How to replace terrain tiles?

[Animalpower]

How to replace terrain tiles on entire map that is grass into snow to get a winter atmosphere. I need the grass to turn to snow after 180 seconds of gameplay and make it grass again after another 180 seconds. I am sure this is possible, I've seen it on some map.

 

[GIMLI_2]

well there is an option to do it with triggers, as you can change terrain in certain locations.
but i dont think thats a good solution