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

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.

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.

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.

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.

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 it’s probably going to get cold. Relative humidity matters more if it’s hot, because human’s cooling strategy- sweating, is more efficient in dry air.

I don’t think it is mentioned in Australia either so not necessarily a feature of warm climates. It seems to be something specific to the USA.

The typical morning show two-day weather forecasts probably won’t mention anything but temp, wind, if it’s going to be sunny, or if it’s going to rain. Both Ireland and Australia have national weather services that produce detailed forecasts that discuss dew point and RH, for those interested.

It’s almost midnight and still over 80 F out there. What is this “cold” of which you speak? It sounds nice.

I guess that’s the point. It’s not particularly interesting for Australians so it’s not part of the basic weather reports. All sorts of detail is there if you want to look for it.

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.

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.

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!)

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.

That’s correct, and makes sense as a reason why it wouldn’t be reported.

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.

@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.

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! :wink:

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 liquid–vapor 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 air–water 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.”

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).

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. :smiley: )

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.