Eleven hours on a warehouse roof in March

In March we put a weather station on the roof of a cold-store warehouse near the Gulf Coast. It took two technicians eleven hours, a crane booking and a conversation with a fire officer. The station is about the size of a shoebox on a tripod. It now supplies a quarter of the wind readings for the cells around it, so the day was worth spending.

We picked the roof because the nearest official station was 38 kilometres inland and sits behind a tree line. Wind readings from a sheltered mast are lower than the wind a coastal building feels, and the difference is large: during a gale the inland station recorded 112 km/h while a handheld meter on the warehouse edge read 139. A policy with a strike at 150 should not depend on the inland number.

The mast is the policy

Siting is the unglamorous centre of an index. A standard anemometer measures wind at 10 metres above open ground, with nothing taller within ten times its height. A rooftop has neither. So we mounted the sensor on a 3.5-metre mast above the parapet, away from the plant room that creates turbulence on the lee side, and wrote down the correction for the roof. That correction is part of the station’s record and is published with every reading.

Eleven hours

The first four hours were permits and cabling. The sensor went up at noon, then spent two hours as the wind rose to 41 km/h and we checked whether the mast moved. It did, by a few millimetres, so we added a third guy wire. The last three hours were calibration: we held a reference meter next to the sensor and compared 190 paired readings, which matched within 0.8 km/h.

What the station was worth

In the first month the station sent its readings to the cell and became one of three sources for the wind field. During a squall line in April, it was the only instrument within 30 kilometres that saw 128 km/h, which the radar had underestimated by about 20. The calculation agent weighted it highest, and that was correct, because the roof was where the damage happened.

What we ask of policyholders

Some policyholders offer us a rooftop. We say yes about once in five times, because we need to check the siting, the power and the access, and we do not mount a station where someone has to climb a ladder in a gale to service it. If you would like to host one, tell your underwriter. The station stays ours and the readings stay public.