Three anemometers on one bridge, forty metres apart, deciding whether high sided vehicles can cross. On a westerly they agree to a knot. On an easterly the middle one reads twenty higher and the bridge closes.
I am the bridge's maintenance engineer and the wind monitoring is mine.
Why there are three of them.
A single sensor deciding whether to close a motorway is a single point of failure, so there are three, spaced along the deck, and the control logic uses the median. If one fails or disagrees badly the system flags it and falls back to the other two.
They are cup anemometers with heated bearings, calibrated annually, and their outputs are logged every ten seconds along with a direction from a vane at each mast.
The thresholds are in the operating regime: at a mean of forty knots the bridge closes to high sided vehicles, and at sixty it closes to everything.
Mast 2, in the middle of the span, reads about twenty knots above the other two when the wind is between about seventy and a hundred and ten degrees.
On anything else, all three agree within a knot or two, which is what three good instruments on one deck should do.
Twenty knots is the difference between an open bridge and a closed one.
Because the logic uses the median of three, a high reading on one does not close the bridge on its own, and that is exactly why the system is built that way. But it flags, and a flag brings me out, and a flagged sensor in an easterly is a bridge running on two.
What has been done, and this is a structure with a proper budget.
Mast 2's anemometer has been replaced four times since 2017, including once with a sonic anemometer, which has no moving parts at all and measures the wind by timing sound between transducers.
The sonic did it too.
The mast was checked for alignment and for vibration. The cabling and the logger were replaced when the sonic went in.
And in 2021 the bridge's designers were asked, because the obvious explanation is aerodynamic: a structure disturbs the air around it, and a sensor in the wrong place reads the disturbance rather than the wind.
They modelled it. Their report says that for easterly winds the deck geometry produces a local speed up at the parapet of the order of five percent at mast 2, which is about two knots at the threshold, and that a twenty knot enhancement is an order of magnitude beyond anything the structure can do.
What the direction vanes say, which nobody looks at.
Each mast has its own vane. On a westerly all three agree on direction to a few degrees.
On the easterlies where this happens, mast 1 and mast 3 agree with each other and with the met station eleven miles away.
Mast 2's vane sits about thirty degrees off the other two, steadily, for the whole event, and comes back into line when the event ends.
So the air at the middle of that span, during those winds, is both faster and coming from somewhere else.
What we do about it.
Nothing has changed in the operating regime and I would resist changing it. The median logic is correct and it protects the bridge from exactly this, whatever this is.
Mast 2 is on the record as a sensor with a known directional anomaly, and it has been through two independent safety reviews with that wording, and both accepted it.
The closures we make are made on the other two and on the met station, and no lorry has ever been blown over on this bridge.
What I do.
Annual calibration, the monthly check, and the walk out when a flag comes up.
The walk out is the thing I have changed.
The maintenance walkway runs under the deck and you come up through a hatch at each mast, and I have gone up mast 2's hatch in the dark in a gale more times than I can count, because that is the job.
I do not go up in an easterly now.
In November 2022 I was on the walkway at about two in the morning with the flag up and the wind about thirty five knots from the east, and I had the hatch open at mast 2 with my head and shoulders out of it.
At deck level in a thirty five knot easterly you are leaning into it and it is coming across the deck from your right.
At mast 2 it was coming from behind me, out of the middle of the span, from a direction with nothing in it but four hundred feet of air and a river, and it was doing perhaps fifty five, and I got down the ladder and closed the hatch and finished the shift in the cabin.