Hey all! My first year training/racing with power (powertap). I’ve been told that humidity affects power readings, but am still unclear about the how and why of this phenomenon.
I would guess that it would take more watts to achieve the same speed when it is humid out because the air is more dense. This is the reason that the hour record is always attempted at altitude, right?
I’ve also heard the inverse- that it takes less wattage to go the same speed when it is humid. This seems counterintuitive to me.
Hey all! My first year training/racing with power (powertap). I’ve been told that humidity affects power readings, but am still unclear about the how and why of this phenomenon.
I would guess that it would take more watts to achieve the same speed when it is humid out because the air is more dense. This is the reason that the hour record is always attempted at altitude, right?
I’ve also heard the inverse- that it takes less wattage to go the same speed when it is humid. This seems counterintuitive to me.
Can anybody clear this one up?
Thanks!
Chris
Humid air is less dense than dry air at a given temperature. Even so, the density of the air isn’t going to affect your power output, just how fast you go for a given power.
The only reason humidity would affect your power is if you have “cooling issues”.
Hey all! My first year training/racing with power (powertap). I’ve been told that humidity affects power readings, but am still unclear about the how and why of this phenomenon.
I would guess that it would take more watts to achieve the same speed when it is humid out because the air is more dense. This is the reason that the hour record is always attempted at altitude, right?
I’ve also heard the inverse- that it takes less wattage to go the same speed when it is humid. This seems counterintuitive to me.
Can anybody clear this one up?
Thanks!
Chris
The molecular weight of water (H2O) is 18 g/mol. OTOH, the molecular weight of nitrogen (N2) (which makes up 79% of the atmosphere) is 28 g/mol. Thus, as humidity goes up, air density goes down, since progressively more nitrogen molecules are displaced by water molecules. This effect, however, is much smaller than that of atmospheric pressure (altitude) or temperature.
So with all other things being constant (which I realize never happens) you should be faster at a given power output on a humid day, right?
So more than likely any decrease in wattage will be from purely physiological reasons- i.e. it is harder to ride/run in humidity?
Right on both counts (although I suppose that some people might have psychological problems with high humidity, i.e., they could push themselves just as hard, but do not do so.)
So with all other things being constant (which I realize never happens) you should be faster at a given power output on a humid day, right?
So more than likely any decrease in wattage will be from purely physiological reasons- i.e. it is harder to ride/run in humidity?
Right on both counts (although I suppose that some people might have psychological problems with high humidity, i.e., they could push themselves just as hard, but do not do so.)
Personally, if the temps are going to be hot (i.e. 80F to 95F) I, psychologically at least, prefer high humidity over extreme dryness. It seems to me that as long as you’re moving, it feels good to feel the cooling effect from the air passing over the unevaporated sweat. In contrast, in desert conditions at the same temps, I feel like every molecule of moisture is being drawn out of my body while a hair dryer is being blown in my face…
But, like I said, that’s as long as I’m MOVING. When you stop in humid conditions, that’s when it becomes “oppressive”.
Then again, I grew up in Wisconsin, so maybe that explains that preference…along with a bunch of other stuff
Humid air is less dense, so from an aero standpoint you go faster in humid weather. It seems counterintuitive, but heres my understanding of it (keep in mind that its been a while since I took chemistry).
For any molecule, a specific number if them will fit into a given volume at a specified temp. A water molecule (h2o) is bigger then a nitrogen or an oxygen, so less of them will fit into your given air volume. A single water molecule displaces more than one oxygen or nitrogen, leaving you with less dense air. When it is humid, the water molecules displace a sufficient amount of oxygen and nitrogen to make the air less dense.
It is not a question water being heavier, it is how many molecules of gas can fit in your specific air volume.
Especially when comparing to N2 or O2, as water is lighter that either of them …
it is how many molecules of gas can fit in your specific air volume.
Which is actually a constant, defined by temperature and pressure, within various nonidealities (pair-potentials and such nonsense). The molar volume of a gas is 24.5 l at room temp and atm. pressure. Any gas.
AC’s post (the other AC) scores the highest for “most correct answer per line of text” ratio.
it is how many molecules of gas can fit in your specific air volume.
Which is actually a constant, defined by temperature and pressure, within various nonidealities (pair-potentials and such nonsense). The molar volume of a gas is 24.5 l at room temp and atm. pressure. Any gas.
Actually, it’s 22.4 L/mol for ideal gasses, and many of them (e.g., CO2, which occupies 22.26 L/mol) don’t exhibit such ideal behavior.
it is how many molecules of gas can fit in your specific air volume.
…which is 6.02 x 10^23 per liter, if you’re talking about an ideal gas and if I remember Avogadro’s number correctly.
I noticed you said psychologically…but shouldn’t you prefer dry heat vs. humid heat at a given temp as with dry heat your sweat will evaporate faster and you’ll be cooler?
Another factor is that water molecules displace oxygen molecules, so that while you will be “pushing less air”, you will also be aspirating less oxygen in those conditions. Evidently though, not enough to counter the effects of thinner atmosphere.
FWIW, at our Wednesday night time trials, we very often see PR’s set after a storm front rolls through. Lower barometric pressure, cool temps and high humidity I suppose.
Actually, it’s 22.4 L/mol
I said at room temp. Maybe you work in a fridge, and that’s fine, but this 22.4 l you’re quoting is for 0 Celcius (273 K).
for ideal gasses
Like I said … “within various nonidealities” …
Dre’
Point(s) taken - obviously I read your post too rapidly, and got stuck on the unfamiliar (to me, anyway…I’m used to working with volumes at STPD) value of 24.5.
I noticed you said psychologically…but shouldn’t you prefer dry heat vs. humid heat at a given temp as with dry heat your sweat will evaporate faster and you’ll be cooler?
Yep…and maybe it’s just me, but I prefer the feeling of sweat on my skin being cooled by any little breeze to having the sweat instantly evaporate and the wind feel like I’m in a convection oven. I may not actually be cooler, but I’m happier
Dry 90 feels much cooler than humid 90 - to me.
Ahh…the old “but it’s a DRY heat” saying. That may be true if you’re sitting or standing still. But, when you’re moving, I’ll go for the humidity any day. But, as I said, perhaps it’s just me…
Another factor is that water molecules displace oxygen molecules, so that while you will be “pushing less air”, you will also be aspirating less oxygen in those conditions.
But the air in our alveoli is always 100% saturated, regardless of the relative humidity of the air we breathe in. IOW, we’re faced with this (rather small) dilutional effect at all times.
Doesn’t the thinner air at altitude only become a net improvement if you are able to train at altitude (or enrich your blood to an equivelant level through “some other method”).
I ask because last weeked I did a HIM at ~6500 feet, while I live and train at about 1500 feet. I actually did not notice too much difference on the bike in terms of HR @ wattage I could maintain. Due to the hilly nature of the course there is no way to tell if I was getting an increased velocity for the wattage I was producing.
The run was another matter…very painful although for more reasons than just altitude.
