflex)
#### Post date: [July 13, 2010, 11:22pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/12 "2010-07-13T23:22:10Z")
> [@tim-n-va](#):
>
> No real experience here but did a few scans of some websites.
>
> I could just be mis-reading but it seems to me that while the temperature can’t go below the dewpoint, that is not a barrier on the temperature. If the temperature gets to the dew point, dew will form and there will be less moisture in the air and the dew point will be lower.
Yeah for example I checked [wx.ca](http://wx.ca/?service=page/Forecast) and the lows are pretty consistently below what I assume are the daytime dewpoints (often 8-9 degrees C below) - so unless the dewpoint calculations or the low forecasts are wrong on that page then the dewpoint must change over the course of the day (or you would have \>100% relative humidity, that can’t happen right?).
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### Author:  [@Uncertain](https://boards.straightdope.com/u/Uncertain)
#### Post date: [July 14, 2010, 2:21am UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/13 "2010-07-14T02:21:35Z")
> [@kenobi\_65](#):
>
> You might have a relative humidity of 90% at 7 a.m., and of 50% at 1 p.m. Did it get less humid out? No…you’re in the same airmass, and the dewpoint didn’t change…it’s just that the air warmed up (as it does during the day), and that caused the “holding capacity” of the air to increase.
>
> As ftg notes, dewpoint is usually a better indicator of how humid the air **feels** to you.
But id did get less humid out in an important sense. At 50% RH you get more cooling due to sweat evaporation than you do at 90% RH. At 100% RH you get no evaporative cooling, but at 50% you’d get some, even if the dewpoint remains the same.
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### Author:  [@FluffyBob](https://boards.straightdope.com/u/FluffyBob)
#### Post date: [July 14, 2010, 3:16am UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/14 "2010-07-14T03:16:50Z")
I just wanted to thank you, Delayed Reflex, for that great site. Cool and very detailed weather site; and the weather station is just a few blocks away from my home.
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### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [July 15, 2010, 3:24pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/15 "2010-07-15T15:24:36Z")
> [@Uncertain](#):
>
> But id did get less humid out in an important sense. At 50% RH you get more cooling due to sweat evaporation than you do at 90% RH. At 100% RH you get no evaporative cooling, but at 50% you’d get some, even if the dewpoint remains the same.
At 100% RH you still get evaporative cooling from the skin, if the skin is warmer than the air.
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### Author:  [@barbitu8](https://boards.straightdope.com/u/barbitu8)
#### Post date: [July 15, 2010, 8:51pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/16 "2010-07-15T20:51:58Z")
> [@Napier](#):
>
> At 100% RH you still get evaporative cooling from the skin, if the skin is warmer than the air.
I think that would be conductive cooling and not by evaporation. If the air is fully saturated, it cannot absorb any more moisture.
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### Author:  [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)
#### Post date: [July 16, 2010, 12:21am UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/17 "2010-07-16T00:21:12Z")
Skin warms the air, air can hold more moisture, air takes some moisture from the skin, skin cools down. Granted it wouldn’t be very effective.
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### Author:  [@Chefguy](https://boards.straightdope.com/u/Chefguy)
#### Post date: [July 16, 2010, 1:10am UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/18 "2010-07-16T01:10:03Z")
Also, as the dewpoint approachs the ambient temperature (I think within 4 degrees), you’re likely to have fog. Or hoarfrost, depending on how cold it is.
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### Author:  [@Napier](https://boards.straightdope.com/u/Napier)
#### Post date: [July 16, 2010, 7:22pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/19 "2010-07-16T19:22:27Z")
> [@barbitu8](#):
>
> I think that would be conductive cooling and not by evaporation. If the air is fully saturated, it cannot absorb any more moisture.
Sure it can, because you warm it adjacent to the skin. When it gets further away it cools and the water can condense. That’s why you see wispy fog rising off of hot wet things when conditions are cold and humid.
100% RH, by the way, doesn’t mean it can’t absorb any more moisture. It means that it is at the absolute humidity that equilibrates at that temperature with a flat water surface. If you avoid flat water surfaces, for example high in the atmosphere or in a cloud chamber or condensation nuclei counter, you can get a good deal higher than 100% RH.
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### Author:  [@barbitu8](https://boards.straightdope.com/u/barbitu8)
#### Post date: [July 16, 2010, 7:46pm UTC](https://boards.straightdope.com/t/dewpoint-vs-relative-humidity/546539/20 "2010-07-16T19:46:20Z")
I suppose that this is the same mechanism by which you can see your “breath;” namely that the air immediately near your mouth is warmed, but is cooled nearly instantly resulting in condensation.
I remember a science teacher in grammar school who could cause condensation from his mouth inside. I imagine he was able to warm his breath sufficiently and provide enough moisture so that even inside it cooled to the ambient room temperature quickly and condensed. I don’t know. WAG.
Evaporation, however, is not the only cooling mechanism available for people. Conduction and convection are also factors.
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