Since 天 refers to "day," 夏天 accordingly still remains the same meaning as 夏. The only difference is people prefer using the short form in written, while 夏天 is more frequently used in oral Chinese. The same principle also applies to other seasons' expressions.

By the way, Chinese don't pay special attention to prepositions, thus, in this specific case, 夏天 may have the direct meaning of "in summer," which highly depends on the context.

All you need to know is 夏天 equals to 夏/(in) summer, and 冬天 equals to 冬/(in) winter.


It can't be too hard even for beginners, right?