Navigation of aircraft and sail-powered vessels is profoundly affected by the
wind, (and motor-powered vessels are significantly affected). Therefore wind
speeds have traditionally been expressed in the same speed units used in
navigation (knots, km/h, mph) to simply "wind triangle" calculations.
Most weather stations are now automated. What comes out of the box is an
averaged, rounded number processed in accordance with the Federal Meteorlogical
Handbook. Regardless of how it is measured internally, the "best" number
available is what comes out of the box, and any other units are obtained by
further conversion and rounding. For the US, the boxes are primarily set up to
produce METAR reports for aircraft and the units are knots. Given the
variability of the wind, rounding errors to other units are pretty minor, but
they are there.
You should be able to obtain a device that reads in meters per second, however.
Wind turbines are commonly rated in those units (perhaps to obscure the fact
that typically a 14 m/s second wind is the minimum to reach rated power, and
that corresponds to a gale in archaic terms.)
Keep in mind that if it is mounted on your bike, a wind indicator will read
apparent wind resulting from the "wind triangle" of real wind and your motion.
You will need apparent wind speed and direction, and bike speed and direction
to resolve. Instrumentation to do so is common on sailboats.
--- On Sun, 3/15/09, James R. Frysinger <[email protected]> wrote:
> From: James R. Frysinger <[email protected]>
> Subject: [USMA:43872] Re: wind velocity
> To: "U.S. Metric Association" <[email protected]>
> Cc: "U.S. Metric Association" <[email protected]>
> Date: Sunday, March 15, 2009, 8:34 AM
> I will try to answer one of your questions...
>
> Mark Simon wrote:
> > The weather report predicts there will be a west wind
> with velocity of
> > 28 km/h. I am going to ride my bicycle about 18 km
> towards the west
> > today. I cannot visualize 28 km/h, but I can visualize
> 7.8 m/s. This
> > means to me that I will get a better workout, going to
> my destination,
> > and should have an easier ride home. Would somebody
> please explain the
> > logic of measuring velocity in km/h?
>
> There likely is no logic in reporting wind speeds in
> kilometers per hour, except that people are used to
> hour-based rates in these reports. Meteorologists actually
> measure wind speeds in meters per second.
>
> Historically, sailors were the ones concerned with wind
> speeds because they accounted for windage in their dead
> reckoning. Those wind speeds were estimated in knots
> (nautical miles per hour), the units used for ship's
> speed. That practice of using hour-based wind speeds carried
> over into practice on shore in coast-predominant nations, I
> imagine, such as the UK, coastal Europe, and the fledgling
> U.S.
>
> Jim
>
> I do not understand why this is
> > done. I live in Southern California, and traffic is
> unpredictable
> > here. When I drive, I am only concerned with safety,
> and leave
> > additional time to get to my destination. If were are
> going to convert
> > to the metric system in the United States, why not
> consider using m/s
> > as the basic unit? I am usually only concerned with my
> velocity when
> > the traffic light turns yellow in front of me, and I
> have to make a
> > decision whether to decelerate or continue driving. If
> my velocity is
> > measured in m/s, that would make calculations simpler.
> In Europe, with
> > its denser populations, does measuring velocity in
> km/h work well in
> > calculations? I wager that highways are engineered
> using m/s
> > calculations.
> > Is it possible to get a velocimeter for a bicycle that
> measures in m/s?
> >
> >
> >
> >
>
> -- James R. Frysinger
> 632 Stony Point Mountain Road
> Doyle, TN 38559-3030
>
> (C) 931.212.0267
> (H) 931.657.3107
> (F) 931.657.3108
