I know that the past temp data has been changed but the details were fuzzy. Here is an explanation sort of.
The methods used to make the changes are shrouded in mystery. Tony Heller goes into more detail on the time of observation bias adjustment.
Apparently the old stations used min/max thermometers which are reset by hand once per day. If reset in the heat of the day they can double count that hot day.
He says that the amount changed to correct for the different time of day (tod) measurement is suspect. Instead he says only look at the morning tod measurements over time. They show long term cooling.
Then there is some other adjustment which he thinks fills in missing data for a site not reporting .
Finally he says that some stations have come and gone over the past so they try to make up data to fill in using unknown methods.
Instead he just used stations which have been reporting for 100 years. This shows slight cooling.
So the adjustments make the past cooler and the present hotter.
https://youtu.be/Sf4gC9E_3iUAlterations To The US Temperature Record (Part 3)
12,886 views•Oct 20, 2020
https://realclimatescience.com/alterations-to-the-us-temperature-record/Also
https://newtube.app/TonyHeller/VvRXsbQ part 3
https://newtube.app/TonyHeller/qGpHf5a part 1
https://newtube.app/TonyHeller/wQDLvfx part 2
