The first version of HadCRUT initially spanned the period 1881–1993, and this was later extended to begin in 1850 and to be regularly updated to the current year/month in near real-time.
A more complete statistical model of uncertainty was introduced with this revision, including estimates of measurements errors, biases due to changing exposure and urbanisation, and uncertainty due to incomplete coverage of the globe by observations of temperature.
[7] The Climatic Research Unit had as an early priority the objective of filling gaps in available information "to establish the past record of climate over as much of the world as possible, as far back in time as was feasible, and in enough detail to recognise and establish the basic processes, interactions, and evolutions in the Earth's fluid envelopes and those involving the Earth's crust and its vegetation cover".
[8] Access to weather station temperature records was often under formal or informal confidentiality agreements that restricted use of this raw data to academic purposes.
From the 1990s onwards the unit received requests for this weather station temperature data from people who hoped to independently verify the impact of various adjustments, and after the UK Freedom of Information Act (FOIA) came into effect in 2005, there were Freedom of Information requests to the Climatic Research Unit for this raw data.
In a decision announced on 27 July 2011 the Information Commissioner's Office (ICO) required release of raw data even though permissions had not been obtained or in one instance had been refused, and on 27 July 2011 CRU announced[9] release of the raw temperature data not already in the public domain, with the exception of Poland which was outside the area covered by the FOIA request.