The largest component of the USHCN upwards adjustments is called the time of observation bias. The concept itself is simple enough, but it assumes that the station observer is stupid and lazy.
Suppose that you have a min/max thermometer and you take the reading once per day at 3 pm. On day one it reads 50F for the high – which for arguments sake occurred right at 3 pm. That night a cold front comes through and drops temperatures by 40 degrees. The next afternoon, the maximum temperature is also going to be recorded as 50F – because the max marker hasn’t moved since yesterday. This is a serious and blatantly obvious problem, if the observer is too stupid to reset the min/max markers before he goes to bed. The opposite problem occurs if you take your readings early in the morning.
I had a min/max thermometer when I was six years old. It took me about three days to realize that you had to reset the markers every night before you went to bed. Otherwise half of the temperature readings are garbage.
USHCN makes it worse by claiming that people used to take the readings in the afternoon, but now take them in the morning. That is how they justify adjusting the past cooler and the present warmer.
They should use the raw data. These adjustments are largely BS.
