> subtract the average temperature from the observed temperature for the same month How are we computing average? The average daily high? The average daily l

> subtract the average temperature from the observed temperature for the same month How are we computing average? The average daily high? The average daily low? The average of the daily midpoint? The average at 9am? The average at whatever time the person got around to recording it? These things affect what one can see in the data and how it relates to reality. My point is that even simple sounding things aren't necessarily.