I came across this handy link the other day courtesy of the UK Met Office.
http://www.metoffice.gov.uk/media/pdf/k/5/Fact_sheet_No._17.pdf
I was often curious to get more detail on how the auto stations work. This applies directly to the UKMO stations (the vast majority of which are automated now). I gather the TUCSON stations would be similar but not identical to this set-up. eg. the TUCSONs dont measure snow depth.
It is a bit of an eye opener and found out a few things I didnt know. Because of regular maintenance issues, they do not use a grass surface for measuring the "grass" minimum temperature. Instead at most automatic stations the natural grass surface under the grass minimum thermometer has been replaced by an artificial equivalent.
But I dont think we are likely to hear Gerry Murphy say it was down to -6° last night on the astro turf!
There are some good photos too. And you can see how some people are upset with the change to a platinum resistance thermometer (PRT) used for the measurement of air temperature at all synoptic stations and all supplementary stations that employ an automatic system. The thermometer is exposed in a Stevenson screen , at a height of 1.25 m above the ground and aspirated only by natural ventilation through the side louvers. The four liquid-in-glass thermometers in the screen are used solely for check readings. Measurements taken every 15 seconds are averaged to provide the underlying 1 minute data. Maximum and minimum temperatures are defined as the extreme 1 minute values during the relevant period. Daily maximum and minimum temperatures normally refer a 24?hour period 0900 UTC to 0900 UTC. The 1 minute average is done so as to equate as best as possible to the old LIG thermoms. It is of course highly likely that a value higher than the 1 min average may have occurred during the 15sec sampling - but those values are never used to calculate the max/mins.
Some people in the UK would be very much against this method especially say when dealing with a long temp data series such as the Central England Temperature.
But it seems that is where we are now anyway.
