"It is colder in winter than in summer."
Translation:Il fait plus froid en hiver qu'en été.
Basically, you can't have en + «definite article».
en l' and en la become en, while en le becomes au and en les becomes aux. This explains en hiver vs. au printemps.
Thanks - it's infuriatingly frustrating that just as I think I'm clever and remembering that French will want the definite article, there's another rule that means that it is not needed. Aaargh.
I think I remember saying "in spring" as "au printemps." Why is it different here? Is there a rule that I am forgetting?
As historicbruno said above, en + le does contract to au, while in this example, en + l' contracts to just en.