Degree Days Explained

you might want to start understanding what degree days are here:

Heating degree dayWikipedia

Degree days are calculated as the integral of the difference between the outside air temperature and a base temperature over time.

If you can define a base temperature (the outside temperature above which a building needs no heating or cooling) then you can use this to estimate degree days. The building requires heating if the outside air temperature falls below the heating base temperature, and the heating degree days accrue; if the outside air temperature rises above the cooling base temperature, the structure requires cooling, and the cooling degree days accumulate.

The base temperature does not necessarily correspond to the desired building internal temperature, but is the homeostatic temperature between the interior and exterior. Base temperature might depend, among other factors, upon the use of the building, the internal heat gains and the level of insulation.

example deegree days for New York, Downtown Manhattan, NY, USA

example deegree days for Palermo Boccadifalco Airport, ITALY