Topic: HDD/CDD

[Jáchym]

Hi guys,

I want to ask you for some help with understanding the concept of degree days. We don´t use it in my country so Im not sure I understand it correctly.

I have read about it in Wikipedia, this is how I understand it:

First you choose a base temperature. Next, you take the average daily T and if it is above the base T, you would have 0 HDD and the difference would be the number for CDD, likewise, if the avg T is below the base, you would obviously not be cooling (i.e. CDD=0) and HDD would be equal to the difference between the base and the avg T.

So far so good.

Now looking at some of the files that are generated on some of your pages using Ken´s script, I see for example this (I used Ken´s data for May 2016):
                                   HEAT  COOL        AVG
    MEAN                              DEG   DEG         WIND                 DOM
DAY TEMP  HIGH   TIME   LOW    TIME   DAYS  DAYS  RAIN  SPEED HIGH   TIME    DIR
 1  69.1  88.4   1:10p  51.6   6:30a   3.5   7.6  0.00   0.5  13.0   5:50p   SSW
 2  62.6  78.6   1:05p  47.6   6:20a   6.0   3.6  0.00   0.7  12.0   3:05p     N
 3  64.1  78.6   4:15p  53.6   2:10a   4.2   3.3  0.00   0.4  11.0   1:45p     N
 4  63.0  75.6   3:45p  55.7   4:30a   4.4   2.4  0.00   0.5  11.0   3:45p     N
 5  60.8  70.9   3:00p  54.5   4:45a   4.8   0.6  0.00   0.6  13.0   2:50p     S
 6  56.3  62.9  11:00a  50.7  11:40p   8.7   0.0  0.15   0.2  11.0   2:30p    NW
 7  56.7  63.9   3:40p  50.2   4:10a   8.3   0.0  0.06   0.5  11.0   2:40p   ESE
 8  61.5  74.6   3:15p  53.3   3:25a   5.1   1.5  0.00   0.5  11.0   5:10p     N
 9  62.3  76.5   2:25p  55.1   4:35a   5.0   2.4  0.00   0.8  12.0   3:45p     N
10  63.7  81.1   2:55p  54.8   7:05a   4.9   3.6  0.00   0.6  11.0   3:20p     N
11  62.7  78.5   2:35p  51.3   4:45a   5.7   3.4  0.00   0.5  11.0   2:55p     N
12  63.1  76.0  12:35p  54.2   1:20a   4.9   3.0  0.00   0.6  11.0   3:40p     N
13  60.5  73.5   2:45p  52.0  12:00m   5.7   1.2  0.00   0.5  13.0   3:55p     N
14  62.3  79.2   3:00p  48.3   2:45a   6.0   3.3  0.00   0.6  16.0   5:30p     N
15  63.6  77.4   4:30p  54.5   6:50a   4.3   2.9  0.00   1.0  14.0   4:40p    NW
16  64.9  81.2   4:05p  49.5   6:15a   5.0   4.9  0.00   1.1  13.0   3:05p     N
17  71.0  90.7   4:25p  52.0   5:55a   3.7   9.7  0.00   1.3  14.0  11:55a    NW
18  73.1  91.6   2:05p  56.2   6:15a   1.8   9.9  0.00   0.9  13.0   2:25p     N
19  62.2  78.3  12:55p  52.8  12:00m   5.4   2.6  0.00   1.2  17.0   4:30p     W
20  57.2  67.8   2:10p  51.1   5:55a   7.8   0.0  0.00   1.8  17.0  10:55a    NW
21  57.1  65.6   6:05p  48.9  12:00m   7.9   0.0  0.02   0.6  13.0   1:20p   WNW
22  56.2  70.9   1:30p  45.1   5:45a   9.2   0.4  0.00   0.8  16.0   3:20p    NW
23  59.2  74.0   4:50p  46.5   3:30a   7.6   1.8  0.00   1.1  16.0   4:10p    NW
24  59.9  73.7   4:45p  49.8   2:55a   6.3   1.1  0.00   0.6  16.0   3:45p    NW
25  54.6  57.2   8:15a  54.2   2:45a   3.6   0.0  0.00   0.0   5.0  12:05a   WNW

Now, what is confusing to me is that based on my understanding of it, a day has a particular avg temperature. If it is above the threshold you would have the difference between the base T and the actual value as cooling deg day, likewise, if the average is below the base temperature, you would have the difference being equal to heating degree days. Now based on this, obviously it is either that or that, the other one would be zero (if it is below the base, you are not cooling, if it is above, you are not heating). So where do those numbers in the above file come from. For example, on the 1st, it shows HDD as 3.5 and CDD as 7.6. The mean is 69.1. So shouldnt it be 0 HDD and 4.1 for CDD? even a conversion to C wouldnt help, that would be multiplied by 1.8, which in case of 4.1 is 7.4, not 7.6 and still, why is HDD simply not 0...

and my next question, is this always calculated in deg F? Because in deg C the differences would be different

[ericfynne]

To do it "properly", you don't just take the average temperature, because it is quite reasonable that you might require both heating and cooling at different times in the same day. You do the comparisons at intervals during the day (say once a minute or whatever) and integrate the results. If all you have is the average, then using that is the best you can do, obviously.

Eric

[ericfynne]

The Wikipedia pages on HDD and CDD do discuss this, by the way.

[Jáchym]

OK thanks, I looked at Wikipedia only for the basic explanation and then read a more sophisticated algorithm given here:

http://www.vesma.com/ddd/ddcalcs.htm

One more question, this will obviously depend on which degrees you use, F or C, in case of the U.S., do you always use deg F?

[SLOweather]

For completeness, and to muddy the water even more there are also "Growing Degree Days" . The Wikipedia entry shows them in C, but they are also used in F. And, I think that some plants may have different threshold temperatures.

Note also that the formula is a little more complex as well.

A while back I did some research using data from one of my Weatherelement stations, comparing GDD calculated over the course of a day from regular updates from the station to the simpler one using the average of the daily high and low temperatures. As I recall, they results were closer than I imagined they would be.

[Jáchym]

Here in my country we use the so-called "temperature sums", similar concept to HDD/CDD, but calculated slightly differently. It is mostly used in agriculture, because the average T is not a very good indicator of potential for growth of various crops.

[hankster]

One more question, this will obviously depend on which degrees you use, F or C, in case of the U.S., do you always use deg F?

Shouldn't matter if its F or C as long as the same base temperature is used. 65F = 18.3C

[Bushman]

More info:  http://www.degreedays.net/

[Jáchym]

Thanks guys, I think I finally understand the concept It is in fact very similar to the temperature sums we use here.

The only difference is that we use two types of temperature sums:
- sum of active temperatures - this is a sum of all daily average temperatures that are above certain base value
- sum of effective temperatures - this is a sum of the differences between average daily temperatures and the base value on days when the average exceeds this base value
- vegetation thermal constant - this is simply sum of all temperatures, no base value - this also tells you how severe the winter is

Example:
base T = 10°C
daily avgs: 8, 11, 13, 12

sum of active temperatures (Tsum10 active)= 11 + 13 + 12 = 36
sum of effective temperatures (Tsum10 effective) = 1 + 3 + 2 = 6
vegetation thermal constant = 8 + 11 + 13 + 12 = 44