DIY · Card 88 · Weather · Measurement
Five paper cups, two straws and a pencil. Set it spinning in the breeze, count the turns for thirty seconds, and work out how fast the wind is blowing, just as weather stations do.
01The brief
Turn the wind into a number.
⅓
roughly how fast the cups move compared with the wind
0–12
the steps of the Beaufort scale, from calm to hurricane
8×
the power in the wind when its speed doubles
You can feel the wind, see it in the trees and hear it in the chimney, but how fast is it really? A spinning set of cups turns that question into something you can count.
This is the same instrument that sits on top of weather stations and wind turbines, made from things in the kitchen cupboard. It sits well beside the jar barometer and the box kite.
02Kit
Paper cups, straws, a pencil and a pin.
Tick off what you have
0 of 5 ready
03Method
Hub, arms, cups, pin, then count.
Step 1 of 8 · Minute 0
You need five paper cups, two drinking straws, a pencil with a rubber on the end, a drawing pin, a hole punch or sharp pencil, tape, a marker, and a watch or phone with a stopwatch.
It doesn’t spin.
Why
Too much friction at the pin, or the cups face different ways.
Fix
Loosen the pin slightly in the rubber, and check every cup opening points the same way round.
It wobbles and catches.
Why
The arms are unbalanced.
Fix
Make sure the straws cross exactly at the middle and all four cups are the same distance out.
Readings jump around.
Why
The wind is gusty.
Fix
Count for longer, a full minute, and average several counts.
04Measuring wind
The open cup facing the wind is pushed hard; the rounded back of the cup opposite is pushed only gently. That difference keeps the arms turning, faster in stronger wind.
Set the wind, then measure it yourself:
The cups turn about 111 times a minute; count the yellow one.
Your measurement
Cups move at
0.5 m/s
Wind about
5 km/h
Beaufort
1 · Light air
The open cup facing the wind catches more push than the rounded back of the cup opposite, so the arms turn. Each turn, a cup travels 2π × 12 cm = 0.75 m. Thomas Robinson assumed the cups move at a third of the wind’s speed, so the wind is about 3 times the cup speed. Real instruments are calibrated, and the factor is usually between 2 and 3, so treat your answer as a good estimate.
And here is what each wind speed looks like, on Francis Beaufort’s scale:
Moderate breeze
Dust and loose paper blow about; small branches move.
Wind speed
20–28 km/h
Power through 1 m²
178 W
The power carried by the wind is ½ ρ v³: double the speed and it carries eight times the power. That is why a gale (force 8) can snap branches that a fresh breeze (force 5), only twice as slow, barely moves, and why wind turbines are put where the wind is strongest.
05The big idea
Count it
to know it.
Science often begins by turning a feeling into a measurement. ‘Quite windy’ means different things to different people; ‘30 kilometres an hour’ means the same to everyone.
The trick is to find something that changes steadily with what you want to measure, here the spin of the cups, and then calibrate it. Thermometers, speedometers and scales all work the same way.
06The math
1 · Cup speed
v_cup = 2π r n ⁄ t
r the arm length, n the turns counted in t seconds.
2 · Wind speed
v_wind ≈ K · v_cup
K about 3 by Robinson’s rule; calibrate for accuracy.
3 · Wind power
P = ½ ρ v³
Through each square metre; ρ is air density, 1.2 kg/m³.
With cups 12 cm from the centre, one turn is 2π × 0.12 ≈ 0.75 m. Twenty turns in 30 s is 15 m ⁄ 30 s = 0.5 m/s at the cups, so the wind is about 3 × 0.5 = 1.5 m/s, or 5.4 km/h: force 1 to 2, a light breeze.
A 10 m/s wind (36 km/h, force 5) carries ½ × 1.2 × 10³ = 600 W through every square metre. At 20 m/s, a gale, it carries 4,800 W: twice the speed, eight times the power.
07Real life
From weather stations and wind farms to the surface of Mars.
Weather stations
Official wind measurements are taken 10 metres above open ground, away from buildings and trees, usually with a three-cup anemometer and a weather vane side by side.
Wind turbines
Every wind turbine has an anemometer on top. It tells the turbine when there is enough wind to start turning, and when it is so strong it must stop to avoid damage.
Airport windsocks
Pilots glance at windsocks to judge wind speed and direction for take-off and landing. A fully stretched sock usually means a strong wind.
Record gusts
On Mount Washington, USA, in 1934, an anemometer recorded a gust of 372 km/h, a world record for 62 years until a cyclone in Australia beat it in 1996.
Sound-based anemometers
Modern sonic anemometers have no moving parts. They time sound pulses between sensors: sound travelling with the wind arrives a little sooner than sound travelling against it.
Wind on Mars
Rovers on Mars carry wind sensors too. The Martian air is so thin that even a strong wind there pushes far less than a breeze on Earth.
1846
Thomas Romney Robinson, director of the Armagh Observatory in Ireland, wanted a reliable way to measure wind. In the 1840s he mounted four hemispherical cups on crossed arms on a vertical spindle and connected it to a counter.
He believed the cups always moved at exactly a third of the wind’s speed, whatever their size. Later tests showed the factor varies with the design, so every instrument must be calibrated. But his cup anemometer was simple, sturdy and accurate enough to become the standard, and versions of it still spin on weather stations today.
08Is and isn’t
The cups spin as fast as the wind.
They go round at only about a third of the wind’s speed, because the backs of the cups are pushed the wrong way too.
An anemometer shows which way the wind blows.
Cups spin the same way whatever the wind direction. Direction needs a weather vane.
The wind is strongest near the ground.
It is weakest there: the ground, trees and buildings slow it down. Wind gets stronger with height.
The Beaufort scale was made for land.
Beaufort wrote it for sailors, describing how a warship’s sails behaved. Descriptions of trees and smoke were added later.
09Go further
A
A wind diary
Measure the wind at the same time every day for two weeks and graph it. Which day was windiest?
B
Add a vane
Make a weather vane from a straw with a card arrow and tail, so you can record direction as well as speed.
C
Calibrate it
On a still day, hold your anemometer out of a car window at a steady 20 km/h (with a grown-up driving). How many turns does it make?
D
Three cups or four?
Build a second anemometer with three cups. Does it spin more smoothly?
E
Height test
Measure the wind at ground level and from an upstairs window. Is it stronger higher up?
10Questions
Punch four holes round the rim of a paper cup and push two straws through so they cross inside. Tape a paper cup on each straw end, all facing the same way round, then pin the crossed straws to the rubber of a pencil with a drawing pin so they spin freely.
Count how many times a marked cup goes round in 30 seconds. Multiply by the distance a cup travels in one turn, 2π times the arm length, to get the cup speed, then multiply by about 3 to estimate the wind speed.
Only the open cup facing the wind is pushed strongly; the rounded backs of the other cups are pushed too, the wrong way. The cups settle at a speed where the pushes balance, roughly a third of the wind speed.
Thomas Romney Robinson, an Irish astronomer at Armagh Observatory, in the 1840s. His four-cup design, now usually made with three cups, is still the standard way weather stations measure wind speed.
A scale of wind strength from 0 (calm) to 12 (hurricane force), devised by the British naval officer Francis Beaufort in 1805. It describes what the wind does, from smoke rising straight up to trees being uprooted.
Safety · A grown-up should push in the drawing pin. Never go outside to measure the wind in a storm, near power lines or on a roof, and never hold an anemometer out of a moving car unless a grown-up is driving safely.
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