NOAA has done some spectacular data tampering with the Boulder, Colorado temperature record, knocking as much as 7.8 degrees off the older measured temperatures, to turn a cooling trend into a warming trend.
A good example of this was May, 1980 – from which NOAA has knocked 7.3F off the measured minimum temperatures. The adjusted mean temperature is very close to the coldest daily temperature that month, and colder than every day but six.
The adjustments are clearly bogus and without any scientific basis, but this is standard practice at the US Government.