Sunday, May 24, 2009

Early Birds South - water vapor in the air = faster ride?

Usual morning procedure... and leaving at 6:30am for Manorville on dry roads but quite some fog in this Island. Water condensing even on warmer or similar warm surfaces due to this mornings over saturation plus water droplets in the air... and thus his glasses were covered with droplets in very short time -- impairing vision a bit.

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Shortly after start a flat caused the nicely sized peleton to stop and wait. Good chance to dry up these glasses again -- but did not helped long, not even a minute...

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However, good enough and once in a while a wipe with the glove/finger did the trick. Flying on Dune Road, it was really water in the air -- condensing water dripping from the helmet, kind of strange... and over the bridge, this time really easy with more or less no wind.

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From here the moisture slowly got cleared up by the sun. 7-11 stop as usual....

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And flying back at good pace as well, finishing up with some pretty good fast group of 3, then four in a little tiny break away until the finish -- nice team work and fast.

May be a little advantage by the high humidity and high amount of water vapor -- did you know the density of very moist air is significent less than of dry air?

Follow the ride here.

South Stats of today (incl. Py's to/from):

Time and Distance cumulated at various speed zones

3 comments:

  1. doesn't lower density also mean you get less o2 when you're breathing? Maybe that makes up for the lack of air resistance.

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  2. Not in this case: O2 (20%) and total pressure stays more or less the same (not like high altitude), but water molecules (H2O: only 18g/mol) mixes in and mostly pushes N2 (28g/mol) away (also a little O2, but less in total as N2 is the major air component)!

    The air pressure stays (only number of molecules count, not size or mass!!) the same, but H2O is lighter than N2, thus simply the air density (m/V) goes down... :-)

    See also:

    http://en.wikipedia.org/wiki/Density_of_air

    http://en.wikipedia.org/wiki/Density_of_air


    This is also why fog/humid air or clouds move up (and down if cold) -- but this is getting more complex due to temperature gradients....!

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  3. Example:

    http://en.wikipedia.org/wiki/Humidity

    10g H2O / kg air at 50% RH, 25C -> 1%
    20g H2O / kg air at 100% RH, 25C -> 2%

    assuming this replaces N2 1% of all N2, this would lower the air density (the mass to push, at same pressure) by 1% x 18/28 = 0.64%

    This goes now linear into the Wind-Resistance:

    http://de.wikipedia.org/wiki/Winddruck

    F = A \cdot p_\mathrm{Wind} = A \cdot c_\mathrm{w} \cdot \frac {\rho}{2} \cdot v^2 (in \frac {\mathrm{kg\,m}}{\mathrm{s}^2} bzw. N)

    However, do not to need the cw number of a cyclist -- it simply will let you move 0.64% easier at any condition!

    To calculate this into speed gain at same power input I need a few more numbers....

    ReplyDelete