The Köppen third letter marking a hot summer is “a.” It means the warmest month averages at least 22°C (71.6°F) in the standard Köppen–Geiger scheme.
This letter appears in temperature-based groups such as C and D. Thus, Cfa is a humid subtropical climate with hot summers, while Dfa is a humid continental climate with hot summers. The letter “a” describes summer heat, not rainfall or winter severity.
A common mix-up is confusing “a” with “b.” The letter “b” means the warmest month averages below 22°C, but at least four months average above 10°C, so it marks a warm rather than hot summer. The letter “c” indicates a cooler summer, with only one to three months above 10°C.
Köppen’s system combines these temperature letters with precipitation letters. For example, the “f” in Cfa means no dry season, while the final “a” supplies the summer-temperature information.