I’m afraid I’m 3 years late in noticing this, but it must be 3 years since I last tried to look up any degree day data online. The online home for degree day data has been with Vilnus Vesma since before I had my own connection to the internet at work (anyone else remember having to log on to the one computer with a dial up modem attached?). It appears from his website he has licensed this data in 2003 and you now need to subscribe to it. You can still get regional data for 1 year for free, so not too much of a big deal.
Vilnus explains what a degree day is and why you might need it:
When you use fuel or electricity for space heating, demand will tend to vary according to how cold the weather is. This of course is what makes the consumption seasonally variable to such an extent; and the erratic nature of the weather, even averaged over a month, is what makes fuel consumption so hard to manage. In the UK, for example, it is possible for one November to be twice as cold as another, and hence for heating fuel demand to vary by a factor of two between corresponding months. But can we really say that one month is ‘twice as cold’ as another? Does it mean anything?
Yes, it does. When the outside air is above a certain temperature your building won’t need heating. This is what we will call the ‘base temperature'; in the UK it’s common to assume a value of 15.5 Celsius, but this is just a historical convention. If the average outside air temperature on a given day is below this base temperature, you will need heat; and your heat requirement that day will be in proportion to the temperature deficit in degrees. Add up the daily temperature deficits over a month, and you get cumulative degree-days (degrees multiplied by days, do you see?). And these cumulative degree days are, by definition, proportional to cumulative heat requirements over the same period. Thus a month in which 360 degree-days are clocked up is ‘twice as cold’ as one with a total of 180 degree-days.
As good an explanation as I have seen in a long time.
I’ve always found it a bit strange that this data comes from what seems to be one man with a passion, rather than BSRIA or CIBSE. Thinking about it, his site gave an insight years ago into what the internet (and in particular, blogging) has become – millions of ordinary people, passionate about a topic, sharing it with the world.