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is there less wind resistance when its humid(19 posts)

is there less wind resistance when its humidishmael
Jun 7, 2002 8:56 AM
I've read here that there is less resistance when its humid, this makes no sense to me, can anyone explain it?
good questionDougSloan
Jun 7, 2002 9:36 AM
"Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air. How can the air become lighter if we add water vapor to it?"

http://www.usatoday.com/weather/wdensity.htm
Great explainationPack Meat
Jun 7, 2002 9:43 AM
Water vapor rises from the boiling pot of water so it must be lighter than ambient air. Baseball announcers are always talking about the ball traveling better in the summer when the humidity is higher.
temperature, tooDougSloan
Jun 7, 2002 9:51 AM
Hotter air or vapor is less dense, too.

In fact, in some desert areas, planes have a concern taking off when it gets really hot, the effect on density is so great. Palm Springs is a good example, but sometimes it happens here in Fresno, too. We're talking up around 115 F, though.

There is a big difference on density with altitude, too. My rough calculation shows a 14% decrease in density going from sea level to 5,000 feet. Wow! You can descend almost 14% faster at a mile high, and even better at 10,000 feet. That's why records are set in Mexico City, and how riders can hit over 60 mph on some roads.
new product ideamr_spin
Jun 7, 2002 10:12 AM
Device that generates hot and extremely humid air. Mount on post 1 meter in length, attached to bars, directly in front of rider. Best used only for straight line time trials.

It's all natural, so it's better than EPO!
Devise that generates hot air??? Didn't that use to be calledelviento
Jun 7, 2002 10:19 AM
a jet engine??? I bet it's better than EPO.
how about a vacuum cleaner? nmDougSloan
Jun 7, 2002 10:26 AM
Great explaination .... notnn23
Jun 7, 2002 10:45 AM
Actually water vapor rises from a boiling pot hotter (100 degree celcius) and thus lighter than the cooler heavier air.

Yeah, but even it were the same temp, it would still rise. Evaporation would be a closer example.

The original explanation of H2O gas (water vapor) being lighter than O2 or N2 is true though.
Oh yeah... hot air rises, right , I'm so surePack Meat
Jun 7, 2002 2:10 PM
If that were true than if you could just capture the hot air in, I don't know, a big nylon bag for example then you could, like tie a weight to it and just float away, whatever man, get a clue.

I actually was stating that the explaination above mine was great.
re: is there less wind resistance when its humidnetso
Jun 7, 2002 9:42 AM
It seems to me that if a wind is blowing at 10mph, then the effect would be of a 10mph wind. If the resistance would be less the wind gauge would read less. At least it makes sense to me.
So is it eaiser to travel through a humid 10mph headwindPack Meat
Jun 7, 2002 9:46 AM
versus a dry 10 mph headwind. I'd have to say the humid for the same reason it's easier to go faster at higher altitude (not taking into consideration the effects on the body) than at sea level.
fewer O2 molecules, thoughDougSloan
Jun 7, 2002 9:54 AM
There is some offset from fewer oxygen molecules per volume, though, in humid air or at altitude. On steep climbs, higher humidity or altitude hurts more than helps, as air resistance becomes neglible at slow speeds anyway.
Who are you calling a slow climber?(nm)bnlkid
Jun 7, 2002 10:02 AM
OT but similar question...biknben
Jun 7, 2002 10:13 AM
Why does air temp. decrease as elevation increases.

If temp increases as I move closer to the equator, why not when I increase elevation.

Answer may be obvious to some but I've never bothered to ask.
The thinner air holds less heat. Fewer O2 molecules, more spacebill
Jun 7, 2002 10:22 AM
between them, so that, even if the molecules are each holding the same amount of radiant heat energy, there is less energy held by the molecules within a given volume of space, because there are fewer molecules in the same volume of space. Also, fewer molecules means less friction between molecules. Don't remember enough physics to say which is more significant.
this is the way I remember itDougSloan
Jun 7, 2002 10:32 AM
Temperature is the average kinetic energy of molecules in a given volume. The fewer molecules due to less density, the lower the average energy.

Conversely, you cram more molecules into a given volume, each having x energy, the total energy in the volume increases, and thus the average, too.

That sort of make sense?
equator explanationDuane Gran
Jun 7, 2002 1:15 PM
Others have explained the relatinship between temp and elevation. The reason it is warmer closer to the equator is because the shines more directly near the center of the earth (longitude-wise) than toward the poles. In the northern hemisphere the Earth is actually closer to the Sun in the winter, but the angle provides less direct sunlight.
you sure?DougSloan
Jun 7, 2002 1:19 PM
I thought it was because people eat more beans in tropical areas?
There is less wind resistance when hot, or low air density.Humma Hah
Jun 8, 2002 5:26 PM
"Dynamic pressure", the pressure of the wind on your body, increases directly with air density (ro) and with the square of wind velocity (v)

When the air is hot, density is lower. Low atmospheric pressure also reduces density, and this tends to come with bad weather. High pressure usually comes with clear, dry air.

Water vapor has a molecular weight of 18 g/mole. Oxygen (32 g/mole) and nitrogen (28 g/mole) have heavier molecules. Water vapor thus reduces the average molecular weight, and so the density, of a given volume of air. But not by much ... there's never really very much water vapor. Temperature and air pressure are bigger factors. Any reduction in density is more than offset by the discomfort of humidity.