Much has been written re the long term temp variation of the climate but as it appears were talking about around 1 deg C change over quite a long time, pre industrial era, just how absolute accurate where the thermometers in use?.
Course these days there are some very accurate ones around but back then in the 1800's how accurate was a standard one back then anyone any idea?.
Lets not take into account as to where the measurement was made either in town out in the country at the sea side etc, but the gear in use to measure?.
Essentially temperatrurre was *defined* buy mercury thermometers.
That is 50C is the difference in length of a mercury column half way beytween the boiling pint at seal level, and 1000mb, and the freezxing point of water.
Unless the column was not constant diameter, this is a pretty good estimate.
Secondly averaging 100 thermometers all measuring the same things, nets yuou a very much higher accuracy assuming te thermometers are all randomly inaccurate.
HOWEVER the bad news is that often and to this day, the 'average' daily temperature is talen as beiing the 'average' between the maximum and minimum temperatures recorded in a single day.
This tends to be higher than the true average, as sunlight and temperaure peaks quite sharply around midday and a little after. And will peak higher in the vicinity of concrete, asphalt and other man made materials.
Utbaisation also tends to incerase temperatires genuinely overall as well as te peak.
Ive got a mercutry thermometer in my victorian or edwardiam barometer andd I'd say its accutrate to quarter of a degree or so, How well calibarted is another matter, but it tends to agree will what the local airfields are reporteing on the internet
Well one should. Because the issue is less how inaccurate the thermometers were, but how they were utilised.
By about 1850 Negretti & Zamba had perfected weather thermometers that were almost as good as the ones we have today. Their centenary catalogue is online. Their best antique kit sells for good money at auction.
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Fitzroy was instrumental in getting their kit widely distributed and used to help try and save lives at sea by warning of impending storms.
You can find complaints from the astronomer royal about the failings of earlier kit (mostly failure to automatically record data reliably rather than of calibration). Calibrating between melting ice and boiling water was fairly reliable if you measure the atmospheric pressure.
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