Differences between the two words for each season, such as 冬季 (dōngjì) and 冬天 (dōngtiān) 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.
冬季 is more formal than 冬天. When used to refer to the season itself, the former is more commonly found in literature, the latter in vernacular.

However, when used to refer to something of that season, i.e. in a noun phrase, the 冬季 form is almost always used. For examples, "winter fashion" is 冬季服装, "Winter Olympics" is 冬季奥运会*. You would never use 冬天 for these, unless rephrasing them in a very informal way e.g. 冬天穿的衣服.

*: if they don't get abbreviated awaylike 冬装 or 冬奥, that is.