There seem to be two words for each season. Each pair has the first character in common but for the second character one will have 季 (jì) and the other will have 冬天 (tiān).
For example "winter" is:
- 冬季 (dōngjì)
- 冬天 (dōngtiān)
What is the difference between the two? I can see the first uses the character for "season" and the second uses the character for "heaven", but this doesn't seem to tell me anything useful.