Hoar frost, minus 6.4, and a thermometer that lied
2025-12-28 · 3 min read
The lowest temperature the station has recorded was on the morning of the 28th: minus 6.4 °C at 07:10, under a clear sky after a windless night, with everything in the garden furred white. Hoar frost, the proper kind, with crystals you could see individually on the washing line, each one a little feather about 5 mm long. I went out in a dressing gown and boots with a hand lens and stayed until my fingers stopped.
The interesting part came when I checked the reading against the old spirit thermometer that hangs on the north wall of the shed, which read minus 3.5. A difference of nearly 3 degrees between two instruments in the same garden. My first thought was that the station was wrong, since the shed thermometer had been there for ten years and is a simple thing with no battery to go flat.
The station was right. The explanation is in where the two instruments are. The station is on its pole at 2.5 metres in the open, with a radiation shield around the temperature probe, a stack of little white plates that let air through and keep direct radiation off. On a clear, still night the air near the ground cools fastest, by radiating to the sky, and the temperature at 1.5 to 2.5 metres in an open garden is a reasonable measure of that. The shed thermometer is screwed to a wooden wall that was itself radiating heat it had stored during the day, and the shed is 30 cm from the house, which leaks warmth through its brick. It is measuring the shed, not the air.
I tested this later in the day when the sun was up and the air had risen to 1 °C by the station: the shed thermometer read 4. Same offset, same reason, from the other side. It had been lying to me by about 3 degrees for ten years and I had believed it because it looked trustworthy. The frost, for the record, was gone from the lawn by 11:00 but persisted in the shadow of the fence until nearly 14:00, and the grass there was still crisp underfoot when the station read 2.3 °C. The air temperature and the ground temperature are different numbers, and I have added a cheap soil probe to the list for the spring.