The spells Darkness and Fog Cloud basically have the same purpose: Denying vision within an area. One does it by darkness, the other by obscuring it with fog, but the result is the same: The creation of a “heavily obscured area”.
Let’s compare those two spells:
|Level||2nd level evocation||1st level conjuration|
|Duration||10 min||1 hour|
|Area||15ft sphere||20ft sphere|
Looking at those numbers, Fog Cloud seems superior in every aspect. So why is Fog Cloud a 1st level spell while Darkness is a 2nd level spell? Why would a character who has access to both ever choose Darkness over Fog Cloud? Is there something I am overlooking?