For years I assumed (and I was probably not alone) that relative humility is responsible for how muggy things feel. They say it’s 100% humidity in New Orleans on a good day.
But weather men where I live (i.e., Detroit) say it’s actually the dew point that’s of greater importance. A dew point if 60° is when you start to feel it (I am of course talking about degrees Fahrenheit, for you non-US dopers).
Two questions: what ‘exactly’ is the dew point? And what does it have to do with how muggy it feels?
As you can see, I want it explained in plain English (I am sure the local meteorologists must’ve taken up the topic by now too).
The warmer that the air is, the more water vapor it can hold. Air that is 40 degrees F can hold a lot less water vapor than air that is 80 degrees F. Because relative humidity is based on the current temperature (i.e., it’s measuring “at this current temperature, what percent of the air’s capacity for water vapor is currently being used to hold water?”), a relative humidity of 60% at 40F is very different from a 60% relative humidity at 80F.
Dew point tells you how much water vapor is currently in the air, and it’s the amount of moisture in the air that determines how noticeable it is – that is, how “muggy” it feels:
Generally, a dew point that’s below 50F is very comfortable for most people, and if it’s well below 50F, it may even feel noticeably dry (i.e., your skin feels dry, etc.).
Once the dew point reaches the high 50s (F) to around 60F, the humidity starts to feel noticeable to many people.
A dew point in the 60s is noticeably muggy.
A dew point in the high 60s to 70s is oppressively humid.
YMMV on all of the above, depending on if you’re accustomed to a humid/tropical climate or not.
The dew point is a little more abstract than the relative humidity, but it’s an effective way of telling you how much moisture is present in the air because it means the same thing no matter how warm or cold it is outside. A 40°F dew point is comfortable whether the air temperature is 60°F or 100°F. This consistency allows us to index the dew point to comfort levels, giving us a quick understanding of how muggy or pleasant it is outside.
OK, so there’s some amount of water in the air. You can measure that amount of water in many ways. The most obvious would be something like the number of grams of water per cubic meter of air.
But that’s not the most useful way of measuring it. Because there’s a maximum amount of water you can have in the air. Try to put too much water in the air, and it’ll just end up condensing on any surface in contact with the air. And this maximum amount depends on temperature.
So another way to measure the amount of water in the air, is to take the number of grams per cubic meter or whatever that are in the air, and divide that by the maximum number of grams per cubic meter air at that temperature could hold before condensing out. That ratio is the relative humidity.
Another way you could do it would be by taking a container of air as it is right now, with whatever water content it has, and cool it down until it starts condensing. The temperature at which that’ll happen is the dew point.
Now, the relative humidity is what determines how effective sweating can be. At 100% relative humidity, sweat is useless. But that’s not really what you want to know, because sweat isn’t always equally necessary: You don’t care that sweating is useless, if it’s cool enough out that you’re not sweating anyway.
Dew point also has another point of relevance to meteorology: In most circumstances, in most parts of the world, the temperature will drop overnight. But by how much? If there’s water in the air, that will decrease the amount that the temperature can drop, because it’ll have a hard time dropping below the dew point: It can only drop below the dew point by decreasing the water in the air (i.e., decreasing the dew point below the air’s temperature), and that’s slow.
As it relates to “mugginess” this is the money shot right here:
Expanding this just a bit …
Humans cool themselves by evaporating sweat. Mugginess is the subjective sensation that your evaporative cooling is losing effectiveness and if that gets bad enough you’ll be in danger of overheating.
Dew point as a single number has the nice result that it encompasses both your need to cool and the effectiveness of that cooling. Relative humidity only captures the second of those two factors.
As @kenobi_65’s scale of DP values and their subjective effects show us.
Point of interest: the Cave of the Crystals in Mexico features temperatures of 136 degrees along with relative humidity of around 90%. According to the Dew Point Calculator, This corresponds to a dewpoint of 132F. Not only does sweating not work in a place like this, but you will find that your nice cool body actively condenses water out of the air. This is the exact opposite of sweating, and will quickly bring your skin temp up to the dewpoint temp, i.e. 132F. This would soon result in death, except that spelunkers are fitted with ice-loaded refrigeration suits that keep them safely cool for half an hour or so.
Here is a very interesting academic paper / article on the parts of the world that are approaching and now occasionally and very briefly exceeding long-term survivable dewpoints. https://advances.sciencemag.org/content/6/19/eaaw1838
It also talks about how as those events become more common and longer-lasting those parts of the world will simply become uninhabitable for ordinary civilization, just as Antarctica is for a different reason.
The difference is that millions of people already live in the soon to be too-hot/humid places and they will need to abandon everything they can’t move and then settle someplace else. It will be slow, but it will be inexorable.
Parenthetical – it’s common to hear someone say, in reference to a hot and humid day, “It’s 90 degrees with 90% humidity.” In reality, such a situation would be exceptionally uncommon, if not unheard of, at least in the U.S.
In most of the U.S., it’s extremely uncommon for the dew point to go above the high 70s; cities in the Midwest typically have their all-time high dew point somewhere in the 80s, and the highest dew point ever recorded in the U.S. was 90, at several different locations (two of those three locations were on or near the Gulf of Mexico; the third was Appleton, Wisconsin, where they just apparently had a very bad day).
An oppresively high dew point (but not unheard of, or record-setting) in the U.S. would be, let’s say, 75 degrees. That’s tropical-level humidity; if accompanied by an air temperature of 90 degrees, that’d give you a “heat index” (the heat+humidity “cousin” of the wind chill index) of 101F. But, the relative humidity in those conditions would “only” be 62%.
Here is a way to visualize this. You know instinctively, that air can hold more water vapor when hot. So imagine that each flute in this pan flute, represents the max amount of water, air can hold at a given temperature.
Just for convenience sake, lets arbitrarily pick the leftmost maximum water to be at 50F, next at 55F, 60F, 65F, 70F, 75F, 80F, 85F, 90F, 95F, 100F and 105F
Relative Humidity : Say your temperature is 105F, and that flute is half full. You have 50% relative humidity. Say if temperature is 80F, and the flute is half full, you have 50% relative humidity.
Dew Point : Say your temperature is 105F, and the 105F flute is half full. You go left to find the flute that will be full with the amount of moisture you have. The 65F flute is 50% the length of the 105F flute, so the Dew Point is 65F. If it was three quarters full, then the dew point will be 90F (the 90F flute is 3/4 the size)
It’s 9 am here and the temperature is 32 C, or 90 F (not exact conversions because it’s not 32.0 C) and the relative humidity of 76%, which calculates to a dew point of 27 C or 81 F. It will still get hotter today.
I think most people say 90 degrees and 90% simply because they haven’t lived someplace like Taiwan.
As well, if the difference between temp & dew point at the ground = airport is small (2C or especially 1C), there is a risk that any additional cooling will quickly result in fog and low clouds. Which if it materializes affects airport arrival rate, legal and prudent requirements for alternate airports, more fuel, delay possibilities, and worst case closes the airport altogether. That’s our list of worries.
For a lightplane pilot it’s all that plus the whole “Can I really fly in IMC well enough to survive trying?” thing we’ve talked so much about.
Immediately pre-sunrise and post-sunset are notorious for rapidly forming ground fogs in the regions prone to that. Both due to radiative cooling of the low altitude air versus the cold of dark space and also the common effect of the wind calming then which reduces boundary mixing and encourages the lower couple hundred feet of the atmosphere to settle with the coldest air at the bottom.
I know you are all trying to be helpful, but I still don’t get it. I will try to explain why the dew point confuses me so much.
Perhaps I would understand it better, if only the dew point were expressed in some units other than degrees. But it is indeed expressed in degrees. Or rather, it is expressed in imaginary degrees. That is: “Right now, the temperature is abc degrees, but if it were xyz degrees then the moisture would start to condense.”
But at the moment, the air temperature is NOT xyz, so the statement “if it were” is meaningless to me. The actual temperature is not xyz, it is abc, so please tell me about the actual humidity in the actual real world, not in the imaginary world where all else is equal except that the temperature is xyz.
I accept that the warmer air can hold more water vapor. And therefore, there is less water in the air at “a relative humidity of 60% at 40F”, and more water in the air at “60% relative humidity at 80F.”
But when you express it in those terms, I have to point out that I have no idea which of those two is less comfortable. What I do know is that when the weather is 40F, it is almost never uncomfortably humid, but at 80F it can easily be uncomfortably humid. So perhaps the best way to express humidity might be in absolute terms: “Right now there are x grams of water vapor per cubic meter of air.”
Maybe that would help, and maybe it wouldn’t. All I’m sure of is that when the weather report used to speak about relative humidity, I understood it; but now when they talk about the dew point, it feels like a technical term intended for people other than me.
The dewpoint does express absolute humidity in the same way as "there are x grams of water vapor per cubic meter of air.” It’s like expressing length in centimeters instead of inches. It’s possible to draw a chart that converts between the two. In fact, it’s been done: here’s that chart. It’s not a linear relationship like inches/centimeters, but for every single dewpoint, there is a unique value of absolute humidity.
Wow! That’s amazing! I had no idea that the correlation would be so simple! I will print that out and see if it helps me over the summer.
Now will try to think in terms of “40 and below is negligible, while 70 and up is oppressive” (or something like that), and try to prevent my brain from thinking in terms of “if the temperature were actually xyz”.
I’m actually a little surprised that the graph doesn’t seem to have any notable feature (a discontinuity in slope, or the like) at the freezing point of water.
Tangentially: if you’d like to read a horrific (fictional) description of an extremely high dew point causing mass death, check out the first chapter of Kim Stanley Robinson’s novel Ministry for the Future. You can probably finish it pretty quickly standing in front of the shelf in the bookstore; the rest of the book is good too, but I’m never gonna forget that chapter.
