The last week was particularly cold in comparison to all my time here at Halley. We had a few days when the outside air temperature, without factoring in the wind chill, would drop to below -50°C (where below means in the direction of -60°C). Early this morning it seems we’ve seen the lowest temperature since the Met records started at Halley, at -55.4°C (exceeding the previous record low of -55.3°C by 0.1°C). This still needs to be confirmed, but we’re claiming the new record anyway!
Thermometers and measuring the temperature
It seems that there is an awful lot more to measuring the outside temperature than just sticking a thermometer outside and reading it. We have a number of thermometers installed in what’s called a Stevenson Screen (which is a characteristic white met instruments housing you can see as a standard part of any weather station) plus two temperature sensors connected to what we all call JAWS, which stands for Just Another Weather Station, which is an automatic weather station we’re running at Halley.
The reason there’s so many thermometers and temperature sensors is that, as with any instrument measuring anything at all, they each show a slightly different value. We need to constantly cross-reference them with each other.
The two temperature sensors, which are connected to JAWS, are of different types. One is just a temperature sensor, while the other one is a combined temperature and humidity sensor. Also, one only goes down to -39.5°C.
The three thermometers we have are of different types. Two of them are scaled to -55°C, while the third one, called a minimum temperature thermometer, scales down to -70°C and also has a little mark that’s pushed as the temperature drops, indicating the minimum recorded temperature so far (hence the name). In addition, the minimum thermometer is mounted horizontally. This morning it showed -55.4°C, which is lower than anything previously recorded at Halley. The three thermometers are within 0.2°C from each other, which really gives us no reason not to believe that there really was a drop in the temperature to that value.
As I mentioned above, all these are kept inside the Stevenson Screen, which shields them from precipitation, wind or heat radiation, while still allowing the air to flow freely around them. The thermometers should also be placed at a specific height above the ground – the Stevenson Screen should be mounted so that the bulbs of the thermometers are 1.25m off the ground surface.