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How to describe climate data / calculate degree days?

asked 2014-09-12 08:12:18 -0600

nheeren's avatar

updated 2020-01-20 13:38:38 -0600

For a publication I am simulating a reference building for different sets of parameters and discuss the resulting differences in space cooling and heat demand. Amongst others, different locations / climates are used. Therefore I would like to shortly describe the climate data being used for the thermal simulations.

So I was thinking to provide heating degree days (HDD) and cooling degree days (CDD). However, I am unsure which is the correct algorithm to calculate those. A typical formula that can be found basically says: image description

If the daily mean temperature is below a threshold temperature (T threshold usually 12°C), sum up this day's temperature differences between mean (T ext, mean) and indoor temperature (T int, usually 20°C). For CDD it is the same but with a higher threshold temperature (T threshold).

Is this a good formula to use or are there other (standardized or more accepted) ones around? Would you take a different approach (than degree days)? Thanks!

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answered 2014-09-13 23:30:51 -0600

Joe Huang's avatar

updated 2014-09-18 18:49:32 -0600

If you want to be faithful to the original derivation of degree-days, the equation would be to calculate the daily mean as the average of the max and min temperature of the day, and then subtract that from the base temperature (65F in the US that then got translated to 18.3 C elsewhere), and then sum the positive values over all the days.

image description

Some tidbits on how this "classical" formulation evolved:

  1. It was used to estimate the amount of coal needed in Chicago (?) in the 1930s.

  2. Max and min temperatures were used because hourly temperatures were unavailable.

  3. 65 F can be interpreted as the balance point temperature (BPT) of a building, below which heating would be required. As buildings have gotten tighter, there was talk 20+ years ago that lower BPTs should be used, but that became irrelevant with the proliferation of simulations, leaving 65 F degree-days as just a general climate index.

  4. Using hourly values, i.e., degree hours/24, will give slightly higher values (3-8%), but these paradoxically often correlate not as well to building energy use, because they ignore building thermal inertia.


In answer to the previous question, I've never bothered to check if the equation has been defined in some standard, although it has been described in a lot of technical literature (just do a Google search). Frankly, I don't understand the compulsion for seeing methods standardized.

While I'm on this subject of building energy-related climate indicators, I'd like to repeat that degree days is just the earliest indicator developed and only addresses sensible heat loss/gain, i.e., temperature. There have been similar indicators proposed for humidity (Latent Enthalpy Days), solar (Heating/Cooling Insolation Days), even infiltration (Infiltration DD's), but they've not been widely used or adopted. For a description of these indicators, please look at this old paper that I wrote 28 years ago (if nothing else, it might impress you with its age!)

Another note for the curious: I recalled that the equation in the original question had two differing temperatures. This actually works well for getting cooling degree days/hours to correlate to building loads. The logic is that the two temperatures correspond to the building's BPT with and without ventilation, i.e., windows open or closed. There's even a term for it, "Vented Cooling Degree Hours" (invented by someone else), but described in my 1982 paper. Alas, it never caught on with the building energy community, and a colleague at UC Berkeley likened it to "tuning up a 1950's Thunderbird", i.e., refining an obsolete technology.

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Thanks. Also nice supplementary info. Has the formula been defined in some standard?

nheeren's avatar nheeren  ( 2014-09-14 09:46:22 -0600 )edit

I added the equations in your answer @Joe Huang if you don't mind. (The X+ notation may seem unusual but it is what I've seen often used in papers about M&V, such as in the Bonneville Power Administration "Regression for M&V: Reference Guide)

Julien Marrec's avatar Julien Marrec  ( 2014-09-15 05:04:48 -0600 )edit

@Julien Marrec, Thanks for adding the equation, which is what I intended to but was unable to input on the bulletin board.

Joe Huang's avatar Joe Huang  ( 2014-09-15 12:30:27 -0600 )edit

@Joe Huang, I've merged your two answers into the first. You can always add to an existing answer by using the edit button below the answer.

I also fixed the link. You can create links using the earth with the arrow icon button.

Neal Kruis's avatar Neal Kruis  ( 2014-09-15 12:46:56 -0600 )edit

@Neal Kruis it'd be great to have some sort of LaTeX way to input formulas (not sure how hard it is...) in the future if we see that often equations are included (and it might very well be the case). There's a good thread here:

In the meantime, there's a handy online WYSIWYG editor when you don't have a TeX editor handy to generate images (you can get the link right away) of equations you type in LaTeX:

Julien Marrec's avatar Julien Marrec  ( 2014-09-15 13:06:20 -0600 )edit

answered 2014-09-13 09:28:58 -0600

updated 2014-09-13 10:33:30 -0600

To answer your equation about the equation, your equation is correct except the standard definition uses the same number for the base and threshold i.e. you would do HDD12 or HDD20 not HDD20/12. You sum up the difference in temperature between the base and the actual when the actual is above or below the base. You also need to use the average daily temperature. Most calculators will actually do the sum on an hourly basis and then divide by 24 to get the daily numbers.

An important question to ask is what base do you want to use? Here in the US where we do it in Fahrenheight, typical numbers for building scientists are HDD50/CDD50 and HDD65/CDD65. You can find lots of data at

Your question about metric is not clear enough. What is the paper about and what are you trying to describe about the climate? Are you trying to just compare the required heating and cooling potential? Then HDD and CDD are fairly decent and well accepted.

If you want to succinctly discuss the effect on building design and overall energy including things like humidity, then I think talking about the ASHRAE climate zones is better (i.e. things like 2A, 4C, 7B, etc). These are more comprehensive, but coarse, distinctions. But, then you need to reference a paper that describes how these were determined rather than give an equation.

If you have a discussion of renewable energy in the paper, then neither describes the solar or wind potential at all and you would want another measure like average solar energy potential Wh/m²/day.

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@Ralph: Thanks! I updated the question - i hope my goal is now clearer. Climate zones are not really defined here. Good point about the solar potential. But I guess this is secondary in my case.

nheeren's avatar nheeren  ( 2014-09-14 09:36:31 -0600 )edit

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Asked: 2014-09-12 08:12:18 -0600

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Last updated: Sep 18 '14