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How much faster can warm humid air be than cool dry air
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Hadn't been on my TT bike since early spring. I decided to ride today on a gently rolling 22mile out and back today that I ride often. Today had a gentle breeze, 93+ degrees and very humid. 24.5 mph on 202 watts. No aero helmet and normal kit. Flip back the calendar to April and the same ride with aero helmet and speed suit takes 238 watts. There was a touch more wind that day though.

I was just curious.

Thanks
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Re: How much faster can warm humid air be than cool dry air [Piche] [ In reply to ]
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Most of it is covered by "a touch more wind" as your drag is proportional to the cube of relative wind speed (speed between body and air, not air and ground). The atmospheric pressure is also your big drag countributor. Humidity is the last thing on this list.
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Re: How much faster can warm humid air be than cool dry air [devashish_paul] [ In reply to ]
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Gotcha. Thanks
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Re: How much faster can warm humid air be than cool dry air [Piche] [ In reply to ]
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1 sec per km per 6°C (10F) temp change is what I use. Density altitude is what matters here. It’s an aviation term used to correct for non standard temp and pressure. Humidity does have an effect but it’s regard to be fairly minimal. Temperature matters more than changes in atmospheric pressure. Assuming a constant altitude that is.
Last edited by: J7: Jul 18, 20 12:05
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Re: How much faster can warm humid air be than cool dry air [J7] [ In reply to ]
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thanks as well
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Re: How much faster can warm humid air be than cool dry air [Piche] [ In reply to ]
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https://www.engineeringtoolbox.com/...ity-air-d_680.html#:~:text=When%20vapor%20content%20increases%20in,more%20dense%20that%20humid%20air!

Air density was probably about 5% lower, and since that is in the aero drag part, the humid days can get you 1% to 2% faster.
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Re: How much faster can warm humid air be than cool dry air [Piche] [ In reply to ]
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Air density \rho (kg/m^3) is given to a *very* good approximation by the ideal gas law,

\rho = p/(R T),

where p is the pressure in pascals, R the specific gas constant (about 287 J/kg/K for dry air and slightly larger by maybe up to 2% for very humid air), and T the temperature in Kelvins.

Because the power lost to drag scales linearly with air density but as the cube of the ground speed (in still air), it will take about a 3% decrease in air density to give a 1% increase in speed, if power is held fixed.

Typically, as noted by another J7, (in mid-latitudes) temperature affects air density more than pressure at a given altitude, with humidity in third place. Temperatures vary by ~20 C seasonally, which is a ~7% impact on air density. Pressures might vary by 20-30 hPa between strong high and low pressure systems, which is a 2-3% impact on density since reference sea-level pressure is about 1000 hPa (Higher wind speeds encountered in strong low-pressure systems tend to make this less effective at increasing speed though!).

Also J7’s 1”/km/6C rule of thumb can be tested by this logic but probably is a bit high. 6C warming would be about a 2% decrease in density, which would be give a 2/3% increase in speed at constant power. At 40kph or 90”/km, 2/3% is 0.6"/km -- but 1"/mile faster for each 10F warming at 24 mph would be about right.

(edited to correct a dumb math error and revise the rule of thumb down a bit!)
Last edited by: twcronin: Jul 19, 20 14:26
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Re: How much faster can warm humid air be than cool dry air [twcronin] [ In reply to ]
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Thanks for doing the math on this. I did the math on it about 13 years ago after a sizzling hot day produced some stellar times on the Charlotte Motor Speedway 10 mile time trial. After posting the 1s/km/6°C rule of thumb I was convinced for sure someone was gonna call on me to produce the actual calculations. You saved me some work, thanks!
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Re: How much faster can warm humid air be than cool dry air [twcronin] [ In reply to ]
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I always love it when peeps are so smart they seem like they are speaking another language. It gives me faith in the human race.

The more people I encounter the more I love my cats.
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Re: How much faster can warm humid air be than cool dry air [devashish_paul] [ In reply to ]
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devashish_paul wrote:
The atmospheric pressure is also your big drag countributor. Humidity is the last thing on this list.


I think it's more accurate so say that air density can affect aerodynamic drag significantly. Air density is a function of atmospheric pressure, temperature, and humidity. All three can have significant effects on air density.

For any volume of gas, there are an equal number of molecules (Avogadro's). When air is humid, water vapor (a gas) displaces dry air molecules. Because (counter-intuitively) water vapor has less molar mass than dry air, the density of that volume of gas decreases the more water vapor there is in it.
Last edited by: trail: Jul 18, 20 20:08
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Re: How much faster can warm humid air be than cool dry air [Slug] [ In reply to ]
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Agreed
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Re: How much faster can warm humid air be than cool dry air [trail] [ In reply to ]
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trail wrote:
devashish_paul wrote:
The atmospheric pressure is also your big drag countributor. Humidity is the last thing on this list.


I think it's more accurate so say that air density can affect aerodynamic drag significantly. Air density is a function of atmospheric pressure, temperature, and humidity. All three can have significant effects on air density.

For any volume of gas, there are an equal number of molecules (Avogadro's). When air is humid, water vapor (a gas) displaces dry air molecules. Because (counter-intuitively) water vapor has less molar mass than dry air, the density of that volume of gas decreases the more water vapor there is in it.
All true. But, most of us run/bike more slowly on hot, humid days when we're going any sort of distance. It must help if you're running a 100m sprint to have hot, humid days, but it doesn't help my 10k run or 40k bike times.

(Somewhat) interesting connection. Atlanta-Fulton County Stadium was known as "The Launching Pad" when the Atlanta Braves played there. The high temperatures, high humidity, and (relatively) high field elevation (1050 feet above sea level) generally made the air less dense there than at other MLB fields. Once you hit a ball, having less dense air will allow it to travel further for a given initial velocity. Hank Aaron would have hit a bunch of home runs anywhere, but the fact that he played there (from 1966) probably helped him.

"Human existence is based upon two pillars: Compassion and knowledge. Compassion without knowledge is ineffective; Knowledge without compassion is inhuman." Victor Weisskopf.
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Re: How much faster can warm humid air be than cool dry air [J7] [ In reply to ]
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Realized I made a dumb mistake on the math and edited the post -- probably should be more like 1"/mile/10 F temperature increase!

I think a substantial covariate in parts of the world that might lead to a bigger signal is that the hottest summer conditions come along with relatively weak large-scale winds, so temperature might appear to have a slightly larger role.

I also don't know the answer to this, but does temperature have a substantial effect on rolling resistance?
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Re: How much faster can warm humid air be than cool dry air [Slug] [ In reply to ]
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Thanks, but LOL I made a boneheaded math mistake so the number I came up with at the end should have been 0.6 seconds per km per 6 C warming rather than a full second! So maybe don't place too much faith in humanity to get it right the first time ;)
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Re: How much faster can warm humid air be than cool dry air [Piche] [ In reply to ]
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Straight up math using me as an example

93F (33.9C), 80%RH 1 atmosphere, Bike and rider at 74.84KG, CDA .22, crr .005 (Air density 1.1313)

It should take me 204 watts to go 24.5 mph

50F (10C), 10%RH 1 atmosphere, Bike and rider at 74.84KG, CDA .22, crr .005 (Air density 1.246)

It should take me 220 watts to go 24.5 mph
Last edited by: jaretj: Jul 19, 20 15:39
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Re: How much faster can warm humid air be than cool dry air [Slug] [ In reply to ]
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Slug wrote:
I always love it when peeps are so smart they seem like they are speaking another language. It gives me faith in the human race.

HG Wells put it somewhat differently



"What's your claim?" - Ben Gravy
"Your best work is the work you're excited about" - Rick Rubin
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Re: How much faster can warm humid air be than cool dry air [jaretj] [ In reply to ]
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jaretj wrote:
Straight up math using me as an example

93F (33.9C), 80%RH 1 atmosphere, Bike and rider at 74.84KG, CDA .22, crr .005 (Air density 1.1313)

It should take me 204 watts to go 24.5 mph

50F (10C), 10%RH 1 atmosphere, Bike and rider at 74.84KG, CDA .22, crr .005 (Air density 1.246)

It should take me 220 watts to go 24.5 mph

How many more watts you think you'd make for AP over a 40k between those two days?

The math is real. For me the physiology sucks.

My body heats up and I can't produce. I'm not a pro. I'm just a hobbyist. Hot weather on longer or repeated efforts affects my power big time.

I could probably make the 16w up given a nearly 45 degree temp difference.
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Re: How much faster can warm humid air be than cool dry air [Alvin Tostig] [ In reply to ]
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Alvin Tostig wrote:
But, most of us run/bike more slowly on hot, humid days when we're going any sort of distance. It must help if you're running a 100m sprint to have hot, humid days, but it doesn't help my 10k run or 40k bike times.


True. But on the other hand, I often race track (cycling) at the LA velodrome. And despite having pristine pine boards, is a slow-ass track. I chalk it up to its overly refrigerated and dry air (I get a cough every time I spend a day there). And of course being at sea level. It's 3/3 for the things that increase air density.
Last edited by: trail: Jul 20, 20 8:42
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Re: How much faster can warm humid air be than cool dry air [burnthesheep] [ In reply to ]
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burnthesheep wrote:
jaretj wrote:
Straight up math using me as an example

93F (33.9C), 80%RH 1 atmosphere, Bike and rider at 74.84KG, CDA .22, crr .005 (Air density 1.1313)

It should take me 204 watts to go 24.5 mph

50F (10C), 10%RH 1 atmosphere, Bike and rider at 74.84KG, CDA .22, crr .005 (Air density 1.246)

It should take me 220 watts to go 24.5 mph

How many more watts you think you'd make for AP over a 40k between those two days?

The math is real. For me the physiology sucks.

My body heats up and I can't produce. I'm not a pro. I'm just a hobbyist. Hot weather on longer or repeated efforts affects my power big time.

I could probably make the 16w up given a nearly 45 degree temp difference.

Perhaps I don't understand what you are saying but the power required to go a specific speed is independent of the distance.

If you give me a certain speed, temp, humidity, air pressure, CDA, crr, mass of rider+bike I can figure out any ideal power/speed you'd like. You could even include slope and powertrain losses if you'd like.

I'm not saying I can/could do 220 watts for a 40K right now but that's what's required for me to go 24.5mph at the conditions specified above.

Edit: You could also figure it out here https://analyticcycling.com/ForcesPower_Page.html
Last edited by: jaretj: Jul 20, 20 9:16
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Re: How much faster can warm humid air be than cool dry air [jaretj] [ In reply to ]
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according to this calculator https://www.omnicalculator.com/physics/air-density
the density (holding temp, and baro press constant at 25 and 100.5 kP is
25% RH density= 1.17078
50% RH 1.16852
100% RH 1.16028
so between 25% and 100% is about 1% difference so it is probably a slight influence but dry air cold air is slightly slower than warm humid air. You can play with the density calc on the link to get an idea of the range.
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Re: How much faster can warm humid air be than cool dry air [s5100e] [ In reply to ]
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Thanks for the calculator, I'm sure it will help a lot of people here but calculating air density is part of my job.
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