Same for Environments: some Ancient Ones force you to discard some kind of Environments (Ithaqua forces you to discard Environment - Urban cards, for example); some other dynamics coming with expansions influence Environments as well.
Ithaqua actually affects Weather environments, not Urban. But can I check something on this, as I played Ithaqua yesterday in my first solo game (4 investigators though) and your post makes me wonder if I treated them correctly.
Ithaquas special power (from the wiki) is as follows:
Icy Winds - While Ithaqua stirs in his slumber, any investigator in a street area at the end of the Mythos phase loses 1 Stamina. In addition, all Weather cards are discarded without their special effects taking place.
Now I treated that as 'If you get a weather card, you perform the gate/clue/monster movement as indicated, then discard it before resolving it's special effect - and leaving any previous environbment card in play'.
But in your reply to the OP you say 'forces you to discard Environment - [Weather] cards' making me wonder if they should be immediately discarded (without resolving anything on the card) and another card drawn for gate/clue/movement.
Which is it?
(Hope the above makes sense)