A degree day is a measure of coldness. I take temperature readings each day and have calculated the D-Days for each week over the past three years. A week on week comparison can't ever say much but a cumulative plot will show a trend over time.
The cumulative plot is just the total D-Days for the current week added to the the total for the year so far. The total at the end of each year could be roughly the same but the shape of the curve will show how the patern of coldness progressed over the year.
My heating year starts in September but the data starts at the end of May / beginning of June. What I wanted to show here was all of the zero heating season at start.
From the plot above and the data on the left you can see that all the years are different. Your energy use should have been smaller last year but it looks like it will be colder this year.
This page was updated on 17 December 2007
by Terry Rawkins