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Old 09-29-2019, 03:41 AM
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Explain "dew point" like I'm a stupid child plz?


I've lived in Dallas for three decades. I am *familiar* with humidity.

That joke about Arizona, "yeah, but it's a DRY heat!" 90 F is not the same as 90 F when it's Phoenix vs. Houston. I do understand.

But I don't understand dew point, no matter how many times I hear a meteorologist explain it. Something about air saturation. I grasp that air can only hold so much water within it.

But how, my sweet Dopers, does that translate into real-world sensation?

I'd like to look at a current local dewpoint reading, and know before stepping outside, "This shall be tolerable" vs. "Drenched in sweat within two seconds."

I've been fooled by "oh it's just 85 F" before.

Sometimes 85 degrees is kinda niiiiice. Sometimes it'll melt you. Please explain how dew point figures into this?
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Old 09-29-2019, 04:25 AM
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Warmer air can hold more moisture than colder air. That means if it's humid, and the air gets cooler, water vapor in the air will start to condense into liquid water. The temperature at which that occurs is the dew point. The dew point depends on the current relative humidity and air pressure. It's called "relative" humidity because it measures how much moisture the air can hold relative to the current temperature. So 90% humidity when it's hot out is a lot more water than 90% humidity when it's cold out, but both are miserable.

When the air is saturated (or close to saturated), sweat evaporates from your skin at a slower rate, which makes you feel hot and gross.

So if it's 90F outside, and the dew point is 85F, that means it's very humid and it will be difficult to remove sweat from your body. Further, it means if it cools off just a few degrees, there will be 100% humidity and it will probably rain.

But if you're just interested in finding out if you'll be comfortable, I think it makes much more sense to just look at the current humidity instead. You'll get a much clearer picture of what's happening at that moment.
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Old 09-29-2019, 06:23 AM
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It’s the split between dew point and actual temperature that indicates humidity, the dew point alone tells you very little. When the temperature and dew point are close, just a few degrees apart, it will be humid. Whether it is humid cold or humid hot depends on the temperature. 95/90 = humid hot, 45/40 is humid cold.
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Old 09-29-2019, 07:31 AM
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Dew point is the temperature at which liquid water will just barely start to condense out of air, given a flat water surface to condense on.

Relative humidity is the amount of water vapor in air, relative to (usually as a percentage of) the amount of water vapor that would barely start to condense out of air, given a flat water surface to condense on.

Both of these rely on the premise of a flat water surface for the condensation to occur on. You could also state them in terms of just starting to evaporate from the surface to the air. If the only water surfaces are curved, it takes more or less water vapor in air to equilibrate with the surface. Positive curvature, as in the surfaces of droplets, require more water vapor to condense, leading to supersaturation in clouds. Negative curvature, as in a capillary pore, require less water vapor, which is why you can make a desiccant out of a chemically inactive but finely porous substance that water can wet. This is called the Kelvin effect.

Dew point is significant to us because we generate waste heat and must export it, and we can use evaporation to do it, but if the dew point is above our body temperature we can't do so.

Last edited by Napier; 09-29-2019 at 07:32 AM.
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Old 09-29-2019, 08:18 AM
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As stated in Wikipedia, and this is the bottom line:

Most inhabitants of temperate areas will consider dew points above 70蚌 oppressive and tropical-like
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Old 09-29-2019, 08:18 AM
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It's also relevant for what you expect the temperature to do overnight. It'll probably cool off some overnight... but it's very difficult for it to cool off below the dew point. Or more precisely, it ordinarily can't drop below the dew point at all, so the only way it can cool off is if it also decreases the dew point first.
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Old 09-29-2019, 08:33 AM
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Depending on your own tolerances, if you pay attention to the dew point + temp you can get a pretty good idea of how it feels outside doing various activities.

For me, a dew point in the 50s is tolerable for working outside if it's not too hot. Once it gets well into the 60s then it's little work outside in the afternoons and only do heavier work in the early morning. Once it's in the 70s it's time for plan B. YMMV

Right now it's 73 with a dew point of 69. I could do a little bit this morning but it's not going to be fun.

Other humidity measures fluctuate with temps so it's harder to calculate ahead of time what it might feel like later based on what such a measure is now.

Dew point does change during the day due to large scale weather effects such as fronts moving thru, wind changes, etc. Of course a drenching thunderstorm ramps it up for a while but then you can see the vapor rising and you know how bad it is. Sometimes you don't need a weatherman to know which way the dew point goes.
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Old 09-29-2019, 09:09 AM
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Explain "dew point" like I'm a stupid child plz?


Quote:
Originally Posted by friedo View Post
Warmer air can hold more moisture than colder air. That means if it's humid, and the air gets cooler, water vapor in the air will start to condense into liquid water. The temperature at which that occurs is the dew point. The dew point depends on the current relative humidity and air pressure. It's called "relative" humidity because it measures how much moisture the air can hold relative to the current temperature. So 90% humidity when it's hot out is a lot more water than 90% humidity when it's cold out, but both are miserable.

When the air is saturated (or close to saturated), sweat evaporates from your skin at a slower rate, which makes you feel hot and gross.

So if it's 90F outside, and the dew point is 85F, that means it's very humid and it will be difficult to remove sweat from your body. Further, it means if it cools off just a few degrees, there will be 100% humidity and it will probably rain.

But if you're just interested in finding out if you'll be comfortable, I think it makes much more sense to just look at the current humidity instead. You'll get a much clearer picture of what's happening at that moment.
+1, but I'd preface the explanation with, "Pay attention and stop licking the window".
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Old 09-29-2019, 10:09 AM
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Originally Posted by friedo View Post
Warmer air can hold more moisture than colder air. That means if it's humid, and the air gets cooler, water vapor in the air will start to condense into liquid water.
An everyday example of this is a glass of water with ice cubes in it on a warm day. As it sits on your table, it cools the air around it, and moisture begins to form on the glass.
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Old 09-29-2019, 10:25 AM
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. Please explain how dew point figures into this?
My Engineering mentor used to say - Unless you can explain it to your grandmother, you dont really understand it

So in that spirit - let me give it try.

So imagine air to be tennis balls and water vapor to be marbles. Imagine you have a container with 100 tennis balls. Now imagine that you add the marbles to fill up the space between the tennis balls.

As you probably understand, the space between the tennis balls expands when the temperature goes up, so you can fit more marbles at a higher temperature.

So you do an experiment to fit in max marbles at different temperatures and find the following :

Temp = 60F, Max marbles = 20
Temp = 70F, Max marbles = 25
Temp = 80F, Max marbles = 30
Temp = 90F, Max marbles = 35
Temp = 100F, Max marbles = 40


The temperatures above are dew points. So at 70F, 25 marbles will be accommodated by the 100 tennis balls. Add one more and it will stick out like a sore thumb.

So if someone tells you, that there are 30 marbles per 100 tennis balls, you can immediately say : Hey that means the dew point is 80F. Now the actual temperature is 90F so there is room for 5 more marbles.

Now your body needs to lose water vapor (marbles) to the air (tennis balls) to keep cool. If the air is already at its dew point or It is maxed out, then it cannot accept the water from your skin. So it will feel muggy.


So lets take HOUSTON. The temperature is 100F and the dew point is 90F. So this means there are 35 marbles but it can accommodate 40. So there is room for only 5 marbles - so the sweat on your skin doesnt evaporate fast and it feels muggy.

Now lets take Arizona. The temperature is 100F and the dew point is 60F. So this means there are 20 marbles but the air can accommodate 40. So there is room for 20 more marbles. Your sweat evaporates fast and you feel good.

Please, let me know if my explanation worked for you.
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Old 09-29-2019, 12:42 PM
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Originally Posted by ftg View Post
Depending on your own tolerances, if you pay attention to the dew point + temp you can get a pretty good idea of how it feels outside doing various activities.
To that end, dew point is a much much easier gauge of how humid it will feel than relative humidity. So many people erroneously say things like "It's 95 degrees out with 95 percent humidity". Horse hockey. In reality that "95 percent humidity" is in actuality 45 to 50 percent. 45 percent humidity in the winter feels fine. 45 percent humidity in the summer feels oppressive.

So much easier with dew point:

50s and lower feel great

60s progressively stickier

70s are oppressive

80s would probably kill me.
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Old 09-29-2019, 03:08 PM
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To that end, dew point is a much much easier gauge of how humid it will feel than relative humidity. So many people erroneously say things like "It's 95 degrees out with 95 percent humidity". Horse hockey. In reality that "95 percent humidity" is in actuality 45 to 50 percent. 45 percent humidity in the winter feels fine. 45 percent humidity in the summer feels oppressive.

So much easier with dew point:

50s and lower feel great

60s progressively stickier

70s are oppressive

80s would probably kill me.
This is an excellent post, and pretty much what I would have added as my contribution.

It's very common for people who don't have a good grasp of weather to say things like the above "It's 95 degrees with 95% humidity" -- it gets the point across (i.e., "it's damned hot and it's damned humid"), but it's almost always inaccurate. For example, if it's 95F, and the dew point is 80F (which is a tropical-rain-forest-level dew point ), the relative humidity would only be about 60%.

For reference, this article, by Tom Skilling, chief meteorologist at WGN-TV in Chicago, notes that the all-time highest verified dew point readings in the U.S. have been in the high 80s (with a few unverified reports of the very low 90s). Chicago has never seen a dew point above 83F.

In using this online calculator for humidity and dew point, in order to achieve "95 degrees with 95% humidity," you'd need to have a dew point of 93 degrees, which has never actually happened in U.S. weather records.

The most useful use of dew point for a layman, IMO, is giving you an understanding of how humid it actually feels to you, and BrickBat lays out, above, about how I'd describe it, as well. A dew point of 50, or below, is a pleasant humidity (unless, possibly, the air temperature is very close to the dew point). Once your dew point gets to the high 50s or low 60s, it's definitely noticeable (at least to me), and in the high 60s to low 70s, it's oppressively humid. YMMV, of course, and if you are accustomed to a warm and humid climate, you may well be more tolerant of high humidity.

Last edited by kenobi 65; 09-29-2019 at 03:11 PM.
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Old 09-29-2019, 10:43 PM
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The tennis ball analogy gives a false picture of what is going on. There is no shortage of space for the water molecules and the saturation point has nothing to do with the properties of air. It has to do only with the properties of water. For a given temperature, 100% relative humidity is simply the amount of water vapor at which the rate of evaporation equals the rate of condensation. This is essentially independent of whether or not there is any air present. To elaborate, if you put water in a vacuum chamber, it will evaporate until a certain pressure of water builds up at which evaporation and condensation are in balance (equilibrium). That pressure is a function only of temperature. If you put air in the chamber and let it come to equilibrium, the amount of water vapor does not change (for all practical purposes). The only effect is that the total pressure has gone up, because you have to add the pressure of the air to the "partial pressure" due to the water, also known as the vapor pressure of water.

The notion that "the air can only hold a certain amount of water vapor" is false. The point is that, if you add more vapor above 100% humidity, it will condense. Again, it has nothing to do with air.
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Old 09-29-2019, 11:20 PM
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The only effect is that the total pressure has gone up, because you have to add the pressure of the air to the "partial pressure" due to the water, also known as the vapor pressure of water..
That is also an incorrect understanding. Equilibrium happens with respect to fugacity not partial pressures. It just happens that the partial pressure of water vapor is approximately the same as its fugacity at low pressure.

Anyways, the intent of the tennis balls model is to illustrate the concept behind dew points and not to technically describe vapor-liquid equilibrium.

Just as George Box said - All models are wrong;
Some are useful.
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Old 09-29-2019, 11:58 PM
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Originally Posted by am77494 View Post
So imagine air to be tennis balls and water vapor to be marbles. Imagine you have a container with 100 tennis balls. Now imagine that you add the marbles to fill up the space between the tennis balls.

As you probably understand, the space between the tennis balls expands when the temperature goes up, so you can fit more marbles at a higher temperature..
Why do you make the bolded assumption? That's where your explanation loses your grandmother. Sorry.

A lot of the explanations here have statements like this which make assumptions or take things for granted, and thereby make the explanations fail.
". . . it measures how much moisture the air can hold relative to the current temperature. So 90% humidity when it's hot out is a lot more water than 90% humidity when it's cold out . . ."
What is that supposed to mean? How much it can hold? Or does hold? And if it's more water, then why have the same measurement? What's the point?
" . . .the amount of water vapor in air, relative to (usually as a percentage of) the amount of water vapor that would barely start to condense out of air, given a flat water surface to condense on . . ."
Water condensing on a "flat water surface"? What is that supposed to mean? How do you differentiate between the the water forming the surface itself and the water condensing? How does water condense on water? And why do we want to measure that?

This is not to say the explanations are wrong--just rhetorically faulty.

Last edited by guizot; 09-30-2019 at 12:00 AM.
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Old 09-30-2019, 12:12 AM
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That is also an incorrect understanding. Equilibrium happens with respect to fugacity not partial pressures. It just happens that the partial pressure of water vapor is approximately the same as its fugacity at low pressure.
At the low partial pressures relevant to this discussion, the vapor pressure is almost precisely equal to the fugacity. That is why it is an excellent approximation to consider water to be an ideal gas, and certainly for purposes of explaining it to your grandmother, the ideal gas approximation should capture the essential behavior. It is very misleading to attempt to explain it by invoking the volume of the gas molecules, when the essential physics is much simpler and has nothing to due with the properties of air.
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Old 09-30-2019, 08:13 AM
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I see that Purplehorseshoe hasn't yet responded, hopefully they've grokked.

However as someone who is 50 something and absolutely never understood Dew Point I now get it has to do something with the ratio of humidity and temperature, (which I kind of always understood) but with all "dew" respect to the posters, beyond that, I'm still have no f'ing clue what practical use it is to me.

I just checked my Dew Point this morning, the actual temp is 15C (60F), the Dew Point is 7C (45F) moving to 12C (54F) later today - so based on all your explanations WTF am I supposed to do with that? It's truly meaningless to me. Does it "feel" like it's 45F right now? Should I dress like it's 45F? But that actually temp in the afternoon is supposed to be in the low 20's (low 70'sF), will I feel cold still or do I feel overdressed?

Next - if Dew Point does all this, then WTF is the:
1) Humidex?
2) Realfeel index?
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Old 09-30-2019, 08:32 AM
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I also do not know what practical effect the dew point has when the temperature is 60F or below. However, to me, the dew point will tell me approximately how "muggy" it will feel to me when it is warmer than that, because mugginess is a combination of heat and humidity. 50% humidity at 95 will feel muggier than 80% humidity at 65 to me, and indeed, the 50% air at 95 will hold more water at least according to a quick perusal of wikipedia.

As a counterexample, 0% humidity at 110 will feel a lot less "muggy" than 60% at 80 but 110 would still feel "hotter".
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Old 09-30-2019, 10:24 AM
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Originally Posted by GMANCANADA View Post
I just checked my Dew Point this morning, the actual temp is 15C (60F), the Dew Point is 7C (45F) moving to 12C (54F) later today - so based on all your explanations WTF am I supposed to do with that? It's truly meaningless to me. Does it "feel" like it's 45F right now? Should I dress like it's 45F? But that actually temp in the afternoon is supposed to be in the low 20's (low 70'sF), will I feel cold still or do I feel overdressed?
FWIW, I'd suggest taking another read through what BrickBat and I posted above for some general guidelines on how noticeably humid it is at different dew point levels.

If your current dew point is 45F, and it's increasing to 54F today, it's still not likely to be noticeably humid to you (though, depending on how much you, personally, notice humidity, maybe you'll be starting to feel like it's a bit humid this afternoon).

Quote:
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Next - if Dew Point does all this, then WTF is the:
1) Humidex?
2) Realfeel index?
Those are attempts by various weather forcasting companies to create a measure that shows the combination of heat+humidity, similar to the concept of wind chill. I have no idea how scientifically tested those are, which is why I tend to ignore those, and just look at the dew point number.
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Old 09-30-2019, 11:18 AM
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I noted an interesting phenomenon yesterday with high humidity and a constant dew point. In the morning it was 72 with a dew point of 67 and a decent breeze. It felt like it should be nice, but I was sweating like crazy with only a little bit of physical exertion. Not too surprising since the relative humidity was 84%. By the afternoon it was 90 degrees with a similar breeze, and it actually felt better since the relative humidity was now only 47% with the dew point remaining constant. Still needed to find shade, and the breeze helped immensely, but that lower relative humidity meant sweat actually evaporated rather than just soaking me all over.

I suspect this only occurs at a relative narrow band of dew points and temperatures, though I'll admit I was surprised the dew point was that high. I'd put that in the "just plain nasty" range no matter the temperature. For the first couple hours of the morning I kept saying "it feels nice out, but I'm still soaked, WTH?" By the afternoon, as long as it was shady then it actually felt ok. Maybe if I was just sitting around it would've changed the perception in a different way. That just goes to show how a basic heat index calculation may not be super informative since it doesn't take into account how you're exerting yourself.
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Old 09-30-2019, 04:57 PM
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Reading this discussion, it occurs to me that neither dewpoint nor humidity (RH%) is ever mentioned in weather forecasts or weather reports in Ireland. I wonder if it's a feature of warmer climates, where it is more relevant to comfort.
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Old 09-30-2019, 05:24 PM
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Perhaps that, and perhaps also that (at least, I would expect) the humidity in Ireland would be rather consistently high. If the humidity never changes, then whatever it is is what you're used to.
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Old 09-30-2019, 09:48 PM
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If you live in Texas, dont mind the dew point if its below 60 deg f. But if the dew point is 85 deg f in July, be careful out there. In cold climates like Minnesota they look at very low dew points. That could mean frostbite.
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Old 09-30-2019, 10:47 PM
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In cold climates like Minnesota they look at very low dew points. That could mean frostbite.
Low dew point / low humidity doesn't lead to frostbite; it's caused by exposure to low temperatures.

The issue that low humidity causes is that very dry air can cause discomfort (dry skin, dry mucous membranes). In the wintertime, in cold climates, when you heat up cold (and dry) outside air, unless you add moisture to the air (through a humidifier), you wind up with extremely low relative humidity indoors.
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Old 09-30-2019, 11:05 PM
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I mean that if you live in the upper Midwest and the dew point is -40 deg f in January, be careful out there because its probably going to get cold. Relative humidity matters more if its hot, because humans cooling strategy- sweating, is more efficient in dry air.
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Old 10-01-2019, 12:11 AM
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Reading this discussion, it occurs to me that neither dewpoint nor humidity (RH%) is ever mentioned in weather forecasts or weather reports in Ireland. I wonder if it's a feature of warmer climates, where it is more relevant to comfort.
I dont think it is mentioned in Australia either so not necessarily a feature of warm climates. It seems to be something specific to the USA.
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Old 10-01-2019, 12:26 AM
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The typical morning show two-day weather forecasts probably wont mention anything but temp, wind, if its going to be sunny, or if its going to rain. Both Ireland and Australia have national weather services that produce detailed forecasts that discuss dew point and RH, for those interested.
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Old 10-01-2019, 12:32 AM
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If you live in Texas, don’t mind the dew point if it’s below 60 deg f. But if the dew point is 85 deg f in July, be careful out there. In cold climates like Minnesota they look at very low dew points. That could mean frostbite.
It's almost midnight and still over 80 F out there. What is this "cold" of which you speak? It sounds nice.
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Old 10-01-2019, 12:42 AM
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The typical morning show two-day weather forecasts probably wont mention anything but temp, wind, if its going to be sunny, or if its going to rain. Both Ireland and Australia have national weather services that produce detailed forecasts that discuss dew point and RH, for those interested.
I guess thats the point. Its not particularly interesting for Australians so its not part of the basic weather reports. All sorts of detail is there if you want to look for it.
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Old 10-01-2019, 09:49 AM
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I mean that if you live in the upper Midwest and the dew point is -40 deg f in January, be careful out there because its probably going to get cold. Relative humidity matters more if its hot, because humans cooling strategy- sweating, is more efficient in dry air.
That's true -- the air temperature really can't drop below the dew point temperature (at least not without precipitiation, or a change in the air mass). So, on a clear, still night, one can usually expect that the overnight low temperature will get close to the dew point temperature*. And, thus, if the dew point is extremely low, the overnight low will very likely be extremely low, as well.

* - and, by the way, this is another useful thing that a layman can use dew point for. On a hot day, if you're hoping for it to cool off overnight, take a look at what the dew point is: that'll tell you the lowest that the overnight low is likely to be (and it might not even get that low). So, if the dew point is 70F, it's unlikely to get any cooler than 70 overnight.
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Old 10-01-2019, 02:24 PM
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However as someone who is 50 something and absolutely never understood Dew Point I now get it has to do something with the ratio of humidity and temperature, (which I kind of always understood) but with all "dew" respect to the posters, beyond that, I'm still have no f'ing clue what practical use it is to me.
When dew point & temp approach each other, like when you walk outside in the morning (coldest time of the day) the air will be saturated. This means that condensation will form on things like car windshields & even just in the air (fog). Many private pilots are not allowed to fly in those conditions because of the lack of visibility & the worst fog will even ground/delay commercial flights. As a photographer, I had a sunrise shoot setup a few weeks ago. I was watching the weather forecast for days; at first it was going to be cloudy but that got better & better every day; everything looked good except dew point. Sure enough, pea soup fog with well less that 100' visibility that didn't burn off for over an hour. What it might mean to you is the scientific reason behind what your eyes tell you - you need to put your windshield wipers on 'intermittent' or put on your headlights so that other cars can see you just a little bit further in the poor conditions when you're driving to work.

As the sun comes up & heats the air, the air temp will rise more that the dew point will, "drying out" the air. With all other conditions remaining the same, you'll get morning fog/dew on more days than you will in the afternoon/evening. Also, warm air can hold more moisture so the same 10 or 20 spread between dew point & temp in the summer can be oppressive while you're putting lip balm on your lips in the winter to keep them from cracking because it's so dry out.
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Old 10-01-2019, 02:31 PM
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I also often find it confusing to ascertain the significance of dew point, as well as distinguishing it from actual vs relative humidity.

I'm pretty sure I recently read that clouds form at the point where rising warm air cools to its dew point (but I suspect I got that wrong too!)
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Old 10-01-2019, 05:06 PM
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I've lived in a variety of climates:

Warm and dry - California. Feels comfortable all the time.

Cool / cold and dry - Midwestern USA winter, also Santa Fe. Really dry indoors.

Warm and humid - Midwestern, Southern and Eastern USA summers. The idea of relative humidity starts to really play out, with warm rains bringing RH to near 100%, and even 70 F feels really uncomfortable.

Cool and humid - Pacific NW during the rainy season. Somehow, having high humidity during the cool (40 or 50F) winter weather makes it feel even colder than actually freezing weather. I think the clamminess of the air pulls body heat out of you somehow. It can be really unpleasant.

Verdict: High humidity makes both hot and cold weather feel worse.
  #34  
Old 10-01-2019, 07:12 PM
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Perhaps that, and perhaps also that (at least, I would expect) the humidity in Ireland would be rather consistently high. If the humidity never changes, then whatever it is is what you're used to.
That's correct, and makes sense as a reason why it wouldn't be reported.
  #35  
Old 10-01-2019, 08:54 PM
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In mild oceanic climates (e.g. Ireland) dew points around 30 (-1 C) can result in dangerous icy driving conditions. In warmer drier climates (e.g. Australia) a dew point of -1 C and some wind in January can lead to explosive wildfires. I think dew point isn’t discussed in daily weather forecasts because most people don’t know what it means.
  #36  
Old 10-02-2019, 07:44 AM
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@Kenobi 65 - I did read the post a couple times and I'm still at a loss as to what from a practical POV I (a regular Joe Lunchbucket - not a photographer who shoots at sunrise or a pilot etc.) would do differently knowing the dew point is 7C versus 13C on a given morning or over the course of a day. What do you and the others that understand this do differently because of it?

Even regarding roads freezing - it seems to me the weather forecaster can just say "the roads might freeze today" rather than leaving it up to people to do all the mental gymnastics around dew points to get there themselves.

I've never once had an issue understanding that extremes of humidity, very low and very high humidity at any temperature range makes things feel worse for people.

Now that I've read the explanations posted (and thank you for that - sincerely), it does strike me that this is way too specific a measurement to be of any relevance for other than a few specialty professions.
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Old 10-02-2019, 08:06 AM
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Originally Posted by GMANCANADA View Post
@Kenobi 65 - I did read the post a couple times and I'm still at a loss as to what from a practical POV I (a regular Joe Lunchbucket - not a photographer who shoots at sunrise or a pilot etc.) would do differently knowing the dew point is 7C versus 13C on a given morning or over the course of a day. What do you and the others that understand this do differently because of it?

...
Apologies for getting off topic, but this reminds me of when I used to run along Chicago's lakefront over lunch. My buddy and I would check the various on-line sites as to temp, but not only were the sites all different (understandable, given the variability near the lake), but the temp alone had quite a limited effect on how it "felt." Maybe we shoulda been looking at the dewpoint!
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  #38  
Old 10-02-2019, 10:05 AM
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[...]
Water condensing on a "flat water surface"? What is that supposed to mean? How do you differentiate between the the water forming the surface itself and the water condensing? How does water condense on water? And why do we want to measure that?

This is not to say the explanations are wrong--just rhetorically faulty.
The point is that relative humidity is not relative to the maximum possible amount of water vapor, a common misstatement. What it is relative to is the equilibrium between humid air and a flat water surface, at which evaporation and condensation are both going on at equal rates. Surface curvature matters. If you have supersaturation, does that mean there is more water vapor than physically possible? Of course not.

Here is something from the Wikipedia article on "Kelvin equation":
"The Kelvin equation describes the change in vapour pressure due to a curved liquidvapor interface, such as the surface of a droplet. The vapor pressure at a convex curved surface is higher than that at a flat surface. The Kelvin equation is dependent upon thermodynamic principles and does not allude to special properties of materials. It is also used for determination of pore size distribution of a porous medium using adsorption porosimetry. The equation is named in honor of William Thomson, also known as Lord Kelvin."

And this, from the Wikipedia article on "Relative humidity":
"The relative humidity (...) of an airwater mixture is defined as the ratio of the partial pressure of water vapor (...) in the mixture to the equilibrium vapor pressure of water (...) over a flat surface of pure water at a given temperature (...)." [I skipped over the fonts and formatting issues because they don't seem to paste right here, but the sentence is the first one in the "Definition" section.]

One more item, from the Wikipedia article on "Vapor pressure":
"Meteorologists also use the term saturation vapor pressure to refer to the equilibrium vapor pressure of water or brine above a flat surface, to distinguish it from equilibrium vapor pressure, which takes into account the shape and size of water droplets and particulates in the atmosphere."
  #39  
Old 10-02-2019, 10:21 AM
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I don't think anyone has directly mentioned this advantage of dew point over humidity. Over the course of a normal day humidity may start out as a high percentage, but will decrease through the day as the the temperature increases. Then it will start to rise again when the temp goes down. Dew point, however, will stay constant as it is not "relative" to the temperature. So, that 80% humidity that becomes 50% in the afternoon does not represent a loss of moisture, just a change in temp and the dew point of 70 degrees stays the same (actual forecast numbers for my city today).
  #40  
Old 10-02-2019, 11:24 AM
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Originally Posted by GMANCANADA View Post
@Kenobi 65 - I did read the post a couple times and I'm still at a loss as to what from a practical POV I (a regular Joe Lunchbucket - not a photographer who shoots at sunrise or a pilot etc.) would do differently knowing the dew point is 7C versus 13C on a given morning or over the course of a day. What do you and the others that understand this do differently because of it?
One big example: if I see that the dew point is high (say, 60F or above), or that it's going to be increasing into that level during the day, I might close the windows of my house and turn on the air conditioner before I leave for work in the morning, knowing that, even if it feels fairly comfortable at 7am when I leave the house, it's going to feel uncomfortable by the time I get home.

And, that "uncomfortable" is a function of both the heat *and* the humidity. If it's going to be 80F, but with a low dew point, I don't feel like I need the air conditioner -- open windows, and maybe a fan running, are enough for me to be comfortable. But, that same air temperature, with a higher dew point, tells me that I *will* be uncomfortable. (And, if it's going to be 90F, I'm turning on the A/C, regardless of the dew point. )

Also, as Anglachel notes, relative humdity is a fairly useless measure for those sorts of decisions, as the exact same amount of humidity (that is, the same dew point) will have a lower relative humidity measure during the day, simply because the air has warmed up.

Last edited by kenobi 65; 10-02-2019 at 11:24 AM.
  #41  
Old 10-02-2019, 12:09 PM
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One big example: if I see that the dew point is high (say, 60F or above), or that it's going to be increasing......
It’s probably not intuitive but think of it this way :

Say the temperature is 80F and the dew point is 60F

1. So if a drop of water (1g) cools from 98F (your body temperature) to 80F (the air temperature), you will lose 98-80 = 18 calories

2. If the same drop (1g) were to evaporate, you will lose 540 calories !! That’s about 30 times the amount of cooling than the first case


So evaporation of water from your skin cools you more than an order of magnitude faster than just conductive cooling.


This is the reason why when the dew point is high (hence less evaporation from skin), you’re body is unable to cool fast.

Last edited by am77494; 10-02-2019 at 12:11 PM.
  #42  
Old 10-06-2019, 03:42 AM
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For example, right now the temp is 76 F and the dew point is 67 degrees.

Am I correct in understanding that this means it is humid as fuck outside? Since the two are so close to each other?
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  #43  
Old 10-06-2019, 04:36 AM
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Originally Posted by purplehorseshoe View Post
For example, right now the temp is 76 F and the dew point is 67 degrees.

Am I correct in understanding that this means it is humid as fuck outside? Since the two are so close to each other?
Yes, that can be said, clinically. But really, at room temperature and above, one doesn't really need to concern oneself with how close they are to each other as far as a practical use of dew point as a yardstick as to the effect of felt humidity. That's the simplistic beauty of dew point vs relative humidity: It needs no qualifiers. With ( a dew point of ) the 50s being comfortable, to 60s being progressively uncomfortable, and the 70's being oppressive to fucking unbearable.

Below room temperatures, especially further below, who gives a damn? It's cool/cold: You'll need a jacket or coat regardless or how "humid" it is or isn't. But at say, for example 85 degrees, a dew point of 53 would mean I could wear a long sleeve button down shirt and feel comfortable, at least out of direct sunlight. If, at the same temperature the dew point was 75, I would be in a loose fitting short sleeve shirt, and even then, even in shade, I'd feel clammy and likely have a sweat splotch on my back the size of a basketball.

Last edited by BrickBat; 10-06-2019 at 04:40 AM.
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