I don't know about explaining why it is so, but it's best to use actual data. You can get data from all over the world for free! Check out the National Climate Data Center (NCDC) operated by the National Oceanographic and Atmospheric Administration (NOAA) in the folder Global Surface Summary of the Day (GSOD). I have tools for extracting and analyzing the data that you may be interested in. It's a huge amount of data.

Thank you so much for the input Dr. Dudley Dudley J Benton ,

I was using data from NCDC only (by NOAA). Mauna loa observatory gives us data of CO2 concentration level, which is partially responsible for CDDs and reversely It should also also have an impact over HDDs (but the proportion is not as around while explaining HDD) and hence I wanted a discussion on what other possible factors could be the reason of a slight decline of HDDs over the time/year.

Best Regards,

Abhay

Mattia De Rosa

Dear Abhay,

Thanks for summoning me :)

My opinion is that HDDs and CDDs are related to the ambient temperature at which you should look at if you want to investigate potential factors influencing them (e.g. urban development, climate change, etc.).

Kind regards,

Mattia

Thank you so much for your input Dr. Mattia De Rosa .

Yeah, Climate Change and temperature are closely related to HDDs and CDDs (or we can say degree days are a kind of derived from the temperature only, since we calculate it from temperature data). Other than this we have found that CO2 concentration is also impacting global degree days and we were seeking another CO2 kind of variable in order to explain larger variance of HDDs.

Best Regards,

Abhay

Thanks Sania Binte Mahtab , It's another point of discussion.