# What would cause variable air resistance on a day with no wind?

I commute to work regularly. I find that on some days when there's no perceivable wind, I feel like there's almost no air resistance at all and it's so easy to ride at speed. It's also relatively quiet without the wind roaring in my ears.

On other days when there's no perceivable wind, it feels like I'm riding in soup. It's really hard to get any speed up and the air is quite noisy in my ears.

I've discussed with my co workers and they often say they have the same effect on the same days as I've observed. Although they're also travelling in the same direction as me, so I can't rule out a very slight headwind causing the perceived difference.

Possible explanations:

• There's a slight tail or head wind on these days.
• On days that I'm more rested or energetic I could feel like the ride is relatively effortless.

Are there any other explanations that would contribute? High humidity or a high pressure zones?

• I sometimes get this "effortless" effect but have concluded that its down to a tail wind. I don't even perceive it as a breeze... but when I ride for pleasure my rides are generally loops, so as soon as I change direction I can feel it. – PeteH Dec 17 '12 at 12:39
• Yep, that's a tail wind. It is virtually silent because you're keeping up with the wind and the air is not moving much relative to you. – Angelo Dec 17 '12 at 21:38

TLDR; assuming my calculations below are correct, there's roughly a 10% increase in air resistance between hot, humid days, and cool, dry days. Add in a slight but imperceptible tailwind or headwind, and it's conceivable that you could experience a 4–5mph difference in cruising speed between two days.

Air resistance is the primary force a cyclist must overcome at typical cruising speeds. According to one online calculator, assuming a typical road bike, a relaxed riding position, and a cruising speed of 18mph, 75% of a rider's power is used to overcome drag.

According to your profile, you live in Melbourne, AU, which is essentially at sea level. Using another online calculator, the air density when it's 50˚ and 0% humidity is 1.24kg/m³. When it's 90˚ and 100% humidity, the air density is 1.13kg/m³. So, on a cold and dry day, there's roughly a 10% increase in air resistance when compared to a hot and humid day.

According to the drag equation,

the drag force scales linearly with air pressure. 10% higher density equals 10% higher force necessary to overcome drag. Instantaneous power is determined by the equation

Assuming constant power output, and 75% of that power is dedicated to overcoming air resistance, you end up with roughly a 7% reduction (1 / 1.075) in overall speed. We started with a cruising speed of 18mph, so simplistically your speed on a cold and dry day would wind up being 93% of 18mph, or 16.75mph. I'd say that's enough to notice.

Of course, it seems unlikely that these two days would occur closely together. But if you're comparing a midday ride just before or after a storm to a late evening ride on a dry day a few days before and/or after, it's conceivable that you could wind up somewhere in the ballpark of a 1mph difference.

That said, even a small headwind and tailwind can make a significant difference in your speed. Sheldon's site has graphs showing wind tunnel tests. In particular,

shows that as the wind angle changes from a headwind to a tailwind for a 5mph wind, a rider traveling at a "normalized" speed of 25mph (assuming no wind) would go from roughly 22mph to 28mph. The difference in rider velocity as wind velocity changes seems to be linear, so extrapolating backward, even something like a 2mph headwind vs. tailwind would cause something like a 3mph difference. That's definitely noticeable. Being in a wind tunnel, these tests weren't done with rolling resistance, so 25mph in the wind tunnel is probably equivalent to our earlier assumption of 18mph on outdoor roads.

If you combine the effects, and compare a late evening ride on a dry day with a slight headwind, to a early afternoon ride on a humid day with a slight tailwind, you could conceivably have a difference of 4-5mph between the two days. That's huge.

• Awesome answer, thanks. I think a 10% difference to the amount of power I need to exert to maintain a constant speed would be enough to notice. Great data. – Mac Dec 17 '12 at 22:01
• It's a nitpick on your comment, but it's closer to 7.5% power difference at constant speed, 7% speed difference at constant power. Also don't forget to accept, if you think this is the best answer. :) – Stephen Touset Dec 17 '12 at 23:35
• Yeah I always accept... just giving it a few days to collect feedback :) – Mac Dec 18 '12 at 0:22

On high humidity days, the air has less mass due to the more H2O, which is lighter than typical O2, CO2 and N2 weights. On high pressure days there is more mass for you to push aside. Air temperature also plays a part - hot air is less dense than cold air. Therefore a hot, low pressure, high humidity day requires less mass to be pushed aside.

These make a measurable difference to aircraft, however I have no idea if a cyclist travels fast enough for it to be measurable. I believe its more likely imperceptible wind and your own well being (or lack there of)

• I would say that it makes a difference in cycling. In the olympic velodrome they heated it up so that the air would be more rarified so the cyclists would go faster. – robthewolf Dec 17 '12 at 8:54
• And the world hour record attempts are done at high altitude, and probably in warm air as well, to have as little air resistance as possible. It's possible that the differences are small, in a way that they are important for sport (for a world hour record attempt, a gain of mere meters over the course of an hour could make the difference between success or failure) while being negligible for commuting. – Roel Schroeven Aug 12 '19 at 14:07

Though the physicists tell us we're full of it, many cyclists at least "perceive" that wind resistance is greater on relatively humid but cool mornings.

(And, of course, if you and your co-workers were out drinking together the night before, that could have something to do with it.)

• I lol'ed at the second paragraph – Mac Dec 18 '12 at 22:22