Explain "dew point" like I'm a stupid child plz?

Factual Questions
1

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?

2

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.

3

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.

4

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.

5

As stated in Wikipedia, and this is the bottom line:

Most inhabitants of temperate areas will consider dew points above 70°F oppressive and tropical-like

6

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.

7

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.

8

+1, but I’d preface the explanation with, “Pay attention and stop licking the window”.

9

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.

10

My Engineering mentor used to say - “Unless you can explain it to your grandmother, you don’t 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 let’s 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 doesn’t evaporate fast and it feels muggy.

Now let’s 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.

11

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.

12

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 :smiley: ), 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.

13

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.

14

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”.

15

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.

16

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.

17

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?
18

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”.

19

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).

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.

20

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.