The second half of my post that you quoted contains that exact information – “what level of dew point is comfortable, versus what level is uncomfortable.” Relative humidity may be understandable, but it cannot tell you “is the humidity uncomfortable or not.”
Does humidity also affect lift? It seems to affect the aerodynamics of baseballs.
Yes, it does, thank you. I didn’t really understand it at first (and that’s how I missed it) but now I’m starting to get it.
Lift-generating ability scales with air density: the more dense the air is, the more lift you can generate at a given air speed and angle of attack.
Re: the effect of humidity, it’s not much. This plot compares the density of moist air at arbitrary temperature to the density of dry air at 20C. The plot shows that temperature itself has a much bigger effect on air density: on a 40C day (e.g. Las Vegas on a summer day), air density is about 93% of what it is for dry 20C air, and humidity of 0% or 100% only moves you up or down from there by one or two percentage points. Humidity’s range of effects is less at lower temperatures, where the absolute range of possible moisture content is less.
Glider pilots are very aware of dew points and lift. As mentioned above, the temperature (usually) drops with altitude - about 3.5 F per thousand feet. So, if there is a high dew point any rising air containing water vapor will cause that vapor to condense at a lower altitude than when there is a low dew point. When clouds form close to the ground it hard to get enough altitude to go anywhere. Hanging around at low altitude on a hot day makes for sweaty, uncomfortable flying.
Also, air containing water vapor is less dense than dry air. That’s why a batted ball will go further in humid conditions. It will take a moist field longer to heat up to the point where a thermal is generated but once it gets going it can provide better lift than surrounding dry surface thermals. As long as the air in a thermal is lighter than the surrounding air, it will keep going up. If its going up faster than the glider is coming down, the glider gains altitude. Glider pilots fly from cloud to cloud and how high they climb under the cloud is determined by dew point. Low dew points mean high cloud bases and that’s a good thing. Sometimes the dew point is so low that the thermal tops out before reaching it. This means no visual indicators as to where the next thermal is. It makes the soaring more challenging but not impossible.
Finally, high humidity/dew points means the air is hazy and much of the sunlight’s energy isn’t able to heat the ground to generate thermals/lift. This is a direct impact where the stuff mentioned above is a little bit less so.
Following up on @Machine_Elf’s and @MikeF’s great info.
From the perspective of a powered plane (large or small) pilot the effect of practical levels of humidity on lift is a rounding error.
Switching gears from airplanes to baseball. …
I’d wonder how much baseballs in flight are actually affected by humidity versus how much commentators and baseball superstition says they are. I could easily believe more difference comes from humidity (and temperature and air pressure) affecting the elastic characteristics of the ball and hence the rebound off the bat than from affecting the subsequent flight.
Altitude (i.e. Coors Field in Denver) is an obvious and real factor. The normal range of change in barometric pressure at any given ground elevation is much much less which I would WAG as practically negligible.
But I freely admit that is a WAG; I’ve not looked for serious research on baseball flight versus weather. I’m hoping someone can pitch us a good cite.
Clearly there are boundary effects. A baseball which just barely clears the fielder’s glove at the fence and goes out is at least a 1 run score while a ball that flies a couple inches less that ends in the glove results in at least one out and may end the inning. Small differences in flight produce large differences in game outcome.
So once in awhile more favorable meteorology would make a difference in game play. But in any given instance we can never know whether weather made the difference or whether fielder or batter effort made the difference.
The way for a regular person to think of dew point is “an index number that is a direct measure of uncomfortableness.” Practically speaking, the units aren’t degrees Fahrenheit; they’re subjective discomfort units where more = worse.
Just like you can say ordinary temperatures (in F) are, practically speaking, just arbitrary comfort units meaning this (as adjusted for personal preferences):
Numbers in the 30s = too damn cold
Numbers in the 40s = rather cold
Numbers in the 50s = quite cool
Numbers in the 60s = pleasantly cool
Numbers in the 70s = ideal
Numbers in the 80s = too warm
Numbers in the 90s = hot
Numbers in the 100s = way too hot
You can say (Ref @kenobi_65 above) and again as adjusted for your personal preference:
Numbers in the low 50s (or lower) = comfy
Numbers in the high 50s = a bit uncomfortable
Numbers in the low 60s = muggy
Numbers in the high 60s = definitely unpleasant
Numbers in the low 70s = friggin’ miserable
…
Numbers in the high 80s = fatal in a couple hours. Not that you’ll experience that in the USA until a few centuries from now.
Like the song says “You must try to ignore that it means more that that.” 
:
You may recall in another thread where I mentioned developing a ballistics flight calculator in Excel. I can run some numbers to get a (ahem) ballpark analysis done. Aero drag scales with density though, so a 1% change in density probably won’t make a huge difference, e.g. a good hit might go 355 feet instead of 350 feet. Stay tuned…
Done.
Start with a sea-level game in dry air. Velocity of the ball when leaving the bat is 55 m/s, at an upward angle of 37 degrees. Ball travels 119.99 meters before coming back to earth.
Now crank the humidity up to 100%. Air density drops by 1%. Ball leaves bat at same speed and departure angle, and this time it travels 119.36 meters before coming back to earth.
Difference? 0.63 meters. Not much to write home about.
The ball goes a shorter distance in the less-dense air?
Said another way, probably not enough difference to trot home with. Except very occasionally. 
It’s also not 100% clear to me whether forespin or backspin produces net lift. But I believe backspin does produce net lift, mostly (entirely?) via Coandă effect.
If it does, then increasing density both increases drag which reduces range, and increases lift which will increase range for at least some launch angles.
Nice fielding. I got the numbers switched; ball does indeed go farther in less-dense air.
It’s the Magnus Effect.
Alas, my crappy little spreadsheet is ill-equipped to simulate the Magnus effect. As you’ve noted, decreasing air density would reduce drag, but would also reduce lift, so a 0.63-meter improvement on a muggy day may be an overestimate.
D’oh! Thank you. Coandă was setting off “not quite right” alarms in my head but I didn’t follow up. Dumbguy!
That’s true when the temperature is about 80F, it drops only by 0.3% when the temperature is 40F.
Air at 80F, and 14.7psia :
Density of Air with 0% relative humidity = 1.174 kg/m3 (0.07327 lb/ft3)
Density of Air with 100% relative humidity = 1.159 kg/m3 (0.07235 lb/ft3)
Air at 40F, and 14.7psia :
Density of Air with 0% relative humidity = 1.268 kg/m3 (0.07327 lb/ft3)
Density of Air with 100% relative humidity = 1.264 kg/m3 (0.07235 lb/ft3)
Doesn’t pressure also change ? And air at low pressure can carry more water vapor at the same temperature. How do glider pilots account for that ?
Sorry, I should have specified that my scenario was for a 20C ambient temp.
@Machine_Elf That’s a lot of good work you have done there and I’d love to see your spreadsheet.
This is not my expertise, but I would have guessed that viscosity will play a bigger part than density.
Air at 68F (20C) , and 14.7psia (1 atm) :
Viscosity of Air with 0% relative humidity = 0.01859 cP
Viscosity of Air with 100% relative humidity = 0.01823 cP
The change in viscosity is about 2% (double than that of density). I think the viscosity is accounted in the Cd factor - but would Cd remain constant with changing viscosity ?
Mine eyes glaze over!
Not threadshitting, I promise. This is fascinating.
The interaction of velocity, object size, fluid density, and fluid viscosity can be encapsulated in a single parameter called the Reynolds number (Re):
Two flow scenarios may differ in size, velocity and fluid properties, but if they both have the same Re (e.g. by one of them having twice the velocity and half the object size of the other), then you can confidently analyze them with the same tools.
Viscosity matters more for flow situations with low Re, and fluid inertia (density) matters more for flow situations with high Re. For air, the ratio of density to viscosity is such that you need to have really small objects moving really slowly before viscosity effects start to dominate. A good example is fine dust settling through air under the influence of gravity:
A baseball on a home-run trajectory through air has a very high Re; density-related effects are far larger than viscosity effects, so you can safely disregard the effects of viscosity changes in the analysis of drag here.
The amount of water it carries is the same. But that amount of water would then be a larger proportion of the air, because there’s less of the other components.