Fog patches, moderate or poor, falling slowly...

The word 'depression' is used to describe the weather, or being a bit fed-up.  It is so often mis-used as a word, as a name for a state of being it is wholly inadequate.

Depression lasts for weeks, months, years.  It's not something that can be lifted by a bar of good chocolate, a bracing walk, or some other treat you can give yourself, (substitute new shoes, clothes, haircut, piece of rare vinyl, glass of wine, whatever you like).

It is very difficult for someone with a well mind to understand an un-well one, so tread carefully, be gentle.
Don't give a depressed person over-optimistically titled self-help books that they probably don't have the concentration to read.  Get them a plant to look at and grow instead.
Don't expect them to recover anytime soon, or tell them to 'pull themselves together', 'snap out of it', or ask, 'what have you got to be depressed about?  You've got a lovely house etc etc'.

Melancholy sounds poetic, but it's not.  Depression is not a sign of weakness or failure, a mood, or a choice, (Who would choose illness over health?).  And remember, it is not their fault.