Siting the station: the shed roof was wrong
2024-10-05 · 3 min read
When the weather station arrived in the spring I put it where it was easiest: on a bracket screwed to the shed roof, about 2 metres above the ground, in the corner of the garden by the fence. It stayed there for five months and I logged everything it said faithfully. In September I compared a month of its temperature readings with the official readings for the nearest town and found that my maximums were running 2 to 3 °C high on sunny days and my wind speeds were implausibly low. The shed roof was wrong, and this is why and what I did about it.
Temperature first. The shed roof is dark grey felt, and on a sunny afternoon it was absorbing heat and radiating it straight up into the temperature probe 40 cm above it. The probe has a shield, but a shield only keeps the sun off; it cannot stop warm air rising from a hot surface beneath. I put a second thermometer on a pole in the middle of the lawn one afternoon and the difference at 15:00 was 2.8 °C. The guidance for siting is a grassed surface beneath, at 1.25 to 2 metres above it, well away from walls and hard surfaces. The shed roof failed on every count.
Wind second. The fence on two sides is 1.8 metres high, and the shed itself blocks the third. The anemometer was sitting in a pocket of still air whatever the wind was doing at roof height. The standard is 10 metres above open ground, which no garden can manage, so the compromise is as high as you can get it, as far from obstacles as you can manage, and an acceptance that your readings are relative rather than absolute.
The station now lives on a 2.5 metre pole set in a concrete footing at the end of the lawn, 3 metres from the nearest fence and 6 from the house, with the temperature probe at 1.6 metres on the shaded north side of the pole and the anemometer at the top. The first week of readings in the new spot shows sunny-day maximums within half a degree of the official ones and wind speeds roughly double what the shed gave me. The rain gauge, which went up with the rest, is now also clear of the fence, which had been shielding it from any rain coming from the west. Five months of logged temperatures have a note next to them saying “shed roof, treat with suspicion.” The pole was an afternoon’s work and I should have done it first.