In the information displayed when hovering over the (i) next to Heating Degree Days in the Variable tab, it talks about "CDD". However, reading Spinoni et al. (2015), I believe this should be "HDD" or "HD" instead: "We distinguish between cooling degree-days (CDD), heating degree-days (HDD), and growing degree-days (GDD)."
(This atlas is awesome and I'm nitpicking, thanks for creating this fantastic resource!)