Question about cooling degree days

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mitchell
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Question about cooling degree days

#1 Postby mitchell » Thu Apr 20, 2017 7:38 pm

Can someone explain how "normal" cooling degree days are calculated?

The reason I ask is that for a past couple of weeks, I have noticed that while the normal high and low temperatures at the local NWS station in Georgetown Delaware are 62 and 42, the climate data summary says that the Normal number of cooling degree days is 1 cooling degree day each day. Why would it be normal to have any cooling degree days when the normal high and low temperatures are so cool?
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Re: Question about cooling degree days

#2 Postby mitchell » Thu Apr 20, 2017 7:46 pm

Image
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Re: Question about cooling degree days

#3 Postby RL3AO » Thu Apr 20, 2017 9:59 pm

I think it just means that from 1980-2010, there was an average of 1 CDD on April 11th. Since CDD is based on how much over 65F the day's average temp was (ex: the actual CDD for April 11th was four since the average temp (High+Low/2) was 69F). It doesn't tell you a whole lot because it could have been 4 CDD's one year, 8 CDD another year, and a whole bunch of zeros. It just says the average is 1.
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Re: Question about cooling degree days

#4 Postby RL3AO » Thu Apr 20, 2017 10:04 pm

In case that wasn't clear, he's a made up example.

Last 5 April 11ths and # of CDD's

2013- 0
2014- 0
2015- 5
2016- 0
2017- 0

or

2013- 2
2014- 0
2015- 3
2016- 0
2017- 0

or

2013- 1
2014- 1
2015- 1
2016- 1
2017- 1

All three examples would have an average CDD of 1 but it doesn't tell you anything about the distribution.
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Re: Question about cooling degree days

#5 Postby mitchell » Fri Apr 21, 2017 6:18 pm

interesting....
that explanation makes sense, although they should call it "average" if they are using an average to calculate the value. Normal is different than average.

I always thought the normal Heating and Cooling degree days for each day were based on the normal temperatures for the day, not the actual average of HDD and CDD for that particular day for the 30 year period.

Thanks.
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