This Winter Is 37% Warmer than 2010-11 (by Heating Degree Days)

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Last week was Groundhog Day, and my question that day was, How can Punxatawney Phil (the weather predicting groundhog) or General Beauregard (his Southern counterpart) say anything at all about more winter when we haven't really had a winter at all? Yes, it's been quite warm this winter, especially in comparison to last year. So I thought I'd check to see just how much warmer it's been.

First of all, looking at the number of Heating Degree Days (HDD) is a great way to compare the cold weather of one year to another. There are three nice websites I'm aware of where you can get the Heating Degree Days for your location:

You should definitely check them out.

Heating Degree Days are a way of looking at how much time your area has spent below a given temperature, called the baseline temperature. The most commonly used baseline here in the States is 65° F, so here's how it works:

  • 1 minute at 64° F gets you 1 Heating Degree Minute.
  • 1 hour at 60° F would be 5 Heating Degree Hours.
  • 24 hours at 30° F accumulates 35 Heating Degree Days.

See how it works?

One way that you can use these numbers is to calculate the amount of heating your home needs. Since your heating load depends on how cold it is outside, the more Heating Degree Days you have, the more heating you have to pay for. The basic equation for this is:

Heat flow by conduction, which can be modified to use Heating Degree Days.

The number of Heating Degree Days tells you the ΔT over a given time period.

So, here in Atlanta, we had a total of 3396 HDD in the 2010-11 winter. It was really cold last year, and we actually had snow three times. We normally get one little snow a year, but we even got shut down for a whole week with snow that turned to ice on the roads and then stuck around. Our 30 year average is about 3000 HDD, so last year was colder than normal.

This year however, if we look just at December and January, we find a huge difference:

December 2010: 891
December 2011:506
January 2011: 823
January 2012: 570

The 37% number in the title of this article comes from comparing the HDD of just those two months. We had 1714 HDD in those months in 2010-11 and 1076 HDD this winter. It's great for heating bills and getting outside and doing stuff here. Not so great killing bugs though.

How do your Heating Degree Days this winter compare to last winter?



Photo by AlicePopkorn2 from, used under a Creative Commons license.


Les Watts

Maybe we will have a colder than normal summer!

Tom Delconte

Thanks to the North American Oscillation keeping the jet stream up north, and the catastrophic effects of global warming: the SE corner of Pennsylvania has been positively balmy this winter!

Bob Seaton

In Tallahassee, Florida: 
December 2010: 625 HDD 
December 2011: 253 HDD (down 59.5%) 
January 2011: 564 HDD 
January 2012: 325 HDD (down 42%) 
Seems like every day this winter "it's another beautiful day in the Deep South". Hoping for a cool summer. 

John Proctor

Another thing we just noticed is the trend of cooling degree days to be higher. I am emailing a graph of the cooling degree days for St. Louis, MO since 1960 to Allison.Cooling degree days at base 70 over that time has trended upward at a rate of 6.93 degree days per year, or 49% over 51 years. 

Lucas Durand

Thanks for the links. 
I've been meaning to do this comparison for a while now. 
My location is outside Thunder Bay, ON. 
30 year average is 10324 HDD. 
Last 12 months is 9541 HDD (~8% warmer) 
I'd like to check these numbers again in june after this winter has past as I suspect that "borrowing" the last few months from last year's winter will have skewed the above observation. 
Anecdotally speaking, this has been the most bizarre winter I have ever experienced living here. 
It's supposed to be the deep of winter still, yet I almost expect to see the bears up and about any day now.

David Butler

Here in the high desert of SE Arizona, our winters have been see-saw. December 2011 was 20% colder than normal, but December 2010 was 30% warmer... 
Dec Norm: 535 
Dec 2010: 381 
Dec 2011: 639 
Global warming is causing higher regional variances and more records being set.


New Jersey - Central 
year over year, Dec & Jan, 23% warmer

John Semmelhack

Charlottesville, VA 
2010 - 2,780HDD (a typical year) 
2011 - 2,222HDD (20% warmer)

Craig Lawson

Santa Rosa, California 
2/1/09 to 1/1/10 = 4,220 HDD 2/1/10 to 1/1/11 = 4,098 HDD 2/1/11 to 1/1/12 = 4,183 HDD 
Historic average +- 3,000 HDD 
Where's the heat?

Donald B.

You had to go and jinx it! Now, there's a pair of Design Days just around the bend... 
My 8.0 Ft^3gas / DegreeDay winter record is going to take a beating this weekend. :-(

David Eakin

I've been compiling heating/cooling degree days for several seasons now. I have an Excel spreadsheet that has one column for energy use (either KWH or CCF) and another for degree days. I have an associated graph that displays both so you can see at a glance whether or not your changes (either behavior or building) have made much of a difference.

David Butler

David E: Here's a regression for my heat pump kWh vs. heating degree *hours* 
You should check out the discussion on this blog in the RESNET BPI forum. Only folks who track HDD vs energy will appreciate the nuances currently being debated over there.