If you have such big balls, drag and inertia will be 'damping' the results to an extent – but maybe that'll be a good thing, if you want a coherent impression of how far the wind is pushing things along expressed as distance/time.

I think once the speed has stabilised, you will have a fairly true picture of linear velocity by 'circumference /sec'

If you need to calibrate it, maybe a piece of flat open country, a ?balloon, a stopwatch and two or three helpers 100m apart, to see if you can time a linear 'run' of unfettered wind, would give you a figure compare the results with?

It'd be a fun precursor to going to the pub in any case, if you don't get arrested as hare-coursers..

Noel asked how to calibrate it once made. Obvious choice is to use a wind gauge, but have a look on the net for a simple pendulum type wind gauge perhaps. Go to your local sailing club and ask a few questions see if you could check your anemometer with the club's one ?

Kids wind gauge

Also Martin's idea seems like an interesting one too.

Bob

As Martin suggested but use the GPS spèed reading on one of the mapping apps such as Google maps or waze available on most smartphone. You may need to find a long bit of road where you can stay at a constant speed

Sadly, won’t work – or at least very accurately – except at really low speed. Car will displace its frontal area and change the real speed. Speedo may provide even more error especially at low speed.

What you need is a pitot tube, to calibrate it properly.

I was looking in my copy of 'The Amateur Scientist', C L Stong, Heinneman Educational Books, 1962 when the gloom of yesterday's power cut stopped play! Thank you Storm Eunice, not.

As it's quite good on experimental physics I was hoping the articles on wind tunnels would cover calibration.

Not quite! Stong says:

Air speed through the tunnel was measured with two gauges: a calibrated pendulum vane and an anemometer of the Richards Type. Pressure in the tunnel during the calibration period was measured by a manometer arrangement, built by Phillips*, which utilised a pair of milk bottles. It was extremely accurate but abandoned after it was found too sensitive to temperature changes for prolonged use. (* Phillips was a member of the team, and a former MIT student.)

Several patents exist for pendulum vane gauges, like this example, but the two I looked at don't explain how they are calibrated in the first place.

In principle, air velocity can be calculated by measuring the pressure applied to a hinged vane. The vane has a known area, and the force on it can be measured by causing it to lift a weight, or stretch a spring of known strength. Or a manometer could be arranged to measure pressure difference between the front and back of the vane. A pitot tube connected to a manometer might be convenient way of capturing the pressure difference.

How accurate is another matter! I suspect it's necessary to compensate for air pressure, temperature, humidity, friction. It's relatively easy to measure to about a thou/0.02mm in an ordinary workshop with a micrometer. Much more care is needed to measure accurately and reliably down to a tenth/0.002mm because tiny factors such as worn instruments, temperature, dirt and operator technique add significant errors. I suspect if I were to make a steel block of peculiar width, such as 10.139mm, and ask 100 forum members to measure it, I would get many different answers. Or I might get a cluster of wrong answers claiming 0.14mm because the human brain is biased in favour of round numbers! Same with making an anemometer: I guess ±5% accuracy isn't difficult, but ±0.5% is!

Another fascinating project. Noel gets a Gold Star from me, and I hope he keeps us updated.

Dave

Our old weather station just used a small magnet, supported on one small ball race, rotating above a reed switch. So the read out was really a pulse counter.

The replacement is probably the same. Because there are trees nearby, am convinced that they under read by a factor of about 3 or 4.

The three cups are only about 30 mm diameter plastic (Table tennis ball sized? )

I like the idea of calibrating from a moving car, probably need to be held well away from the bodywork to be clear of any strange eddies.

Storm Eunice en route to Noel and the North Sea, has provided some of the highest readings yet seen, well into double figures.

Howard

I calibrated mine by holding it out of the car window – making sure Karen drove it in both directions along a quiet road. That way it was she who got done when we got caught doing 130mph…

Thanks, Bill.

Mike

My suggestion would simply be to see if yours can be compared to a known one, perhaps in yacht-club, flying-club or similar.

I have read a description of the use of a vane-type anenometer used in an unusual experiment, that in turn led me to consider a particular aspect of wind behaviour. It exploited the point that such a vane will indicate direction as well as force, but only while the wind is moving perpendicularly to the vane.

You sometime hear art-critics waffling about the "resonance" of an auditorium or cathedral – the same people who muddle "acoustic" (adj.) with "acoustics" (noun) – when clearly they mean "reverberation". Yes, the air in the hall can resonate, just as the air in a bottle resonates when you blow across its top, but at fractional-Hz frequency. It can also happen in Nature…

The experiment, carried out in America, investigated the resonance in a cave observed to have a regularly reversing draught in a fairly narrow passage leading from the entrance to a large chamber. The vane, made from a sheet of plastic, was suspended from a spindle carrying a potentiometer as the transducer for a data-logger, so that it would read the wind's direction as a polarity oscillation, as well as its relative speed by displacement.

The acoustics did indeed match the nature predicted by theory, treating the cave as a flue-type organ-pipe with closed end, to reasonable certainty. I think they tried it in other caves but these were more complicated, giving less certain results.

I had noticed the same effect in a cave I helped discover in Norway, a single passage descending steeply to an air-tight choke of sand; and breathing" due to the wind blowing across its entrance. Indeed, we named it "Breathing Cave" , or "Pustehohle" if I recall its translation correctly.

So I tried to apply the American results to an exploration project in which I am involved on the Mendip Hills, but the same fairly simple maths predicted a volume too large to fit under the hill! Despite the apparent regularity of the draught reversals, we decided the cause was simply fluctuations in the wind blowing across the open top of the vertical entrance shaft; lined with concrete pipes for its first 30 feet depth from their rim a little above the field level.

This does though raise an intriguing question:

– Does the wind itself, blowing over a suitable edge, create a high-amplitude, sub-Hertz sound, different from the irregular gusts? In that location the edge is created by the steep SW face of the Mendip Hills, rising from a plain to a gently-undulating plateau surface. (The cave is not far to the East of Cheddar Gorge, so fairly close to the edge of the range; and the Gorge itself could also act as an acoustic lip.) So the cave's draught was influenced by the acoustic wave rather than the simple gusts marking turbulence in the air stream.

As our "dig" has progressed gradually downwards through an interminable mass of irregular boulders the "breathing" effect has largely disappeared; and the very chilly draught now, although weather-controlled, is generally fairly steady in direction and force.

Further to my thoughts are the shapes of some types of cloud cover, forming quite regular, parallel ridges of fluffy vapour separated by clear air. Though I have not often flown, I have seen from aeroplanes, strikingly regular swell effects on cloud cover lying below the flight path. Again, a very low frequency sound marked in clouds either by piling the vapour mass into waves analogous to sea-swells, or perhaps, by the sound-waves' compression and rarefaction controlling the vapour's condensation into concentrated regions.

'

That cave dig has an unlikely model-engineering link. We made for it a simple winch, revolving on plastic bushes on a cross-bar on a scaffolding tripod. The hexagonal side-frames about 3 feet across-corners, are welded from flat steel bar originally rails from a dismantled miniature railway!