I'm getting it from what the author does, rather than what they say. The calculation for high temperatures is based on annual averages. The calculation for low

I'm getting it from what the author does, rather than what they say. The calculation for high temperatures is based on annual averages. The calculation for low temperatures is based on the coldest time of the year. I had missed that sentence, but fail to see how it makes things better. It just means that the author is aware of the problem and has no excuse for using a different methodology for the two cases.