Heating deegree days

General Discussions: ASK John: Heating deegree days

By tree_acres_camp on Tuesday, November 03, 2009 - 01:46 pm: Edit Post

Can you explain in simple terms what are heating deegree days?

By admin on Tuesday, November 03, 2009 - 05:21 pm: Edit Post

A heating degree day is basically a unit of measure for how much heat would be used to keep a building comfortable in cold weather.

A little bit more info:

The way a HDD is calculated is you take the maximum temp for the day and the minimum temp for the day, add the two and then divide by 2. Then you subtract that number from 65 (the base temp). So for example, on a day with a high of 45 and a low of 25 you end up with an average temp of 30. Subtract that from 65 and you end up with 35, which is the HDD number for that day. Utility companies can use the HDD number in their calculations for demands for things like natural gas for heat.


By tree_acres_camp on Wednesday, November 04, 2009 - 07:32 am: Edit Post

Thanks for the info.

By tree_acres_camp on Wednesday, November 04, 2009 - 07:33 am: Edit Post

I have one more question. Is 65 always the (base temp)?

By admin on Wednesday, November 04, 2009 - 08:55 am: Edit Post

Typically 65 is the base temp as it is the temp at which it is thought that a human is comfortable working in. However, you can use what ever base temp you want, it just has to be noted when you are giving out the data.


By frnash on Wednesday, November 04, 2009 - 12:15 pm: Edit Post

A base temperature of 65°F is typically used for both Heating Degree Days (HDD) and Cooling Degree Days (CDD) particularly by the utility companies.

As commonly used, HDD and CDD really are quite a crude measure, as has been demonstrated recently in Phoenix, with pretty good radiational cooling during nighttime hours, the nights and early mornings have been quite comfortable, only to be followed by a relatively brief high temperature spike between 4 and 5 PM, followed by fairly rapid cooling with the earlier fall sunset. Under such conditions, I find the Cooling Degree Days calculation, as follows, to be quite misleading:


CDD = (Minimum Temperature + Maximum Temperature) / 2, rounded up - 65°F
Example (Phoenix, AZ, 11/01/2009):
(55°F + 88°F) / 2 = 71.5°F (Rounded up to 72°F)
Then (72°F - 65°F) = 7 CDD.

More accurately, a degree day should be computed as the integral of a function of time that generally varies with temperature. With today's automatic recording thermometers and computers, you'd think we could approach that ideal by at least using the hourly temperature measurements throughout the 24 hour day instead of the maximum and minimum temperatures.

That, and I always thought it was utterly loony to use a base temp of 65°F for Cooling Degree Days (CDD). Particularly as a measure of electrical energy demand in the arid southwest deserts, where humidity is not a big factor in the perceived comfort level. A base temperature of 80°F would probably be more realistic, yet the 65°F base is still used by the utility companies here! (Come on, who really cools their home to 65°F in Phoenix, AZ?)

By admin on Wednesday, November 04, 2009 - 12:50 pm: Edit Post


I think that the utility companies do use much more complicated formulas and hourly data to compute past weather conditions vs. loads as well as to forecast future weather conditions and loads on an hourly basis.

I think the HDD and CDD calculations are used more in the long term study (a month or more) of weather conditions vs. usages.


By frnash on Wednesday, November 04, 2009 - 01:26 pm: Edit Post

"… the utility companies do use much more complicated formulas and hourly data…"

I would certainly hope so, and maybe they do, but Arizona Public Service at least still reports conventional HDD/CDD data (65°F base) on its electric bills. (If perhaps only for the consumption of the 'unwashed masses'!)

On another note, hopefully good news for the future of the atmospheric models. From US News & World Report:
Click → Supercomputer Achieves a Thousand Trillion Calculations Per Second.


"The National Institute for Computational Sciences’ (NICS’s) Cray XT5 supercomputer—Kraken—has been upgraded to become the first academic system to surpass a thousand trillion calculations a second, or one petaflop, a landmark achievement that will greatly accelerate science and place Kraken among the top five computers in the world."

See also, from Computerworld: NICS joins petaflop supercomputer club.

By booondocker on Wednesday, November 11, 2009 - 08:24 pm: Edit Post

Nash, I'm think'n that Arizon just skips the cooling part in the formula altogether.'s always hotter than hello, without the "o"!

By frnash on Wednesday, November 11, 2009 - 09:00 pm: Edit Post

Hey Phoenix scored 23 Heating Degree Days (HDD) from October 28-30(!), and that was the total for the entire month as well! Of course we also had a total of 321 Cooling Degree Days (CDD) in October. :-)

For the first ten days of November the totals are 91 CDD & 0 HDD!

Phoenix also had record high temps on Nov 3 (96°F) and Nov 10 (91°F)! :-)

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