One mnemonic trick 🙂
Think “D = Deep winters”,
“f = full‑year moisture (no dry season)”,
“a = awesome heat in summer”.

Great question – Köppen looks super cryptic until you know what each letter is doing.

Very roughly, the system uses 1–3 letters:

First letter = main climate group
A – tropical
B – dry
C – temperate
D – continental (cold winter)
E – polar

Second letter = precipitation pattern
f – no dry season
s – dry summer
w – dry winter

Third letter = temperature detail
a – hot summer
b – warm summer
c – cool short summer
d – very cold winter

So your example, Dfa, breaks down like this:
D – continental climate (big difference between warm and cold seasons)
f – precipitation in all seasons, no pronounced dry season
a – hot summer