Cycling and Wind

Mr. Wind

Nobody knows,
Where the wind comes from,
Where the wind goes.
~ A.A. Milne

A friend and I were riding the other day and joking around about how wind is “wind” in winter, but in summer, it’s just a “breeze.” Is that because we’re in better shape in summer, so wind resistance is easier to deal with? Undoubtedly, that’s some of it, but there’s more to it than that.

Enter the Ideal Gas Law, which calculates the density (how much a volume of air weighs) of dry air as a function of its pressure and temperature. Dry air is air with no moisture content — zero humidity.

Plugging some numbers into the Ideal Gas Law reveals that a cubic meter of 77º dry air has a density of 1.18 kg/m3 and a cubic meter of 32º dry air has a density of 1.29 kg/m3. Warm air is lighter than cool air.

Air exerts a force against the bike rider which we refer to as wind resistance. How much force depends on air’s density (which we calculated above), the square of the velocity of the rider, the drag coefficient (based on the shape of the rider and bike) and the cross sectional area of the rider and bike.

Everything else being the same, the wind resistance of 32º air is about 10% higher than the resistance of 77º air. So the perception that “wind” morphs into a “breeze” as the weather warms up is real.

Here’s something that’s completely counter-intuitive. Humid air is lighter than dry air. Read the informative blog referenced below for the reason why.

Hmmm. This means that a hot, humid day is better for cycling than a cold, dry day…at least in terms of wind resistance. How many of us would opt for the hot, humid day? Not many! Humid conditions introduce a lot of physical challenges for the cyclist. They usually override the benefits of that lighter, faster air. But to the extent that we can acclimate to that kind of weather, the easier the cycling will be.

Cycling and wind — fun things to think about the next time the wind or breeze blows!

_________________________________

References

The Ideal Gas Law

Calculating drag

Why Dry Air is Heavier than Humid Air

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Posted in Technical
3 comments on “Cycling and Wind
  1. Carol Linden says:

    Or, as the sage Bob Dylan put it, “The answer, my friend,is blowing in the wind; the answer is blowing in the wind.” Thanks for the explanation and references! I still prefer NO wind!

  2. Bill Reuther says:

    Thanks Georgena!!!! Great stuff!

  3. Mark Holm says:

    Ms Terry’s analysis is incorrect. Viscosity of gasses is quite counterintuitive. Viscosity increases as temperature rises and decreases as temperature decreases. Surprisingly, viscosity is independent of pressure for a gas of constant composition. Also, viscosity increases as molecular weight decreases. Water vapor has a molecular weight of 18 compared to 29 for nitrogen and 32 for oxygen. Because water molecules are lighter than dry air molecules, water vapor is more, not less viscous.

    I have just stated three counterintuitive things: Warmer gasses are more viscous. Pressure does not change gas viscosity. Gases made of lighter molecules are more viscous. All three happen to be true.

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